ACTION 3: REPORT ON WASTE TREATMENT TECHNOLOGIES

NTUA

Establishment of Waste Network for sustainable solid waste management

planning and promotion of integrated decision tools in the Balkan Region

(BALKWASTE)

LIFE07/ENV/RO/686

ACTION 3: REPORT ON WASTE TREATMENT TECHNOLOGIES

REPORT ON WASTE TREATMENT TECHNOLOGIES

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INDEX

TABLES .................................................................................................................... 5

FIGURES ................................................................................................................... 7

PICTURES ................................................................................................................ 9

1. BIOLOGICAL TREATMENT................................................................................ 10

1.1. COMPOSTING ............................................................................................................ 10

1.1.1. Introduction to composting process ........................................................................................ 10

1.1.2. Biology of composting ............................................................................................................. 10

1.1.3. Factors affecting the composting process ............................................................................. 12 1.1.3.1. Feedstock and nutrient balance............................................................................................................... 12 1.1.3.2. Particle size ............................................................................................................................................ 15 1.1.3.3. Moisture content ..................................................................................................................................... 15 1.1.3.4. Oxygen flow ........................................................................................................................................... 16 1.1.3.5. Temperature ........................................................................................................................................... 16

1.1.4. Composting Systems Classification ....................................................................................... 19 1.1.4.1. Windrow Systems ................................................................................................................................... 19 1.1.4.2. Turned Windrow System ......................................................................................................................... 23 1.1.4.3. In-Vessel Systems .................................................................................................................................. 30

1.1.5. Post-Processing....................................................................................................................... 37

1.1.6. Mass and energy balances ..................................................................................................... 37

1.1.7. Market potential for products .................................................................................................. 39 1.1.7.1. Limitations .............................................................................................................................................. 40

1.1.8. Environmental impacts ............................................................................................................ 45

1.1.9. Economic data ......................................................................................................................... 46

1.2. ANAEROBIC DIGESTION .............................................................................................. 47

1.2.1. Introduction to anaerobic digestion process .......................................................................... 47

1.2.2. Biology of anaerobic digestion ................................................................................................ 48

1.2.3. FEEDSTOCK OF ANAEROBIC DIGESTION ..................................................................... 50

1.2.4. Procedures of Anaerobic Waste Fermentation ...................................................................... 52 1.2.4.1. Delivery and Storage .............................................................................................................................. 54 1.2.4.2. Pre-processing ....................................................................................................................................... 55 1.2.4.3. Anaerobic Fermentation .......................................................................................................................... 55 1.2.4.4. Post-processing ...................................................................................................................................... 56


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1.2.5. Process Engineering of Anaerobic Fermentation of Biowaste ............................................. 56 1.2.5.1. Dry and Wet Fermentation ...................................................................................................................... 58 1.2.5.2. Continuous and Discontinuous Operation ................................................................................................ 60 1.2.5.3. Thermophilic and Mesophilic Operation ................................................................................................... 61 1.2.5.4. Agitation ................................................................................................................................................. 62

1.2.6. Anaerobic Digestion Products ................................................................................................ 62 1.2.6.1. Biogas .................................................................................................................................................... 62 1.2.6.2. Digestate ................................................................................................................................................ 65

1.2.7. Market potential for products .................................................................................................. 66


1.2.9. Parameters effecting anaerobic digestion process ............................................................... 71 1.2.9.1. Organic Loading Rate ............................................................................................................................. 71 1.2.9.2. Biomass Yield......................................................................................................................................... 72 1.2.9.3. Specific Biological Activity ....................................................................................................................... 75 1.2.9.4. Hydraulic Retention Time and Solids Retention Time ............................................................................... 76 1.2.9.5. Start-Up Time ......................................................................................................................................... 77 1.2.9.6. Microbiology ........................................................................................................................................... 78 1.2.9.7. Environmental Factors ............................................................................................................................ 78 1.2.9.8. Reactor Configuration ............................................................................................................................. 82

1.2.10. Environmental impacts .......................................................................................................... 82

1.2.11. Economic data ....................................................................................................................... 83

1.3. MECHANICAL BIOLOGICAL TREATMENT (MBT) .............................................................. 86

1.3.1. Mechanical sorting component ............................................................................................... 86

1.3.2. Biological processing compartment ........................................................................................ 87


1.3.4. Market potential for products .................................................................................................. 91

1.3.5. Environmental impacts ............................................................................................................ 91

1.3.6. Economic data ......................................................................................................................... 92

1.4. CASE STUDIES OF BIOLOGICAL TREATMENT SYSTEMS IN THE TARGET AREA .................. 93

1.4.1. General ............................................................................................................................ 93

1.4.2. Mechanical Biological Treatment Plant in the West Attica Region, Greece ................ 94

1.4.3. Mechanical Biological Treatment Plant in Chania, Greece .......................................... 96

1.4.4. Mechanical Biological Treatment Plant in Kalamata, Greece ...................................... 97

1.4.5. Composting Plant for Solid Waste at the Landfill Site in Piatra Neamt, Romania ...... 97

1.4.6. Composting Plant in Vrhnika, Slovenia ......................................................................... 98


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1.4.7 Composting Plant in Puconci, Slovenia......................................................................... 99

THERMAL TREATMENT TECHNOLOGIES ......................................................... 100

2.1. GENERAL ............................................................................................................. 100

2.2. INCINERATION .......................................................................................................... 107

2.2.1. General................................................................................................................................... 107

2.2.2. Types of incinerators ............................................................................................................. 111

2.2.3. Air emissions.......................................................................................................................... 123

2.2.4. Wastewater ............................................................................................................................ 133

2.2.5. Solid residues ........................................................................................................................ 135

2.2.6. Mass and energy balances ................................................................................................... 137

2.2.7. Market potential for products ................................................................................................ 137


2.2.9. Economic data ....................................................................................................................... 139

2.2.10. Applicability in the target area............................................................................................. 139

2.3. GASIFICATION.......................................................................................................... 146

2.3.1. General................................................................................................................................... 146

2.3.2. Feedstock............................................................................................................................... 155

2.3.3. Gasifier ................................................................................................................................... 155

2.3.4. Oxygen plant .......................................................................................................................... 156

2.3.5. Gas Clean-Up ........................................................................................................................ 156




2.3.9. Economic data ....................................................................................................................... 160

2.3.10. Applicability in the target area............................................................................................. 160

2.4. PYROLYSIS .............................................................................................................. 164

2.4.1. General................................................................................................................................... 164




2.4.5. Economic data ....................................................................................................................... 167


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2.4.6. Applicability in the target area............................................................................................... 168

2.5. PLASMA GASIFICATION TECHNOLOGY ........................................................................ 169

2.5.1. General................................................................................................................................... 169




2.5.5. Economic data ....................................................................................................................... 175

2.5.6. Applicability in the target region ............................................................................................ 175

REFERENCES ...................................................................................................... 183


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TABLES

Table 1: Indicative C/N ratios of various organic waste streams .................................. 14

Table 2: Indicative relationships between temperature and time duration during composting for the sanitization of the final product ........................................................................ 18

Table 3: Heavy metal limits for European compost standards (mg/kg dm)................... 41

Table 4: Organic pollutants standards for compost and stabilised biowaste ................ 44

Table 5: Impurities standards for compost and stabilised biowaste ............................. 44

Table 6: EU requirements on pathogens/weeds in compost ........................................ 45

based on the EC eco-label ......................................................................................... 45

Table 7: Characteristics of anaerobic waste treatments (Rilling, 1994) ........................ 57

Table 8: Comparison of one- and two-stage processes .............................................. 58

Table 9: Comparison of wet and dry fermentations ..................................................... 60

(Jördening and Winter, 2005). .................................................................................... 60

Table 10: Comparison of continuous and discontinuous feed...................................... 61

Table 11: Comparison of mesophilic and thermophilic process operation (Jördening and Winter, 2005) ............................................................................................................. 62

Table 12: Mean composition and specific yields of biogas in relation to the kind of substances degraded (Rilling, 1994) ............................................................................................. 63

Table 13: Energy input and output from various biomass resources, (EUBIA) ............. 70

Table 14: Biomass yield coefficients for different biological treatment processes and stages (Young and McCarty,1969; Henze and Harremoes, 1983; van Haandel and Lettinga, 1994) 73

Table 15: Biomass yield coefficients for different types of substrate (Pavlostathis and Giraldo, 1991) 73

Table 16: HRTs of anaerobic systems needed to achieve 80% COD removal efficiency at temperature >20 C (Van Haandel and Lettinga, 1994) ................................................ 77

Table 17: Inhibition of anaerobic digestion by heavy metals (Konzell-Katsiri and Kartsonas,1986)......................................................................................................... 81

Table 18: Operation incineration facilities in the USA ................................................ 115

Table 19: Electrical Energy production from Renewable Energy Sources in USA in 2002 (except hydroelectric) (DOE-EIA, Annual Energy Outlook 2002) ............................... 116

Table 20: Daily average values of air emission limit values ....................................... 123

(Directive 2000/76/EC on the incineration of waste) .................................................. 123

Table 21: Half-hourly average values of air emission limit values .............................. 124


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Table 22: Average values over the sample period of a minimum of 30 minutes and a maximum of 8 hours ................................................................................................. 124


Table 23: Limit values of CO concentrations ............................................................. 125


Table 24: Existing technologies for the management and treatment of gaseous pollutants 129

Table 25: Emission limit values for discharges of waste water from the cleaning of exhaust gases 134


Table 26: Summary of quantities of solid waste, wastewater and gases produced during the operation of an incineration plant .............................................................................. 136

Table 27: Synopsis on data on incineration units in Romania .................................... 140

Table 28: Summary of solid waste, wastewater and air emissions generated during the operation of a gasification unit .................................................................................. 152

Table 29: Commercial Plasma Waste Processing Facilities (Circeo, 2007) ............... 173


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FIGURES

Figure 1: Composting process flow chart (Wastesum, 2006) ....................................... 11

Figure 2: The main stages of the composting process (Wastesum, 2006) ................... 12

Figure 3: Schematic diagram of aerated static pile composting ................................... 20

(Diaz et al., 2002) ....................................................................................................... 20

Figure 4: Approximate dimensions for an aerated static pile (Diaz et al., 2002) ........... 21

Figure 5: Examples of a self-propelled and a towed windrow turners .......................... 27

(Diaz et al., 2002; Recycle & Composting Equipment Pty Ltd, 2010) ........................... 27

Figure 6: Schematic diagram of vertical plug-flow reactors ......................................... 32

Figure 7: Typical examples of Channels or Trenches (Turovskiy and Mathai, 2006; Diaz et al., 2002) 33

Figure 8: Typical mass flow diagram of composting .................................................... 37

Figure 9: Heavy metals limit values for compost in European countries ...................... 42

Figure 10: Anaerobic digestion flow chart (Wastesum, 2006) ...................................... 47

Figure 11: The stages of anaerobic digestion (Wastesum, 2006) ................................ 50

Figure 12: Suitability of waste for aerobic composting and anaerobic digestion (Kern et al., 1996) 52

Figure 13: Possible treatment steps used in anaerobic digestion process of biodegradable organic waste (Rilling, 1994) ...................................................................................... 54

Figure 14: Capacity Range of engines in relation to their electrical efficiency .............. 68

Figure 15: Typical mass balance for an anaerobic digestion system ........................... 69

(Ostrem K., 2004) ...................................................................................................... 69

Figure 16: Typical energy balance for an anaerobic digestion system (Ostrem K., 2004)71

Figure 17: (a) Capital cost and (b) M&O costs curves for European MSW digesters (CIWMB, 2008) 84

Figure 18: Mechanical Biological Treatment flow chart ............................................... 86

Figure 19: Schematic presentation of inputs and outputs of a typical mechanical sorting component with aerobic digestion (Juniper, 2006) ...................................................... 89

Figure 20: Schematic presentation of inputs and outputs of a typical mechanical sorting component with anaerobic digestion ........................................................................... 90

Figure 21: Schematic presentation of inputs and outputs of a typical mechanical sorting component with biodrying ........................................................................................... 90

Figure 22: MBT plant in Ano Liosia ............................................................................. 95

Figure 23: MBT plant in Chania, Crete........................................................................ 96

Figure 24: MBT plant in Kalamata, Peloponnese ........................................................ 97

Figure 26: Composting plant in Vrhnika, Slovenia ....................................................... 99


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Figure 27: Composting plant in Puconci, Slovenia ...................................................... 99

Figure 28: Pyramid of the priorities in the waste management sector ........................ 100

Figure 29: Management practices for municipal waste in the EU countries (Eurostat 2008) 101

Figure 30: Incineration process flow chart................................................................. 109

Indicative incineration facilities operating at European level can be seen in the photos below. 110

Figure 31: Diagrammatic configuration of incineration plant (with energy recovery) in Paris 111

Figure 32: Typical mass-fired waste incineration plant .............................................. 112

(with energy production) ........................................................................................... 112

Figure 33: Typical RDF-fired incineration facility ....................................................... 114

Figure 34: Three types of incinerators: (a) fixed grate (left), (b) rotary kiln (middle), (c) fluidized bed (right) (Finbioenergy, 2006).................................................................. 117

Figure 35: Dioxin emissions in USA (Themelis & Gregory 2002) ............................... 126

Figure 36: Dioxins emission in the USA (Deriziotis, 2004). ........................................ 126

Figure 37: Cyclones (left), electrostatic precipitators (middle) & bagfilters (right) ....... 130

Figure 38: Typical bagfilters ..................................................................................... 130

Figure 39: Typical Electrostatic precipitator .............................................................. 131

Figure 40: dry or semi-dry absorption towers (scrubbing) .......................................... 133

Figure 41: Gasification process flow chart ................................................................ 147

Figure 42: Process for converting waste into energy ................................................. 148

(Vasudevan & Mathew 2007) ................................................................................... 148

Figure 43: Vertical steady bed gasification plants ..................................................... 149

Figure 44: Horizontal steady bed gasification plants ................................................. 150

Figure 45: ITI gasification plant flow diagram ............................................................ 154

Figure 46: Schematic presentation of inputs and outputs of a typical gasification process 157

Figure 47: Mass and energy balance of the ITI gasification plant .............................. 158

Figure 48: Flow diagram of the EfW plant in Celje .................................................... 162

Figure 49: Process scheme of the EfW plant in Celje................................................ 163

Figure 50: Energy and mass balance of the EfW plant in Celje ................................. 163

Figure 51: Pyrolysis process flow chart ..................................................................... 166

Figure 52: Plasma gasification process flow chart ..................................................... 170

Figure 53: Plasma torch operation ............................................................................ 172

Figure 54: Plasma Gasification / Vitrification Process ............................................... 178


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PICTURES

Picture 1: MSW incineration plant in Amsterdam ...................................................... 110

Picture 2: MSW incineration plant in Brescia............................................................. 110

Picture 3: MSW incineration plant in Vienna ............................................................. 110

Picture 4: Incineration Plant in Zorbau, Germany for municipal solid waste and industrial waste 111

Picture 5: Incineration plant in Thun, Switzerland for municipal solid waste and dewatered sludge 111

Picture 6: MSW gasification plant in Chiba (Japan) ................................................... 151

Picture 7: Celje Waste to Energy CHP Plant ............................................................. 161

Picture 8: Molten slag pouring from plasma waste gasification reactor (Pyrogenesis Inc, Montreal, Canada) ................................................................................................... 171

Picture 9: Final inert slag residue can be used in construction applications ............... 171

Picture 10: General view of the demonstration gasification / vitrification unit ............. 176

Picture 11: Another general view of the demonstration gasification / vitrification unit . 177

Picture 12: Feeding system ...................................................................................... 177

Picture 13: Gasification / vitrification furnace............................................................. 178

Picture 14: Cyclone Picture 15: Secondary Combustion Chamber ........... 180

Picture 16: Quench Vessel Picture 17: Scrubber ........................ 181


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1. BIOLOGICAL TREATMENT

1.1. COMPOSTING

1.1.1. Introduction to composting process

Composting is defined as the aerobic, or oxygen requiring process during which

the organic matter is decomposed by micro-organisms under controlled conditions

to a biologically stable end product. During composting the microorganisms

consume oxygen for the bio-oxidation of the organic matter resulting in the

generation of heat, carbon dioxide and water vapor, which are released into the

atmosphere (Ipek et al., 2002; Epstein, 1997). At the same time, the volume and

mass of the organic raw material is reduced significantly transforming it into a

stable organic final product which can be used as soil conditioner, improver as well

as for land reclamation (Hogg et al., 2009; Epstein, 1997; Engeli et al., 1993; Carry

et al., 1990; Toffey, 1990). The rate of the organic matter decomposition depends

upon the evolution of the environmental conditions (e.g. temperature, moisture,

oxygen) which regulate the growth of aerobic micro-organisms. Therefore,

composting is the “controlled” aerobic biodegradation of most organic (biologically

derived carbon-containing) solid matter meaning that the environmental conditions

are controlled throughout the process. In that way composting differentiates from

the decomposition which occurs in nature (Gidarakos, 2007). Nevertheless, the

biochemical process in composting and in the natural decomposition of the organic

matter is the same.

1.1.2. Biology of composting

The composting process can generally be divided into two major stages. The first

stage comprises of the ‘‘active phase’’ of the process which mainly involves the

development of bio-oxidation reactions. Therefore, the readily available organic

matter is used as energy source by microorganisms for their metabolic activities.


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The second phase of the composting process, known as ‘‘curing phase’’, involving

the production of organic macromolecules humus-like substances for the formation

of mature compost (Cooperband, 2000). All reactions are based on numerous

biological, thermal and physicochemical phenomena and involve oxygen

consumption, as well as heat, water and carbon dioxide production. A schematic

representation of the composting process is shown in Figure 1.

Figure 1: Composting process flow chart (Wastesum, 2006)

In the process of composting, presented in Figure 1, microorganisms decompose

the biodegradable organic fraction in order to produce carbon dioxide, water, heat,

and humus the biological stable organic end product. Considering that the aerobic

degradation is carried out under optimal conditions, than the composting process

can be classified into through three separate phases (Figure 2) which may have

considerable overlap based on temperature gradients and differential temperature

effects on microorganisms. The first phase incorporates a mesophilic or moderate-

temperature phase (<40-45oC), which has a duration of a couple of days. The

second stage, named thermophilic phase (>40-45oC), involves the development of

Feedstock (organic matter including carbon,

chemical energy, protein, nitrogen)

----------------------------------- Minerals (including nitrogen and other

nutrients) -----------------------------------

H2O ----------------------------------

Micro-organisms

Organic matter

Minerals

H2O

Micro-organisms

Finished-compost

Oxygen O2

H2O Raw Material

Compost site

Heat CO2


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elevated-temperatures due to enhanced decomposition of the organic compounds

which can last from a few days to several weeks depending on the availability of

organic matter to be exposed to aerobic degradation. The last stage includes the

cooling and maturation phase which results to the biological stabilisation of the

end product. The factors affecting the composting process include: the physical

and chemical properties of the raw material, the level of oxygen, the moisture

content, the temperature and the time over which the composting process takes

place.

Figure 2: The main stages of the composting process (Wastesum, 2006)

1.1.3. Factors affecting the composting process

The main parameters which regulate the composting process include: the physical

and chemical properties of the raw material, the level of oxygen, the moisture

content, the temperature and the particle size of the substrate.

1.1.3.1. Feedstock and nutrient balance

In the composting process it is essential to determine the nutrient content of the

feedstock, since aerobic micro-organisms responsible for the biodegradation of the

organic raw material require various nutrients in order to grow. The main nutrients


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involved are the following in a decreasing order of importance: carbon (C),

nitrogen (N), phosphorous (P) and potassium (K) (Gajalakshmi and Abbasi, 2008).

Apart from micro-organisms growth, nitrogen, potassium and phosphorous are

also primary nutrients for plants and their concentrations influence the quality of

the composted organic material. Of the nutrient elements required for microbial

decomposition, carbon and nitrogen (C:N ratio) are considered to be the most

important, since the majority of organic materials contain ample quantities of

nutrients (EA, 2001). Therefore, the amounts of carbon and/or nitrogen are the

substances most likely to affect the composting process by their presence in

insufficient or excessive quantities. Carbon is both the energy source and the

basic microbial building block of microorganisms, whereas nitrogen is a crucial

component of proteins, nucleic acids, amino acids, enzymes and co-enzymes

needed for microbial cell growth (Gajalakshmi and Abbasi, 2008). Considering the

above, the nutritional balance during composting is mainly de ned by the carbon

to nitrogen ration i.e. C/N ratio. According to literature the optimum C/N ratio for

composting is in the range 25–35 (Gaur, 2000; Golueke, 1992), because it is

considered that the microorganisms require 30 parts of C per unit of N (Alexander

1977; Bishop and Godfrey, 1983). High C/N ratios make the process very slow as

there is an excess of carbonated degradable substrate for the microorganisms

(Bernal et al., 1998; Verdonck, 1988). On the other hand, in low C/N ratios there is

an excess of N in the organic material which in turn provides excess of inorganic N

due to the nitrification process leading to ammonia losses through volatilisation or

nitrogen losses by leaching from the composting mass (Pagans et al., 2005;

Sanchez-Monedero et al., 2001; Reddy et al., 1979). Controlling the C/N ratio,

when mixing different organic materials, is crucial for the successful

implementation of the composting process. In general, "green" organic materials

(e.g., grass clippings, food scraps, manure), contain large amounts of nitrogen,

whereas "brown" organic materials (e.g. dry leaves, wood chips, branches),

contain large amounts of carbon but little nitrogen. Therefore a proper balance of

"green" and "brown" organic waste shall result in appropriate C/N ratios for the

initiation of the composting process. Indicative C/N ratios of various organic waste

are presented in Table 1.


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Table 1: Indicative C/N ratios of various organic waste streams

organic waste

C/N Diaz et

al. (2002)

Diaz and Savage (2007)

Trautmann and Krasny

(1998)

IWMI (2003)

Cornstalks 60-73 - -

Fruit waste 20-49 - 20-50

Rice hulls 113-

1120 - -

Vegetable waste 11-13 - 10-20 13

Poultry litter (broiler) 12-15 15 - 10-18

Cattle manure 11-30 18 -

Horse manure 22-50 25 20-50

Garbage (food waste) 14-16 - 15 10-16

Paper (from domestic

refuse) 127-178 - 100-200

Newspapers - - 400-900

Refuse 34-80 - - 23-66

Sewage sludge 5-16 11 - 6-10

Primary sludge - - - 7-11

Activated sludge - 6 - 6-8

Grass clippings 9-25 - 10-25

Leaves 40-80 - 40-80

Shrub trimmings 53 -

Tree trimmings

Bark

- hardwood

- softwood

16

-

-

-

-

-

-

100-400

100-1200 170-500

Wood chips or shavings

- hardwood

- softwood

-

-

-

-

450-800

200-1300

Sawdust 200-750 200-500 200-750

Straw - 128-150 50-150


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1.1.3.2. Particle size

The particle size of the substrate and more specifically the surface area of the

organic material exposed to microorganisms is another factor which affects the

rate of the composting process. The raw material which is grinded, chipped and/or

shredded acquire reduced particle size or otherwise increased surface area on

which the microorganism can feed thus waste is degraded more rapidly (EA,

2001). In addition, smaller particles also produce a more homogeneous mixture

and improve substrate insulation which promotes the maintenance of optimum

temperatures (O’Leary and Walsh, 1995). However, if the particles are too small,

they might prevent air diffusion through the substrate (Gidarakos, 2007).

According to Diaz et al. (2002), an average particle size of 10mm to 50mm

generally produces the best results. However, certain composting methods that do

not include a turning process require a more robust physical structure to resist

settling (e.g. due to gravity and biodegradation process), so larger particles are

necessary (greater than 50mm) (Diaz and Savage, 2007). GTZ (2000)

recommends chopping all materials to be composted to the length of about 50-

100mm, whereas Obeng and Wright (1987) reported that typical particle sizes

should be approximately 10mm for forced aeration composting and 50mm for

passive aeration and windrow composting.

1.1.3.3. Moisture content

Moisture supports the metabolic and biodegradation processes of the micro-

organisms, since water is the medium for biochemical reactions, transportation of

nutrients and allows the microorganisms to move about (Gajalakshmi and Abbasi,

2008). Generally, the ideal moisture content is considered to be between 40% and

65% for the optimal biodegradation of the raw material. However, the optimal

moisture level is depended upon the composted material and more specifically on

its porosity (Diaz and Savage, 2007). Organic mix with a low porosity requires

higher moisture content than a substrate with a higher porosity level (Diaz and

Savage, 2007). Moisture content which is lower or higher than the optimum range

results in the inhibition of the microbial activity due to early dehydration and the

formation of anaerobic conditions respectively (Gajalakshmi and Abbasi, 2008; de


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Bertoldi et al., 1983). When the moisture content exceeds 70%, O2 movement is

inhibited and the process tends to become anaerobic because the air spaces of

the substrate are filled with water obstructing the sufficient oxygen diffusion within

the organic mass (Tiquia, et al., 2002, 1996). On the other hand, if the moisture

content is lower than required, the microorganisms’ growth and subsequently the

decomposition rate of organic matter are significantly reduced creating a final

product that is physical but not biologically stabilized (Diaz and Savage, 2007a; de

Bertoldi et al., 1983).

1.1.3.4. Oxygen flow

The oxygen that is required for the composting process is essential for the aerobic

metabolism and respiration by the microorganisms, but also for the bio-oxidation of

the organic molecules present in the substrate. Oxygen consumption during

composting is directly proportional to the microbial activity providing a direct

relationship between oxygen consumption, temperature, moisture and aeration

(EA, 2001). Therefore, aeration is a key factor for composting, since proper

aeration controls the temperature, removes excess moisture and CO2 and

provides O2 for the biological processes. According to Miller, (1992) the optimum

O2 concentration is between 15% and 20%. If there is insufficient oxygen, the

process can become anaerobic involving a different set of micro-organisms and

different biochemical reactions which result in the production of methane gas and

malodorous compounds, such as hydrogen sulfide gas and ammonia. Aeration of

the organic substrate is achieved through agitation, active aeration (air blowing)

and/or passive aeration (natural diffusion of air though negative pressure) (IWMI,

2003).

1.1.3.5. Temperature

Temperature is one of the main control parameters of the composting process and

constitutes a by-product of the microbial activity during organic matter

biodegradation. The importance of temperature monitoring lies on the fact that it

reflects the activity of microorganisms in the substrate and it represents an

indicator of the proper evolution and occurrence of the composting process (Diaz


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and Savage, 2007a). According to Hassen et al. (2001) substrate temperature

determines the rate at which biological processes take place and plays an

important role in the evolution and succession of the micro-organisms population.

Microorganisms require a certain temperature range for optimal activity. de

Bertoldi et al. (1983) state that the optimum temperature range for the

maximization of the decomposition rate is 40–65°C. According to Epstein (1997)

and Miller (1992), thermophilic microorganisms become less active at elevated

temperatures between 60-70°C and thus the microbial activity is reduced. At even

higher levels (>70°C) Mena et al. (2003), Fermor et al. (1989) and Finstein et al.

(1986) indicate that the microorganisms suffer the effects of high temperatures

(inactivation or elimination) and the process slows down. At these temperatures

many micro-organisms die or become dormant and the process effectively stops

until the micro-organisms can recover.

While high composting temperatures may inhibit or slow down the biodegradation

of organic waste, elevated temperatures are desirable for the destruction of

pathogens and weed seeds that may be contained in the substrate. According to

Hogg et al. (2002) the key parameter for the sanitization of the substrate is the

temperature-time regime. Indicative relationships between temperature and time

duration during composting for the sanitization of the final product is given in Table

2.


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Table 2: Indicative relationships between temperature and time duration during composting for the sanitization of the final product

Country Composting System

Temperature – time regime

Canada

(CCME,

2005)

Windrows 55°C for 15 days

Static piles 55°C for 3 days

In-vessel 55°C for 3 days

USA

(USEPA,

2003)

Class

compost

B Class compost

Windrows 55°C for 15

days

40°C for 5 days. For 4

hours during the 5 day

period, the temperature

must exceed 55°C

Static piles 55°C for 3

days




must exceed 55°C

In-vessel 55°C for 3

days




must exceed 55°C

UK

(DoE,

1996)

Windrows or Aerated

piled

The compost must be maintained at 40°C for

at least 5 days and for 4 hours during this

period at a minimum of 55°C within the body

of the pile followed by a period of maturation

adequate to ensure that the compost reaction

process is substantially complete


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1.1.4. Composting Systems Classification

The currently available compost systems can be generally classified into two

broad categories the “windrow” and the “in-vessel” composting systems. The main

feature of windrow technology is the accumulation and formation of the organic

substrate into piles. Typically, the piles are usually shaped into more or less

elongated windrows with specified width and height. With respect to the in-vessel

composting systems, the aerobic decomposition of the organic matter takes place

in a bioreactor. It should be noted that many of the current in-vessel systems

involve the parallel use of windrow systems for the curing and maturation phase of

the end product (Dziejowski and Kazanowska, 2002).

1.1.4.1. Windrow Systems

Windrow systems are further subdivided on the basis of the aeration method of the

substrate into “turned windrow” and “forced air windrow or static pile”. The

windrows may or may not be sheltered from the elements. In the windrow

composting process, the mixture to be composted is stacked in long parallel rows

or windrows. The cross section of the windrows is usually trapezoidal or triangular,

mainly depending on the characteristics of the equipment used for the agitation or

aeration of the piles. A variety of factors combine to determine the dimensions of

the area requirement. Among them are total volume of material to be

accommodated during all stages of the compost process, i.e., from the

construction of the windrows through disposal of the stored product, the

configuration of the windrows, space required for the associated materials

handling equipment and the maneuvering thereof, and the aeration system (forced

or turning).

1.1.4.1.1. Forced air windrow – Aerated static system

In a forced air windrow or aerated static composting system, air is either forced

upwards through the composting mass or is pulled downwards and through it

(Shammas and Wang, 2009). In both instances, the composting mass is not


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disturbed. The forced aeration composting systems usually involves a combination

of drawing air into and through the pile, followed by air forcing upward through the

pile. The air that leaves the substrate is either discharged directly into the

environment, or is forced through a cone-shaped biofilter (e.g. finished compost or

other “stable” organic matter). The use of biofilter serves as a mean for the

deodorization of the effluent air stream. According to Bidlingmaier (1996) and

Schlegelmilch et al. (2005) finished compost and other organic materials can

effectively serve as an odor filter. The basic arrangements of an aerated static pile

are shown in Figures 3 and 4. The system includes the following six steps:

1. the mixing of the raw material with of a bulking agent

2. the construction of the windrow (width and height),

3. the decomposition process of the substrate (composting),

4. the screening of the end product and the removal of the bulking agent,

5. the curing phase and

6. the storage of the final product.

The construction of the windrow involves the longitude and parallel placement of a

series of perforated longitudinally orientated air pipes (e.g. 10.2–15.2 cm diameter)

along each compost pile (Diaz and Savage, 2007).

Figure 2: Schematic diagram of aerated static pile composting

(Diaz et al., 2002)


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Figure 4: Approximate dimensions for an aerated static pile (Diaz et al., 2002)

In order to avoid short circuiting of air during air suction or forced aeration, the

perforated pipes must be placed in an appropriate distance from the edges of the

windrow. According to Diaz and Savage (2007) this distance is between 1.5–2.7m.

The pipes are connected to a blower through a length of non-perforated pipe. The

piping network is covered with a layer of bulking agent or finished compost that

extends over the area of the pile in which the raw material will be composted. The

formation of a bulking agent layer is used to facilitate the movement and uniform

distribution of air throughout the organic mass during composting. In addition, the

formed layer enables the absorption of excess moisture resulting from the

composted raw material and thereby minimizing leachate runoff. The organic

waste is stacked on the network piping and to bulking agent layer in order to form

the windrow pile, as shown in Figure 3. According to the specifications provide by

Diaz and Savage (2007), the finished pile should be about 20–30m long, about 3–

6m wide, and about 1.5–2.5m high. Additionally, on top of the formulated pile a

layer of matured compost or synthetic materials (about 15-20 cm thick) is placed in

order to absorb emitted odors from the composting mass and to ensure the

homogeneous distribution of temperature throughout the organic matter. The

aforementioned arrangement aims at achieving the desired temperature level to


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optimize the decomposition rate of the organic material and to obtain the required

temperature-time regime for the sanitization of the organic mass throughout the

pile. Leachate control is provided by sloped and sealed or impervious composting

pads with a surrounding drainage system (Diaz and Savage, 2007).

1.1.4.1.2. Extended Aerated Pile

In case where large amounts of organic material are to be composted, the

extended aerated pile method can be adopted. The extended aerated pile has the

following arrangement: On the first day, a pile is constructed in the same way as

described in for the conventional aerated static pile, with the exception that only

one side and the two ends of the pile are covered with the a finished compost

layer. The exposed side of the pile is covered with a thin layer of compost in order

to prevent the escape of odors. On the second day, a second piping network and

bulking layer is set parallel to the exposed side of the pile erected on the first day,

and the pile is erected in the same manner as was the first pile. This procedure is

repeated for 28 days. The first pile is removed after 21 days; the second pile on

the day after, and so on. An important advantage of this approach is a substantial

reduction in spatial requirements. The land area requirement for systems that use

a single pile is about 7–11 tonnes d.w. of organic waste treated per hectare. The

estimate of about 7 tonne/ha allows for sufficient land area to accommodate

leachate collection, administration, and storage (e.g. raw material, end product).

1.1.4.1.3. Economics

The aerated static pile method is probably the least expensive method of all of the

various types of composting technologies that are currently available. Aerated

static pile method is a low technology which requires minimum capital investment

in terms of equipment (the amount of material handling is limited) as well as in the

operation and maintenance cost (Shammas and Wang, 2009). It is difficult to

present a generally applicable capital cost for static pile composting technology

since the treatment process and the developed compost markets are usually site

specific. With respect to material and operational costs, Diaz et al. (2007) state


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that the cost for composting a mixture of sludge and woodchips is about $50 per

tonne (2005 US dollars), of which about $10 per tonne is for woodchips whereas

according to Mavropoulos et al. (2008) the windrow composting cost of green

waste ranges between 20 to 35 € per tonne.

1.1.4.1.4. Limitations

The aerated static pile method is not the most suitable for all types of raw

materials and under all conditions. Since aerated static pile method does not

acquire a mechanism for the agitation of the substrate during the composting

process, the material used requires having relatively uniform particle size not

exceeding 3.5–5 cm in any dimension (Diaz and Savage, 2007). Granular

materials such as sludge are the most appropriate. In case of organic mixtures of

large particle size that exhibit a wide spectrum of dimensions the composting

process shall probably result in uneven distribution and movement of air through

the pile. This uneven distribution of air through the pile promotes air short-

circuiting and the development of anaerobic pockets of decomposing material.

1.1.4.2. Turned Windrow System

The turned windrow method is the one that traditionally and conventionally has

been associated with composting. The term “turned” applies to the method used

for aeration. Aeration of the windrow is achieved by agitation of the substrate using

tractors with front end loaders or any other appropriate machinery which tears

down the piles and reconstructs them. Turning not only promotes aeration, but it

also ensures uniformity of decomposition by exposing at one time or another all of

the composting material to the particularly active interior zone of a pile. In addition,

the mechanical agitation of the substrate reduces to some extend the particle size

of the organic material, whereas water loss due to evaporation (elevated

temperatures) is accelerated (Cornell University, 2010). The water loss can be

considered as a benefit of the composting method in cases where the moisture

content of the substrate is too high. However, in low moisture content the turning


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of the substrate can be potentially disadvantageous and water addition is required

during the turning process.

1.1.4.2.1. Construction of Piles

As in the case of static pile method, forced windrow piles are constructed by

stacking the prepared feedstock in the form of an elongated pile. The cross section

of the windrows is trapezoidal or triangular depending on the conditions of the area

in which the system is cited (e.g. geared to climatic conditions and efficient use of

the composting working area.). According to Diaz and Savage (2007) in dry and

windy areas the piles usually acquire a trapezoidal shape because the ratio of

exposed surface area to volume is lower with such a configuration. In addition, the

volume of the overall hot zone is greater in a trapezoidal shaped pile in relation to

a triangular or conical cross section, since heat loss is less and windrow volume

per unit pad area is greatest. On the other hand, during wet weather the flattened

top is a disadvantage because water is absorbed into the composting mass

changing the moisture conditions within the organic mass. Although the climate

condition might influence windrow geometry, in practice, the determinant is mainly

the turning equipment (e.g. manual, mechanical type).

In operations in which the turning is carried out mechanically, the pile configuration

that results will obviously be the one imparted by the machine. Ideally, the windrow

should be about 1.5–2.0m high (Diaz et al., 2002). In situations in which it is

practical to perform the turning manually, the height should be roughly that of the

average laborer. At most, it should not be higher than that easily reached with the

normal pitch of the equipment used in turning. Another factor that impacts the

maximum height is the tendency of stacked material to compact. The height for

mechanical turning depends on the design of the turning equipment — generally, it

is between 1.5 and 3.0 m (Diaz et al., 2002). The pile’s width is a function of

convenience and expediency, since it has little effect to the diffusion of oxygen into

a pile and, therefore, it does not contribute significantly to meeting the oxygen

requires within the composting mass. With manual turning, a width of about 2.4-

2.7m is considered to be suitable, whereas the width of the pile with mechanical


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turning depends upon the design of the mechanical equipment (usually 3.0 to 4.0

m) (Diaz et al., 2002). In theory, the length of the windrow is indeterminate. For

example, the length of a 180 tonne conical shape windrow of material at a height

of 1.8m and width of 2.5m would be about 46.0 m. A nearly continuous system can

be set up by successively adding each day’s input of raw waste to one end of the

windrow. The continuous windrow system is employed by adding fresh material to

one end of the pile and removing material from the other as it reaches stability.

1.1.4.2.2. Arrangement of the Windrows

The specific arrangement of the windrows at a composting facility depends upon

the availability of land and the accessibility of the equipment used. Whatever the

arrangement, the windrows should be positioned such that each day’s input can

be followed until the material is completely composted. An important requirement

is the space that is required to perform the turning of each day’s input material,

whether the turning is done manually or mechanically. With manual turning, the

total area requirement is at least two times that of the original windrow depending

upon the type of machinery used, since turning varies with type of machine (Diaz

and Savage, 2007). It is worth mentioning that some machines accomplish turning

in such a manner that as the original windrow is torn down, the new windrow is

reconstructed directly behind the machine. This type of machine requires little

more area than that of the original windrow, whereas other types of machines

rebuild the windrow adjacent to its original position the area requirement of which

is comparable to that described for manual turning. According to Diaz et al. (2002)

a considerable degree of advancement has been made in the design of

mechanical turning machines. Emphasis has been placed on the comfort of the

operator, on the size of the windrow, and on the overall space requirements.

Indicative types of turners are the auger turner, the elevating face conveyor, and

the rotary drum with flails.


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1.1.4.2.3. Methods of Turning

With respect to manual turning of the piles, the most common and convenient

equipment is the pitchfork with four or five tines. In the manual turning of the pile,

when reconstructing building the pile, material from the outside layers of the

original windrow should be placed in the interior of the rebuilt windrow. In this way

during the compost cycle every particle of the material is at one time or another in

the active interior zone of the pile. If this ideal situation is not practical to attain, the

deficiency can be compensated by increasing the frequency of turning. Finally, it is

important not to compact the raw material when constructing the original windrow,

and when rebuilding the pile.

1.1.4.2.4. Frequency of Turning

The turning frequency of the pile is strongly related to the rate of oxygen uptake by

the active microbial population. Practically, there is a compromise between

required turning frequency and technical and economic feasibility of meeting that

the required frequency. Structural strength and moisture content of the substrate

are the most important physical characteristics when it comes to the determination

of the turning frequency. Other parameters involved with the effectiveness of the

turning procedure are the pathogen elimination and the uniformity of substrate

decomposition. Another variable factor is the decomposition duration desired by

the operator. High-rate composting requires very frequent turning, since the rate of

degradation is directly proportional to the turning frequency. The lower the

moisture content of the organic matter and the firmer the structure of the particles,

the less frequent will be the required turning. For instance, when straw, rice hulls,

dry grass, dry leaves, woodchips, or sawdust are used as bulking material and the

moisture content of the mixture is about 60% or less, turning on the third day after

constructing the original pile and every other day thereafter, for a total of about

four turnings, is sufficient to accomplish “high rate” composting. After the fourth

turning, the frequency can be reduced to once each 4 or 5 days. The same

program is applicable if paper is the bulking material, provided that the moisture

content does not exceed 50% (Golueke and McGauhey, 1953). If the composting


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mass gives off fouling odors, it means that the composting process has become

anoxic, which in most cases it is due to the presence of excessive moisture. To

overcome the anoxic condition additional turning, at least once each day, is

required to foster evaporation until odors are disappeared.

1.1.4.2.5. Equipment Used for Turning

There are several types of windrow turners available on the market with higher

capacity than rototillers which are quite satisfactory for relatively small composting

operations (Diaz and Savage, 2007). Windrow turners can be generally

categorized into three main groups divided according to the design of the turner

mechanism. The three groups include the auger turner, the elevating face

conveyor, and the rotary drum with flails. Some types of turners are designed to be

towed and others are self-propelled (Figure 5). The self-propelled types are more

expensive than the towed types. An advantage of the towed type is the fact that

the tractor can be used for other purposes between turnings. In addition, the self-

propelled type requires much less space for maneuvering and, therefore, reducing

the required area, since windrows can be closer to each other. The turning

capacity of the machines ranges from a few tonnes per hour to as much as 3,000

tonnes/h depending on model, whereas the costs (2005 US dollars) of the self-

powered machines range from about $200,000 to 300,000 (Diaz et al., 2002).

Figure 5: Examples of a self-propelled and a towed windrow turners (Diaz et al., 2002; Recycle & Composting Equipment Pty Ltd, 2010)


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1.1.4.2.6. Site Preparation

The composting piles should be placed on a hard paved surface. The pad should

be sufficiently rugged to support the combined weight of the composting mass and

associated materials handling equipment, as well as the maneuvering of the

machines. The main reasons for the paving are: (1) to facilitate materials handling,

(2) to control any leachate that may be formed, and (3) to prevent fly larvae from

escaping the area. In summary, preservation of sanitation and materials handling

are the two key factors. In operations processing less than about 10 tonnes/day,

the paving may consist simply of well-compacted clay as a base with a layer of

packed gravel or crushed stone on the surface. In the event that crushed stone

and gravel are not available, a layer of soil can be used. The soil should be firmly

packed on top of the clay. Of course, when soil is used as the top layer, a problem

arises during the rainy season. Paving is especially essential if mechanical turners

are utilized. The machines are fairly heavy and, accordingly, can operate properly

only on a firm footing. Paving materials in addition to gravel and crushed stone are

asphalt and concrete. Special provisions should be made for collecting the

leachate that might be generated. The fresh leachate has an extremely

objectionable odor and unless controlled, it can lead to the development of

problems. In desert regions, the windrows should be protected from the wind so as

to reduce moisture loss through evaporation. In regions of moderate to heavy

rainfall, the windrows should be sheltered from the rain. If shelters are not

available, the possibility of the windrows taking in an excessive amount of

moisture would be particularly high.

1.1.4.2.7. Economics

The cost of windrow composting depends on factors such as the type and particle

size distribution of the organic feedstock (whether preprocessing is required for

size reduction), the contamination level of the raw material, the requirements for

permits, the labor costs and the use and value of the produced compost. Generally

windrow composting facilities are relatively low-cost processes. If the composted


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raw material is yard waste, then the cost of the process using turned windrow

technology is in the range of $15–30 per input tonne inclusive of capital and

operation and maintenance expenses (Diaz et al., 2007). According to recent data

provided by ARCADIS & EUNOMIA (2010) the windrow composting costs ranges

marginally higher than 20 to 40 €/tonne (net of compost sales) depending upon the

specifications of the composting facilities. Additionally the operating costs is

estimated at 6.5 €/tonne whereas the annual maintenance costs is considered at

3.15 €/tonne.

1.1.4.2.8. Limitations

The major limitation of turned windrow systems probably is related to public health

issues. The limitations are particularly applicable to operations that involve the

processing of sewage sludge or animal waste which incorporate pathogenic

microorganisms. This limitation stems from two basic features when operating

turned windrows. The first feature is related to the fact that elevated temperatures,

which favor pathogens elimination, do not generally prevail throughout a windrow,

since in its outer layers the temperatures are lower than in the active interior zone

of a pile. The second feature involves the recontamination of the sanitized material

when turning the pile. In case when outer layers of the pile do not acquire the

desired temperature level there is a risk of pathogen exposure of sanitized organic

material (Bustamante et al., 2008). However, repeated turning eventually reduces

the pathogen populations to concentrations that are less than infective. This latter

condition is reached by the time the material is ready for final processing and use.

Improper and/or insufficient turning might lead to the generation of fouling odors.

Even with a suitable protocol, some odors are certain to be generated. This

situation is typical for all composting system that involves handling and processing

of organic waste, regardless the method employed (e.g. static, turned windrow, or

mechanized composting). The generation of objectionable odors is mainly

occurring, in nuisance proportions, during the preparation and the active process

of composting. Therefore, appropriate preventing measures can be taken only

during that time. A slower then optimum rate of organic matter decomposition and

the subsequent larger land requirement often have been alleged against the use of


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the turned windrow as contrasted to the “highspeed” composting which is claimed

for mechanical composting. With respect to the composting rate, it should be

emphasized that enhanced composting is achieved only when high-priced land

area and costly machinery usage are involved. If machinery means are not

involved and land area is not critical, rapid composting loses its advantage.

Furthermore, under those conditions, the intensity and frequency of turning can be

reduced. The reason is that very little odor is emitted from a pile of composting

material that is not disturbed. It is mainly during the turning process that foul odors,

if present, are released from the pile. However, it must be emphasized that this

relaxation in terms of turning frequency of the pile is safe only when no human

habitations are nearby (i.e. distance >150m).

1.1.4.3. In-Vessel Systems

In-vessel composting occurs within a contained vessel, enabling the operator to

maintain closer control over the process in comparison with other composting

methods. The in-vessel systems are designed to minimize odors (e.g. biofilter) and

process time by controlling environmental conditions such as air ow, temperature,

and oxygen concentration. In this section the term “in-vessel” or “reactor” is

applied to the unit or set of units in which the “active” stage of composting takes

place. These units are also called bioreactors, since composting essentially is a

biological process. There are several in-vessel systems on the market. The growth

in new designs is partly related to the regulatory requirements enacted by some

European member states and by the EU. The primary objective of the in-vessel

design is to provide the best environmental conditions, particularly aeration,

temperature, and moisture. Nearly all in-vessel systems use forced aeration in

combination with stirring, tumbling, or both.

In general, bioreactors can be divided into two main types (1) vertical and (2)

horizontal (Haug, 1993). Horizontal bioreactors are further categorized into (1)

channels, (2) cells (3) containers (4) tunnels and (5) “inclined” reactors or rotating

drums (Crowe et al., 2002). In-vessel bioreactors can also be classified as a

function of the movement of the material. Consequently, the reactors can be

denoted as static and dynamic.


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The retention time, during which the active phase of composting takes place within

the bioreactors, generally lasts from 7 to 15 days and largely depends upon the

type of substrate used. Since the detention time is rather short, upon completion of

the rapid degradation phase, the material that exits the bioreactor most of the

times is placed in windrows for further maturation. A brief description of each type

of bioreactor is presented to the following paragraphs.

1.1.4.3.1. Vertical Reactors

Generally in vertical in-vessel bioreactors the organic material is introduced

through the top of the system and removed from the bottom of the unit as shown in

Figure 6. The end product is usually is discharged out the bottom of the bioreactor

by a horizontally rotating screw auger. As such, the bioreactors acquire the

configurations to operate in a continuous basis. Air is introduced in these systems

by forced aeration either from the bottom, by means of aeration pipes, traveling up

through the composting mass where it is collected for treatment or through air

lances hanging from the top of the bioreactor. The emitted gas is removed from

the reactors is transported to a gas treatment system. Typically, vertical in-vessel

bioreactors involve some type of cylindrical container or tank and they are

manufactured from steel and concrete, whereas they are thermally insulated. The

capacity of these systems ranges from a few cubic meters to more than 1500m3

(Diaz et al., 2007). It must be stated that most vertical bioreactors are used for

composting solid waste and sewage sludge and they have been plagued by a

number of operational difficulties (Diaz et al., 2002).


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Figure 6: Schematic diagram of vertical plug-flow reactors

1.1.4.3.2. Horizontal Reactors

Horizontal reactors are units that, as their classifications suggest, operate in the

horizontal position. Horizontal reactors can be further classified into: channels,

cells, containers, tunnels and rotating drums.

Channels or Trenches These designs are similar to windrow composting facilities, since the organic

material is placed in piles. The main difference between the channels and

windrows is that in channel composting, the material to be treated is placed

between walls, whereas in most cases the facility is housed inside a building. The

walls vary in height from 1-3m with a distance of approximately 6m apart from

each wall, whereas the piles are about 50m long. The air is introduced within the

organic mass through forced aeration or air suction, while at the same time the

substrate is agitated with mechanical turning. In order to manage the potentially

negative impacts of the emissions from the composting mass, the processing

building is kept under negative pressure. The intake emitted gas is removed from

the building to a biofilter (Misra et al., 2003) or other air pollution control device

(e.g. bioscrubbers) (Shammas and Wang, 2009). Generally, the raw material is

loaded into the trenches by means of a conveyor belt or with automated units that

use Archimedean screws or front-end loaders, whereas in similar manner the end

product is unloaded. Channels can be operated either on a batch basis or on a


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continuous basis. On a batch basis, the incoming material is loaded into the

channel as soon as the first phase of the composting process is finished and

treated material has been removed. In channels which operate in a continuous

mode, the incoming material is loaded on a daily basis. Channels operating on a

continuous basis can be further classified into longitudinal and lateral channels

according to the movement direction of the organic material treated.

Longitudinal Channels: In this type of channel the substrate is gradually moved

from one end to another by the turning machine. Therefore, the processing time is

related to the design of the turner which in turn specifies the movement rate of the

composting mass. Indicatively, during the intensive phase of decomposition (the

first phase of the process), the processing time is about 4 weeks (Diaz et al.,

2007). Longitudinal channels incorporate different channel shapes the most

common of which are the straight, elliptical and U-shaped.

Lateral Movement Channels: In this type of channel the substrate is transferred by

the turning machine laterally to the next row. The loading operations are usually

performed through the use of conveyors. Loading is carried out approximately

every 2–3 days, depending upon the volumetric capacity of the channel. Most of

the designs include forced aeration and rely on the use of conveyors to remove

the composted material and transport it to the maturation area (Diaz et al., 2007).

Figure 7: Typical examples of Channels or Trenches (Turovskiy and Mathai, 2006; Diaz et al., 2002)


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Cells Cells, or biocells, are hermetically enclosed units, generally rectangular in shape,

in which the composting takes place. In that way the environmental conditions of

the composting process can be fully controlled and optimized, whereas the outside

surfaces are thermally insulated to minimize thermal losses during composting.

Biocells operate on batch basis and they can be built onsite or can be

prefabricated. In a typical operational sequence, the substrate is introduced into

the cell by means of front-end loaders or conveyors. Once the unit has been filled,

the biocell is closed and the composting process begins. Typically, the period of

intensive composting lasts for approximately 14 days depending on the type of

treated material. Air supply is provided to the organic matter by means of a forced

aeration system (pipes or channels) through the bottom layer of the cell forcing the

air to move upwards through the organic mass. The gas emitted during the bio-

oxidation phase is removed at the top of the biocell and usually directed to a

biofilter or partially recirculated. Some biocell systems incorporate a heat

exchanger to pre-heat the air prior to introduction into the composting mass. Water

addition to the substrate is performed by means of a hydration system (nozzles

and pipes) which is typically installed at the top of the biocell, whereas the

generated leachate (excess moisture) is collected and recirculated to regulate the

moisture content. Furthermore, some biocell models incorporate screw conveyors

and moving floors aiming to agitate the substrate, while being in the container.

After the end of the intensive composting process, the organic material is removed

from the biocell with a front-end loader. The biocells capacity ranges between 100

and 1000m3, whereas typical dimensions are: 6m wide, 4m high and more than

50m long. The height of the material inside the container must be carefully chosen

in order to limit substrate’s compaction and enhance proper air diffusion

throughout the composting mass.

Containers Typically the top of the container is opened or removed and the raw material is

loaded into the container by means of a conveyor belt or a front-end loader.

Containers usually are rectangular in shape, with volumetric capacities ranging

from 20 to 40m3. Air is supplied to the substrate via force aeration from the bottom


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of the system (e.g. nozzles), while the exhaust gas is removed from the container

and directed to an air control system (i.e biofilter). The containers are usually

equipped with a hydration system for the moisture content control of the substrate,

whereas generated leachate is gravitationally removed through perforated pipes

located at the bottom of the system. The processing time is about 8–15 days, at

the end of which the organic material is discharged from an opening located at the

one end of the container by means of a roll-off collection truck. The containers are

usually installed in parallel system/modules with which a nearly continuous system

can be set up. Each module includes approximately 6 to 8 containers with a total

capacity which ranges from 3,000 to 5,000 tonnes/year of organic matter.

Tunnels Tunnels or biotunnels are insulated, rectangular shaped structures which are

made out of metal, concrete or brick and their typical measurements are: 4–5m

wide, 3-4m high, and up to 30m long. The tunnels operate on a continuous basis

and the substrate is introduced at the one end of the tunnel on a daily basis. The

organic mass is forced forward toward the opposite end of the tunnel by means of

a hydraulic piston or through the reciprocating motion of moving floors. Moisture

and oxygen levels are monitored at all times, whereas water and air are introduced

whenever deemed necessary. Aeration is provided to the organic mass via

compressors which deliver the air through the floor while in some tunnel systems

air is provided by the use of centrifugal fans in order to reduce the noise level that

is generated. Pipes placed on the roof of the unit remove the discharge air through

negative pressure. Some of the units recirculate the process air through reversing

aeration systems which can reach up to 80%. The entire process is automated

and controlled by means of a computer. The retention time of the organic material

is approximately 14 days whereas the overall treatment process lasts about 2

weeks.

Inclined Bioreactor or Rotating Drum Another type of in-vessel composting system is the inclined bioreactor or rotating

drum consisting of a rotating cylinder. The cylinder is built at a slight inclination so

that the substrate is moved from one end to another. The rotating drums usually


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incorporate internal vanes which, combined with the rotating action of the drum,

contribute to the size reduction as well as to the agitation of the organic matter.

This type of reactor normally is used for the active phase of composting and by

carefully controlling the oxygen and moisture contents and maintaining at optimum

or near-optimum levels, the composting process can be accelerated. The cylinder

is approximately 45m long and 2–4m in diameter, whereas the rotational speed is

about 0.2–2 rpm (Diaz et al., 2002). Under normal operating conditions, the

bioreactor is filled to about two-thirds, while the retention time of the substrate is

about 1 week. After the active phase of composting within the rotating drum, the

organic material is cured for a few weeks in windrows. (Diaz et al., 2002).

1.1.4.3.4. Economic Limitations

As has been mentioned in-vessel composting systems reserve the bioreactor for

the active stage of the composting process and rely upon windrow systems for the

curing and maturation phase of the organic matter. The rationale of these systems

is to maintain conditions at optimum levels during the active stage of the process

and thus accelerating the microbial activity rate and consequently shortening the

active phase. The economic gain of in-vessel systems in comparison to windrow

composting is the reduction of residence time and the increase of its processing

capacity as well as the better quality of the end product, since the conditions

during the process are usually optimized and controlled at all times. On the

contrary the capital and O&M costs of in-vessel composting systems are

significantly than those of windrow systems. According to Diaz et al. (2007), in the

early 1970s, capital costs for compost plants in the USA were of the order of

$15,000–20,000 per tonne of daily capacity. The operational costs were about

$10–15 per tonne processed. In 2005 the costs range from about $25,000 to about

$80,000 per tonne of daily capacity weeks (Diaz et al., 2007). According to recent

data the financial cost of in-vessel biowaste treatment in EU member states

ranges from 30 to 41 € per tonne treated. The unit cost includes annualized

capital, O&M expenditure as well as other specific costs or revenues (e.g.

revenues from sale of the energy) without considering taxes or any form of

subsidization of the end products (EC, 2010).


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1.1.5. Post-Processing

Post-processing practices involve the various stages that are employed to in order

to refine the produced compost and to meet market and regulatory standards.

Among the main processing units that are included in compost post-processing

practices are: size reduction, screening, air classification, and de-stoning. It must

be stated that the post processing techniques achieve adequate separation in

cases where the moisture content of the orgaic end product is lower than 30%.

1.1.6. Mass and energy balances

A typical mass balance diagram of the composting process is shown in Figure 8.

According to the figure, for 100 tn of processing MSW, 95tn is assumed to be the

input for the composting process after the mechanical pretreatment. The

degradation process of the organic matter causes the loss of 63tn DS and water,

thus producing a final end compost which accounts to 30tn and 2 tn of residues.

Figure 8: Typical mass flow diagram of composting


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In composting, energy is generated during the biodegradable organic solids

oxidation in the form of heat release (enthalpy of reaction). Also, energy enters the

composting system through the ambient air (sensible and latent heat), and exits

the system through the flue gas (sensible and latent heat and the heat of reaction

associated to the presence reduced gaseous species, namely ammonia (NH3) and

hydrogen sulphide (H2S)). The enthalpy accumulated (EAc) in the composting

materials along the processing time can be evaluated through the following

equation (Neves et al., 2007):

where :

NI is input rate of energy associated to the inflow rate of ambient air:

NE is the output rate of energy associated to the composting flue gas rate:

is the molar saturation ratio of water vapor at the atmospheric pressure p1 (=

1.013 105 Pa); pvs is the water saturation vapor pressure calculated from the

Clausius–Clapeyron equation at the actual temperature of the composting

materials (TR), considering that flue gas is moisture saturated.

NG is the rate of energy released by the biological oxidation of the

biodegradable materials,


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NU is the rate energy transfer between the composting materials at temperature

TR and the environment, at temperature TS (considered constant and equal to

298 K),

where AR is the estimated superficial area of each load of waste ( 0.24 m2) and U

is the global heat transfer coefficient. It was considered that heat is transferred

though conduction, based in a compost thermal conductivity of 0.1 W m-1 K-1.

The temperature of the composting materials can be calculated at any moment by:

1.1.7. Market potential for products

As mentioned above the end product of the composting process is the compost.

Compost has the potential to be used as a soil amendment in various applications.

Compost can substantially improve the fertility, texture, aeration, nutrient content

and water retention capacity of the soil. Due to its beneficial characteristics,

compost has a variety of potential applications and can be used by several market

segments. Some of the markets include:

• agriculture (small- and large-scale);

• landscaping;

• gardening (residential, community);

• nurseries;

• top dressing (e.g. golf courses, parks, median strips);

• land reclamation or rehabilitation (landfills, surface mines, and others); and

• erosion control.


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The markets or uses listed in the previous paragraph are constrained by: (1) the

characteristics of the compost, (2) the limitations applicable to its use, and (3)

pertinent laws and regulations (Alexander, 2000; Harrison et al., 2003).

Results of marketing studies and surveys conducted in several countries have

demonstrated that some of the most critical elements in the use and marketability

of the compost products are: (1) quality, and (2) consistency. The quality of a

specific type of compost is a function of its chemical, biological, and physical

characteristics. Assuming that a composting process is properly carried out, the

quality of the finished product is determined by: (1) the composition and

characteristics of the input material used in the production of the compost, and (2)

the type and thoroughness of the process used to remove impurities. Some of the

physical characteristics that are normally desired for a particular compost product

are color, uniform particle size, earthy odor, absence of contaminants, adequate

moisture, concentration of nutrients, and amount of organic matter (Eggerth et al.,

1989). The size of a particular market for compost depends, to a large extent, on

the quality of the compost and on the types of uses for the material. Composts

from different types of substrates (e.g., yard waste, source-separated MSW) have

different characteristics and consequently have different potential markets

(Franklin Associates et al., 1990).

1.1.7.1. Limitations

The use of compost and therefore the limitation of its use on land is depended on

the potential adverse effects on human health and safety, animal & livestock

health and safety, crop production and the quality of the air, water, and land

resources. The significance of the aforementioned limitation is related to whom or

what is affected and the extent to which they are affected. The limitations

associated with the use of the compost with respect to the health and safety of

humans are related to the harmful substances that may be present in the product.

Among the main substances that are being regulated in orde to prevent potential

adverse effects include the pathogenic organisms, heavy metals, persistent

organic pollutants (POPs) and level of contaminants (e.g. plastic, glass). The level

of their appearance is mainly associated of to the feedstock material to the


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composting process. Although the output from processing mixed MSW is not

considered compost in several countries that are members of the EU, this practice

is still being conducted and considered in other countries.

EU initiatives with respect to the heavy metals maximum consentration in compost

are also seen in Table 3. The second draft EU Working Document on the

Biological Treatment of Biowaste lays down heavy metal limit values for two

classes of compost both towards the high quality end product, while a third class

of material, stabilised biowastes, which is still considered as ‘waste’. In addition

according to the commission decision 2001/688/EC (EC Eco-label) on

“establishing ecological criteria for the award of the Community eco-label to soil

improvers and growing media” environmental performance in regard to the heavy

metal concentration of soil improvers1 and growing media2 have been set. For the

utilization of compost as fertilizer or soil conditioner within organic farming (Eco-

agric), specific compost quality standards for heavy metal concentration are also

provided. Within the Eco-agric (EC 2092/91 - EC 1488/97) only composted source

separated household waste containing only vegetable and animal waste is

accepted.

Table 3: Heavy metal limits for European compost standards (mg/kg dm)

Policy measures Cd Crtot CrVI Cu Hg Ni Pb Zn As

EC Draft W.D.

Biological

Treatment of

Biowaste (class 1)

0.7 100 100 0.5 50 100 200

Draft W.D.

Biological

Treatment of

Biowaste (class 2)

1.5 150 150 1 75 150 400

Stabilised

Biowaste** 5 600 600 5 150 500 1500

1 soil improvers: materials to be added to the soil in situ primarily to maintain or improve its physical properties, and which may improve its chemical and/or biological properties or activity 2 growing media: material, other than soils in situ, in which plants are grown


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EC/'eco-label'

2001/688/EC 1 100 100 1 50 100 300 10

EC/'eco-agric'

2092/91 EC-

1488/97 EC 0.7 70 0 70 0.4 25 45 200

Figure 9 gives a comparative survey on heavy metal limit and guide values for

composts in European countries expressed as relative mean limits as compared to

the maximum concentration of the EC Eco-Label for soil improver (= 100 %).

Figure 9: Heavy metals limit values for compost in European countries

[mean percentage relative to threshold values of the EC Ecolabel for soil

improver]. Countries with more than one compost category or quality class

referring to PTE thresholds are indicated with ‘I / II /III’]

There are thousands of chemically synthesised compounds that are used in

products and materials commonly used in our everyday life. Many of them are

potential contaminants of biowaste, although, due to their low concentration or

easiness to be broken down by micro-organisms, as to the buffering capacity of

soils, they do not cause a threat to the environment. However, there are some


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organic compounds that are not easily broken down during waste treatment and

tend to accumulate and be the source of concern due to their eco-toxicity, the eco-

toxicity of the products resulting from their degradation or to their potential for bio-

accumulation. There are usually three main reasons why an organic compound

may be subject to preventive action:

(a) the break down by soil micro-organisms of the compound concerned is slow

(from some months to many years) and, therefore, there is an actual risk of build-

up in the soil;

(b) the organic compound can bio-accumulate in animals and, therefore, it poses a

serious threat to man;

(c) the degradation products of the organic compound are more toxic than the

initial compound.

Therefore, there is likelihood a very high number of organic contaminants to be

found in compost made from collected and treated biodegradable organic waste.

Each year, the use of new compounds increases by a few thousand. Some of

these compounds break down or undergo a transformation during the composting

operations, while others remain stable. The presence of organic contaminants in

compost used on soils could represent a potential risk to the environment and to

the quality of crops intended for human or animal consumption.

Limits for organic contaminants were proposed only within the second draft of the

Working Document on biological treatment of biowaste, concerning the

polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs)

only (Table 4), and their concentration was set to be in consistence with the

Sewage Sludge Directive (86/278/EEC). In general, organic contaminants are

expected to be at low levels in composts derived form source separated materials

and, therefore, in most European countries there are no set limit values for organic

contaminant in composts.


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Table 4: Organic pollutants standards for compost and stabilised biowaste

Parameter

(mg/kg dm)

Compost Class 1 Compost Class 2 Stabilised

Biowaste

PCBs (mg/kg dm) - - 0.4

PAHs (mg.kg dm) - - 3

Threshold values for these organic pollutants to be set in consistence with the

Sewage Sludge Directive.

Impurities or any inert non organic contraries may be found in composts from

biodegradable municipal waste. The better the performance of separate collection

from households or small enterprises the higher the purity. When developing an

industry standard for compost quality, the presence of foreign matter in compost

should be taken into consideration, since it has a negative impact on consumers

and on the composting industry in general. The consumers look for compost free

of visible foreign matter or otherwise harmful foreign matter. Table 5 presents the

classification of compost according to the level of impurities in compost as has

been laid down by the second working document on Biological Treatment of

Biowaste in EU.

Table 5: Impurities standards for compost and stabilised biowaste

Parameter Compost Class

1

Compost Class

2

Stabilised Biowaste

Impurities >2mm <0.5% <0.5% <3%

Gravel and stones

>5mm

<5% <5% -

From the very beginning of the implementation of compost standards hygienic

aspects have been addressed in order to “guarantee a safe product” and to

prevent the spreading of human, animal and plant diseases. Provisions for the

exclusion of potential pathogenic microorganisms within process and quality

requirements are established at two levels:

• direct methods by setting minimum requirements for pathogenic indicator

organisms in the final product


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• indirect methods by documentation and recording of the process showing

compliance with required process parameters (HACCP concepts,

temperature regime, black and white zone separation,

hygienisation/sanitisation in closed reactors etc.).

Table 6 shows the requirements of the Decision of EC ECO-label.

Table 6: EU requirements on pathogens/weeds in compost based on the EC eco-label

Pathogens /Weeds Approval of technology (AT)

Salmonella sp. absent in 25 g

E. coli < 1000 MPN (most probable number)/g

Helminth Ova absent in 1.5 g

weeds/propagules germinated plants: 2 plants /l

1.1.8. Environmental impacts

Significant environmental benefits can be obtained by the use of compost as a soil

conditioner, a fertilizer, or a growth medium. Those benefits are related to the

nutrients recycling back to the soil which enables the reduction of synthetic

fertilizers. When it is used as daily cover at landfills, it replaces other materials that

would otherwise be used for that purpose.

However, there are also negative impacts on the environment associated with the

production and utilization of compost. These impacts depend both on the

operational composting process and to a large extent to the waste composition of

the input organic streams. Mixed MSW and sewage sludge composting pose

greater risks because these materials typically contain higher concentrations of

heavy metals, POPs and pathogenic microorganisms than do source separated

organic waste.

Negative impacts of composting on the environment can also be caused from

gases that are released due to the improper operation and maintainance of the

compost piles. More specifically, in cases when composting piles are not properly


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operated in that they do not deliver the required oxygen within the organic mass,

anaerobic bacteria are being developed which result in the production of

methane.. The release of methane significantly contributes to the problem of

greenhouse gases in the atmosphere. Additionally, poorly operated composting

facilities are always associated with unpleasant odors and the creation of

nuisance. Other air emissions are generated by the combustion engines used to

power windrow turning machines and grinders.

During composting leachate production is another matter of concern especially in

open composting systems. Leachates are produced from water runoff and

condensation at the compost facilities due to the increased moisture content of the

feedstock material as well as due to the need for water addition to the organic

mass in order to maintain the moisture content at an acceptable level during the

biodegradation process. Leachates occasionally acquire increased levels of

biological oxygen demand (BOD) that may exceed the acceptable discharge limits.

Leachates runoff to surface water can reduce significantly the amount of dissolved

oxygen in the aquatic ecosystem resulting in eutrophic conditions. Sound practice

here is to avoid discharge to water and to capture or direct all leachate to

absorption in sand or soil.

1.1.9. Economic data

The capital costs of the in-vessel composting varies significantly according to the

scale of the facility, the input material that is being treated, characteristics of the

exhaust gases that are being treated and the retention time of the organic matter.

According to the World Bank, the capital cost for the development of an in vessel

composting system is approximatetely of the order of 240€ per tonne of capacity

(35-55 million € for a capacity of 500 tonnes/day) while the operation and

maintenance cost is 20-40 €/tonne. According to recent data ASCARDIS &

EUNOMIA (2010) typical capital costs are of the order of 190€ per tonne of

capacity and suggest operating and maintenance cost of 12.5 and 10.25€ per

tonne respectively.


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1.2. ANAEROBIC DIGESTION

1.2.1. Introduction to anaerobic digestion process

Anaerobic digestion (Figure 10) is defined as the biological process during which

the organic material is decomposed by anaerobic microorganisms in the absence

of dissolved oxygen (i.e. anaerobic conditions). Anaerobic microorganisms digest

the input organic material which is converted through anaerobic degradation into a

more stabilized form, while a high energy gas mixture (biogas) consisting mainly of

methane (CH4) and carbon dioxide (CO2), is generated. Biogas is collected and

utilized as a source of energy, since it can be combusted in a cogeneration unit

and produce green energy. Apart from CO2, CH4 is also considered as a gas

which contributes significantly to the greenhouse effect and hence to global

climate change. The organic material can originate from industrial or municipal

waste, agricultural residues or sludge generated from wastewater treatment plants

(Pavlostathis and Giraldo-Gomez, 1991).

Figure 3: Anaerobic digestion flow chart (Wastesum, 2006)


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1.2.2. Biology of anaerobic digestion

One of the key factors in the success of microbial-mediated processes is an

adequate understanding of process microbiology, more speci cally the study of

microorganisms involved in organic waste decomposition and the subsequent by-

product formation. The anaerobic fermentation process is much more complex

than composting due to the involvement of a diverse group of microorganisms and

a series of interdependent metabolic stages which demand meticulous process

control for stable operation. The anaerobic digestion of organic material is

accomplished by a consortium of microorganisms (bacteria) working

synergistically in the absence of oxygen. These microorganisms use up the initial

feedstock as an energy and biomass source through various biological and

chemical reactions transforming the input organic matter to intermediate molecules

such as sugars, hydrogen and acetic acid before finally being converted to biogas.

The anaerobic digestion process can be generally classified into four distinct

stages which are related to the biological and chemical phases of anaerobic

treatement of biodegradable organic waste as shown in Figure 9:

1. Hydrolysis

2. Acidogenesis

3. Acetogenesis

4. Methanogenesis

Generally the organic input material is composed of large macromolecules which

need to be broken down into smaller chemical components so that the anaerobic

bacteria will be able to access the energy potential of the substrate. Hydrolysis is

the first stage of the anaerobic digestion process in which complex and large

organic polymers are decomposed and dissolved to constituent monomers

(Ostrem, 2004). Therefore the hydrolysis stage involves the breakdown of complex

organic molecules such as polysaccharides, proteins, and lipids into simple

compounds namely sugars, amino acids, and fatty acids by extracellular enzymes

(e.g. cellulase, protease and lipase) and then to soluble products of small enough

size to allow their transport across the cell membrane. Hydrolysis can be a rate-

http://en.wikipedia.org/wiki/Hydrolysis

http://en.wikipedia.org/wiki/Acidogenesis

http://en.wikipedia.org/wiki/Acetogenesis

http://en.wikipedia.org/wiki/Methanogenesis

http://en.wikipedia.org/wiki/Hydrolysis

http://en.wikipedia.org/wiki/Simple_sugar

http://en.wikipedia.org/wiki/Simple_sugar

http://en.wikipedia.org/wiki/Fatty_acid


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limiting step in the overall anaerobic treatment processes for waste containing

lipids and/or a signi cant amount of particulate matter (e.g., sewage sludge,

animal manure, and food waste) (Henze and Harremoes 1983; van Haandel and

Lettinga 1994).

In the hydrolysis phase, acetate and hydrogen are produced which can be directly

taken up by methanogens. Additionally, chemical compounds such as volatile fatty

acids which acquire greater chain length than acetate (e.g. propionic, formic,

lactic, butyric, or succinic acids) must be further broken down into constituents that

can be used by methanogenic microorganisms (Khanal, 2008). The biological

process in which the products resulting from hydrolysis are further breaking down

is called acidogenesis and it takes place by acidogenic bacteria. Volatile fatty

acids are generated along with ammonia, carbon dioxide and hydrogen sulfide as

well as other by-products. The specific concentrations of products formed in this

stage vary with the type of bacteria as well as with culture conditions, such as

temperature and pH (United Tech 2003). The third stage of the anaerobic

digestion is the acetogenesis. During this stage, simple molecules which have

been produced through the acidogenesis stage (e.g. acetate) are further degraded

by acetogens to produce mainly acetic acid as well as carbon dioxide and

hydrogen (Khanal, 2008). The final stage of anaerobic digestion of the

biodegradable organic waste is the methanogenesis phase during which

methanogenic bacteria use the intermediate products resulting from the preceding

stages for the production of methane, carbon dioxide and water. Methanogens are

sensitive to changes and prefer a neutral to slightly alkaline (between pH 6.5 and

pH 8) (Wastesum, 2006). The remaining non-digestable organic material, which

the anaerobic microbes cannot decompose along with the dead bacterial,

constitutes the digestate.

http://en.wikipedia.org/wiki/Acidogenesis

http://en.wikipedia.org/wiki/Hydrogen_sulfide

http://en.wikipedia.org/wiki/Acetogenesis

http://en.wikipedia.org/wiki/Methanogenesis


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Figure 4: The stages of anaerobic digestion (Wastesum, 2006)

1.2.3. FEEDSTOCK OF ANAEROBIC DIGESTION

The feedstock material that is used in anaerobic digestion constitutes the most

important initial parameter when considering the application of anaerobic

treatment. Feedstock include any substrate that can be converted to methane by

anaerobic bacteria thereby it can range from readily degradable organic waste

(e.g. wastewater) to complex high-solid waste. Anaerobic digesters typically can

accept any biodegradable material, but the level of biodegradability is the key

factor for its successful application. Anaerobic microorganisms can dissolve

organic matter to varying degrees of success. More specifically sugars which are

short chain hydrocarbons can be used readily whereas the decomposition process

of cellulose and hemicellulose compounds is significantly longer. Anaerobic

microorganisms are unable to break down long chain woody molecules such as

lignin (Wastesum, 2006). Therefore it can be inferred that the characteristics of the

input material determine in a large extent the methane yield and production rates

within the anaerobic digesters. In order to improve the methane potential in

anaerobic digestion several techniques are being applied by determining various

characteristics of the organic feedstock. Additional variables such as solids

content as well as elemental and organic analyses are considered as useful

methods in the design and the operation of anaerobic digesters.

Another parameter that needs to be considered when it comes to anaerobic

treatment is the moisture content of the feedstock material. Generally, the higher

the moisture content of feedstock the easier its handling and conveyance since

standard pumps can be applied instead of concrete pumps and physical means of

http://en.wikipedia.org/wiki/Anaerobic_digestion#cite_note-44%23cite_note-44


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movement which are more energy demanding practices. However, the increased

moisture content of the organic input material results in higher volume and area

that is required in comparison to the levels of biogas that are generated.

Therefore, the type of anaerobic system that will be developed and applied is

depended on the moisture level of the input material different. Another key

consideration in anaerobic digestion is the C/N ratio of the initial substrate that is

subjected to anaerobic decomposition. The C/N ratio represents the relationship

between the amount of carbon and nitrogen present in the organic material thus

regulating the nutrients take up of microbes and balancing their growth. Optimum

C/N ratios in anaerobic digesters are between 20-30 (Verma, 2002). A high C/N

ratio is an indication of rapid consumption of nitrogen by methanogens and results

in lower gas production. On the other hand, a lower C/N ratio causes ammonia

accumulation and pH values exceeding 8.5, which is toxic to methanogenic

bacteria (Verma, 2002).

The impurities level of the organic input material is another key parameter when

considering the deployment of anaerobic treatment. In cases when the substrate

acquires increased quantities of impurities such as plastic, glass or metals, then a

pre-treatment stage is required in order to increase the purity level of the feedstock

and to prevent potential malfunctions and inefficiencies of the anaerobic digesters

processes (Wastesum, 2006).

The feedstock for an anaerobic digestion plant can be organic waste that has been

separately collected and delivered to the plant ready for processing or, municipal

solid waste or its fraction from a mechanical sorting plant in which the other

fraction is the refuse-derived fuel. A further source of organic waste is ‘green

wastes’ collected at centralized collection points. At least the purity of the raw

material fed into the anaerobic digestion process dictates the quality of the product

coming out at the end of the process. The range of application of the anaerobic

digestion process is very broad. In principle, any organic material can be digested

a list of which is given below (Rilling, 1994):

organic municipal solid waste

waste from central markets (e.g. fruit, vegetable and flower residuals)

slaughterhouse waste (paunch manure)



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residues from the fish processing industry

food waste from hotels, restaurants, and canteens

bleaching soil

drift materials such as seaweed or algae

agricultural waste

manure

beer draff

fruit or wine marc

sewage sludge.

However, the treatment method for each waste stream might be depended on its

moisture level as shown in Figure 12, since composting is widely used for waste

containing high amounts of dry matter, whereas anaerobic digestion has turned

out to be a good alternative for treating wet organic waste.

Figure 5: Suitability of waste for aerobic composting and anaerobic

digestion (Kern et al., 1996)

1.2.4. Procedures of Anaerobic Waste Fermentation

The anaerobic treatment of organic waste generally follows specific steps as

presented below (Rilling, 1994):

1. delivery and storage of biodegradable organic waste

2. preprocessing of the incoming biodegradable organic waste


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3. anaerobic fermentation

4. storage and treatment of the digester gas

5. treatment of the process water

6. post-processing of the digested material.

Figure 13 shows the possible treatment phases carried out in anaerobic digestion

process. Typically, all fermentation processes can be described as a combination

of a selection of these treatment phases. The process technology demanded for

the implementation of the different phases of the treatment varies significantly and

depends on the anaerobic process chosen. In general, the gas production

increases and the detention time decreases with increasing energy input for

preparation of the material and the fermentation itself (mesophilic/thermophilic).


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Figure 6: Possible treatment steps used in anaerobic digestion process of

biodegradable organic waste (Rilling, 1994)

1.2.4.1. Delivery and Storage

The organic substrates are quantitatively and qualitatively recorded by weighing,

are visually inspected at an acceptance station, and are unloaded into a flat or

deep bunker or a collecting tank that serves as a short-term intermediate storage

place and permits continuous feeding to the subsequent pretreatment plant.


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1.2.4.2. Pre-processing

In the pretreatment stage pollutants and inert material are removed from the

organic material, whereas the substrate is homogenized and conditioned. The

pretreatment type depends on the specific system of the anaerobic fermentation

process. More specifically, dry fermentation processes use dry preprocessing, in

which sieves, grinders, shredders, metal separators, homogenization drums,

ballistic separators, and hand sorting sections can be combined. On the other

hand, in wet fermentation processes the organic substrate is additionally mixed

with water, homogenized, and shredded. Organic material that has larger surface

area is more easily broken down by the bacteria.

1.2.4.3. Anaerobic Fermentation

Once the pre-processing procedure has been elaborated any recyclable or

unwanted materials is separated from the incoming waste, whereas the organic

material is shredded and supplied to the digester. In case when organic waste with

high water content, e.g. sewage sludge, is used as raw material the addition water

is not required, whereas for dry substrates, e.g. household organic waste, water is

usually necessary to be added in order to dilute the solids. Waste with low

structure and high water content are best for wet fermentation. On the other hand,

substrates with high structural strength can also be anaerobically decomposed

through dry fermentation processes (RISE-AT, 1998). For anaerobic fermentation

to take place, heat is needed to be adjusted to the required process temperatures

to about 35°C (mesophilic operation mode) or 50-55°C (thermophilic operation

mode), and in some cases water addition is prerequisite. During substrate

digestion the decomposition of the organic matter is carried out in the absence of

oxygen i.e. anaerobically, in closed, temperature-regulated bioreactors. Depending

on the process operation, the material consistency may vary between well-

structured and fluid suspension organic matter. The output of the anaerobic

digester is a wet, organically stabilized residue (digestate) and biogas. After

dewatering of the digestate, a compost like material can be obtained by aerobic

post-treatment. In addition, the wastewater produced during draining can be partly

recirculated into the pre-processing stage to adjust the water content of the initial


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substrate, whereas surplus wastewater has to be treated accordingly (e.g. purified

in specially designed purification ponds) and discharged. Biogas, which constitutes

the main product of anaerobic, is used as an energy source. Biogas is generally

used in decentralized fuel-burning power stations for the production of electricity

and heat in order to cover the energy requirements of the fermentation process

and thus enabling the system to operate in an energy-neutral manner. The excess

energy is marketed by supplying the public heat power needs. In cases when only

easily degradable organic waste components are used in the anaerobic digestion,

the energy is produced with minimal technical expenditure, whereas the energy-

intensive pretreatment stages can be omitted (RISE-AT, 1998).

1.2.4.4. Post-processing

The solid by-product of the anaerobic digestion has to be stabilized, sanitized and

refined prior to its application for agricultural or horticultural uses. Typically, after

dewatering and/or drying, the digestate is composted and matured in order to

become a good quality, marketable compost. The biogas, after drying and, if

required, purification, can be used as an energy source.

1.2.5. Process Engineering of Anaerobic Fermentation of Biowaste

In comparison to the commonly applied composting treatment, anaerobic

fermentation of biowaste is a relatively new and dynamic biological process. With

great scientific expenditure, process developments and optimizations are being

pursued, so it may be assumed that the technological potential of biowaste

fermentation has not yet been fully exhausted. Anaerobic digestion is generally

suitable for the biological treatment of readily degradable substances which

acquire low structure and high water content (e.g. kitchen waste). The anaerobic

fermentation processes of organic solid waste differ in number depending on (i)

the biodegradation stages (one or two-stage), (ii) separation of liquid and solids

(one or two-phase), (iii) water content (dry or wet fermentation), (iv) feed method

(continuous or discontinuous), and (v) agitation method (Rilling, 1994). The most

important characteristics of anaerobic digestion are presented in Table 7.


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Table 7: Characteristics of anaerobic waste treatments (Rilling, 1994)

Stages of biodegradation

One-stage Two-stage

Separation of liquid and

solids

One-phase dry

fermentation

Two-phase wet

fermentation

Total solids content 25%-45% <15%

Water content 55%-75% >85%

Feed method Discontinuous Continuous

Agitation None Stirring, mixing,

percolation

Temperature Mesophilic (30-37oC) Thermophilic (55-65oC)

As stated above the anaerobic fermentation of biowaste can be operated by one-

stage or two-stage fermentation. In the one-stage process (Table 8) all

fermentation stages (e.g. hydrolysis, acidification, acidification and

methanogenesis) take place in one reactor; therefore, optimum reaction conditions

for the overall process are not achieved, due to the different environmental

requirements during the various stages of the fermentation. Therefore, the

degradation rate is reduced and consequently the retention time increases. The

basic advantage of one stage process operation is the relatively simple technical

installation and operation of the anaerobic digestion plant, whereas the costs are

lower. In two-stage processes (Table 8), the hydrolysis and acidification-

acidification take place in one bioreactor, while methanogenesis is carried out in a

separate reactors thus providing flexibility to optimize each of these reactions so

that e.g. mixing and adjustment of the pH can be optimized separately, permitting

higher degradation degrees and loading rates. In two-stage processes the

retention time of the substrate is significantly decreased. However, such systems

involve more sophisticated technical design and operation and subsequently

higher costs.

In the first reactor, organic fraction is hydrolyzed producing dissolved organics,

organic acids, CO2 and low concentrations of hydrogen. The reaction rate in the

first reactor is limited by the rate of hydrolysis of cellulose. In the second stage the


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highly concentrated water is supplied to an anaerobic fixed-film reactor, sludge

blanket reactor, or other appropriate system where methane and CO2 are

produced as final products. In the second reactor the rate of reaction is limited by

microbial growth (Verma, 2002).

Table 8: Comparison of one- and two-stage processes Process Operation One-stage Two-stage

Operational reliability In the same range

Technical equipment Relatively simple Very complex

Process control Compromise solution Optimal

Risk of process instability High Minimal

Retention time Long Short

Degradation rate Reduced Increased

1.2.5.1. Dry and Wet Fermentation

According to the moisture level of the substrate the anaerobic digestion systems

can be classified as dry fermentation processes or “high-solids systems” (dry

solids content >15-20%) and wet fermentation processes “low-solids systems” (dry

solids content <15%) (Jördening and Winter, 2005). However, there is no

established standard for the cutoff point. Table 9 shows advantages and

disadvantages of dry and wet fermentation. With the dry fermentation process,

little or no water is added to the biowaste. As a consequence, the material streams

to be treated are minimized. The resulting advantages are smaller reactor volumes

and easier dewatering of the digestate thus less costly reactors. Operating in dry

fermentation processes places higher requirements on mechanical pretreatment

and conveyance (e.g. pumping denser material), on the gas-tightness of charge

and discharge equipment and on mixing the substrate. Due to the low substrate

mobility in high solid content fermentation, a defined residence time can be

reached by approximating plug flow, which is particularly important for the

sanitization of the organic solid end product in the thermophilic operation process.

The degradation rates in dry fermentation processes are lower than in wet


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fermentation, due to the larger particle size and reduced substrate surface

availability (Jördening and Winter, 2005).

In the case of wet fermentation processes, the organic waste is shredded into

small particle size whereas specific amount of water is added aiming to formulate

sludges or suspensions. The low solid content of the substrate enables the use

standardd mechanical conveyance techniques (pumping) while the removal of

interfering substances can be achieved by sink–float separation (Jördening and

Winter, 2005). Additionally, the agitation of the organic substrate is easily

operated; allowing controlled degassing and defined concentration equalization in

the digester which in turn optimizes the degradation performance of the

microorganisms. The mean substrate concentrations and thus also the related

degradation rates are lower than in plug flow systems, since for completely mixed

systems the concentrations in the system are equal to the outlet concentrations.

Mixing is limited due to the increased sensitivity of methanogenic microorganisms,

while mixing at a lower degree may result in floating and sinking layers.

Homogeneity and a fluid consistency permit easier process control. However, by

fluidizing biowaste, the treated substrate increases and it requires larger area and

volume (aggregates and reactors) for the treatment process to be performed.

Fluidization and dewatering of the fermentation suspension are costly procedures,

since they require considerable technical and energetic expenditures. However, if

the degrees of degradation are the same, recycling the liquid phase from the

dewatering step to the fluidization of the initial substrate, makes it possible to

reduce the wastewater quantity to an amount comparable to that used in dry

fermentation and to keep a considerable part of the required thermal energy within

the system (Jördening and Winter, 2005).


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Table 9: Comparison of wet and dry fermentations (Jördening and Winter, 2005).

Process Mode Dry Wet

Total solids content High 25-45% Low 2-15%

Reactor volume Minimized Increased

Conveyance technique Expensive Simple

Agitation Difficult Easy

Scumming Little risk High risk

Short circuit flow Little risk High risk

Solid-liquid separation Simple Expensive

Variety of waste components Small Great

1.2.5.2. Continuous and Discontinuous Operation

In case when the anaerobic digestion process is in continuous operation mode,

the bioreactor is fed and discharged regularly. Completely mixed and plug flow

systems are operating in that mode during which sufficient substrate is fed into the

reactor to replace the putrefied material as it is discharged. Therefore, in such

systems the substrate must be flowable and uniform to allow its unobstructed

movement, whereas steady provision of nutrients in the form of raw biodegradable

waste enables stable process operation and constant biogas yield. Depending on

the bioreactor type, design and the means of substrate mixing, short circuits may

occur. In such occasions the retention time cannot be guaranteed for the whole

substrate in completely mixed systems. In the discontinuous-batch operation

mode, the digester is completely filled with raw organic material mixed with

digestate provided by another bioreactor and then discharged after a specified

retention time. Batch mode bioreactors are easier to design with a relative lower

cost than plug and flow systems, while they are suitable for dry as well as for wet

fermentation (Table 10) (Jördening and Winter, 2005).


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Table 10: Comparison of continuous and discontinuous feed

Process Operation Continuous Discontinuous

Retention time Shorter Longer

Technical equipment Complex Simple

1.2.5.3. Thermophilic and Mesophilic Operation

In anaerobic digestion, the optimal process conditions are in the mesophilic

temperature range (about 35oC) and in the thermophilic temperature range (about

50-55oC) (RISE-AT, 1998). At this temperatures process condition the methane

fermentation is maximized. Bioreactors designed to operate on mesophilic levels

are heated to 30 - 40oC and this type of systems acquire high stability process,

whereas small temperature deviations have minor effect on the microorganisms.

This is attributed to the fact that a great variety of mesophilic methane bacteria

exists that shows low sensitivity to temperature variation. The main advantage of

mesophilic process operation is the lower amount of energy (e.g. heat) required to

be supplied and the subsequent higher net energy production (RISE-AT, 1998).

On the other hand, thermophilic operation requires temperatures between 50 and

60oC. Under certain conditions the thermophilic process operation enables higher

substrate decomposition rates with subsequent lower retention times. However,

this operation type requires larger amounts of energy to maintain the process

temperature and thus higher energy expenditure. Therefore, the net energy

production is lower than mesophilic operation, whereas the temperature sensitivity

of the thermophilic microorganisms reduces the process stability. In addition,

under thermophilic conditions the sanitization of the substrate might be achieved

for a fixed retention time; otherwise, sanitation has to be achieved in a separate

treatment step or by composting (RISE-AT, 1998). Table 11 lists the advantages

and disadvantages of mesophilic and thermophilic process operations.


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Table 11: Comparison of mesophilic and thermophilic process operation (Jördening and Winter, 2005)

Process Operation Mesophilic (35oC) Thermophilic

Process stability Higher Lower

Temperature sensitivity Low High

Energy demand Low High

Degradation rate Decreased Increased

Detention time Longer or the same Shorter or the same

Sanitation No Possible

1.2.5.4. Agitation

For a high degradation activity of the bacteria, it is necessary to provide the

microorganisms with sufficient degradable substrate, whereas the metabolic

products of the organisms have to be removed (Dauber, 1993). The

aforementioned requirements can be met by mechanical mixing or other agitation

of the bioreactor’s substrate. Another way in achieving the required mixing of the

organic matter is to install a water recirculation system, by which the process

water, which ensures nutrient provision and the removal of metabolic products,

trickles through the biowaste in the reactor (Rilling and Stegmann, 1992). Other

processes use compressed biogas for total or partial mixing of the material.

1.2.6. Anaerobic Digestion Products

The main products resulting from the anaerobic digestion process are the biogas,

the solid end product (digestate) and water.

1.2.6.1. Biogas

Biogas is a mixture of various gases. Independent of the fermentation

temperature, a biogas is produced which consists of 60%–70% methane and

30%–40% carbon dioxide, whereas trace components of ammonia (NH3) and


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hydrogen sulfide (H2S) can be detected. However, the yield biogas depends on

several factors such as temperature, pH and alkalinity, hydraulic and organic

loading rates, toxic compounds, substrate type, and total solids (TS)/volatile solids

(VS) content (Pavlostathis and Giraldo-Gomez, 1991). The caloric value of the

biogas is about 5.5–6.0 kWh m–3 which corresponds to about 0.5 L of diesel oil.

According to Symons and Buswell (1933) the yield and composition of the biogas

can be estimated from the following equation, when the chemical composition of

the substrate is known

242 48248224CObanCHbanOHbanOHC ban

Table 12 shows the mean composition and specific quantity of biogas as

dependent on the kind of degraded substances. For anaerobic digestion of the

organic fraction of municipal solid waste, an average biogas yield of 100 m3 t–1 wet

biowaste and having a methane content of about 60% by volume may be

assumed. The highest yield of methane is accomplished after the bacterial

population has reached its peak and it begins to decrease due to the gradual

depletion of the organic load.

Table 12: Mean composition and specific yields of biogas in relation to the kind of substances degraded (Rilling, 1994)

Substance Gas Yield

(m3kg-1TS)

CH4 Methane

content (Vol. %)

CO2 Carbon Dioxide

Content

(Vol. %)

Carbohydrates 0.79 50 50

Fats 1.27 68 32

Proteins 0.70 71 29

Municipal Solid Waste

(MSW)

0.1-0.2 55-65 35-45

Biowaste 0.2-0.3 55-65 35-45

Sewage Sludge 0.2-0.4 60-70 30-40


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Manure 0.1-0.3 60-65 35-40

The biogas is generally stored in an inflatable bubbles located on top of the

system while in other cases the gas is collected and stored in biogas holders

near to the facility.

1.2.6.1.1. Electricity Supply

The mode of operation of a gas engine depends on type of its use e.g. covers

peak load, covers basic load, supplies its own needs and only feeds the surplus

into the network. The electricity supply mode is determined by the local conditions

as well as the price of electricity. Different plant designs are needed for covering a

constant basic load and for covering peak loads for certain periods of the day.

Peak load covering requires complex and expensive gasholders for longer periods

and larger and more expensive power stations. The worldwide ongoing system of

promoting renewable energy, as from biogas, does not especially consider

whether the power is generated for basic or for peak load and at what time of day

the current is fed into the network. Therefore, biogas plants are normally designed

to cover the basic load, although the produced power depends on the activity of

the microorganism and, as a result, varies. Biogas plants are usually constructed

at places, where the power network is not available and special efforts are

required to connect the central heat and power to the public power network

(Deublin and Steinhouser, 2008)

1.2.6.1.2. Heat Supply

Generally, the economics of biogas industry largely depends on the utilization and

exploitation of the generated heat from biogas combustion. It must be borne in

mind that the heat is produced over the whole year and not only in the winter,

when it can be easily used. The heat could be used for the following purposes

(Jördening and Winter, 2005):

• heating swimming pools and/or industrial plants

• heating stables


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• heating greenhouses

• cleaning and disinfection of the milking equipment

• transformation of warmth in cold e.g. for milk cooling.

1.2.6.2. Digestate

Digestate is the solid end product of the process and contains organic compounds

which are not susceptible to anaerobic microorganisms attack (e.g. lignin). It also

consists of the inorganic remains of the dead bacteria population that have been

developed during the anaerobic treatment (Wastesum, 2006). Digestate is

produced in three different physical states depending on the type of feedstock

material and digestion process. The three forms are namely fibrous, liquor or a

sludge-based combination of the two aforementioned fractions. More specifically in

the case where two-stage anaerobic system is applied, fibrous and liquor

digestates are produced from the different digestion tanks. In single stage

digestion systems the two digestate fractions are being combined and need to be

further processed in case when separation of the two forms is required.

When the digestion is complete, the residue slurry is removed and dewatered to

produce a liquid stream and a drier solid. The water content is filtered out and re-

circulated to the digester, and the filter cake is cured aerobically, to form compost.

The final product is screened for any undesirable materials, (such as glass shards,

plastic pieces etc) before being used on the land and sold as organic soil

amendment to condition and improve soil (Ostrem, 2004). It must be noted that the

produced digestate may contain ammonia a compound which is potential

phytotoxic to plants while testing for pathogenic microorganisms shall be provided

especially in case where the time temperature regime in not sufficient for their

inactivation. However, pathogen destruction can be guaranteed at thermophilic

temperatures with a high SRT (Ostrem, 2004). Therefore, the digestate is

generally composted after the digestion in order to produce high quality end

product. It must be stated that anaerobic digestion does not reduce nutrient

content (NPK value), making the digestate more valuable as a fertilizer (Mahony

and O'Flaherty, 2002).


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1.2.6.3. Wastewater

This wastewater resulting from the anaerobic digestion process comes from the

moisture content of the produced digestate that is treated (e.g. dewatered) as well

as the water that is being produced during the biological reactions within the

digesters.. The wastewater that is being collected is generally recirculated to the

system aiming to adjust and regulate the water content of the feedstock material

whereas its excess is treated accordingly. Typically, wastewater contains high

levels of organic load which is mainly not biodegradable and often it is required to

be further processed (Wastesum, 2006).


The methane in biogas is utilized as a renewable energy source to co-generate

heat and electricity, using generally a reciprocating engine and/or microturbine.

The energy generated (electricity & heat) is used to supply the energy requirement

of the system in order to operate in an energy sufficient and energy neutral

manner whereas the excess electricity can be either provided the local grid or sold

to potential suppliers . The biogas that is produced during the anaerobic treatment

of organic waste is considered to be biogenic meaning that it does not contribute

to increasing atmospheric carbon dioxide concentrations since it is not released

directly into the atmosphere while the emitted carbon dioxide originates from

organic sources with a short carbon cycle.

Biogas may require treatment to refine it in order to use it as an energy source.

Hydrogen sulfide is among the main chemical components that need to be

removed from biogas since it constitutes a toxic product and it is released as a

trace component of the biogas. Environmental legislation puts stringent limits on

hydrogen sulfide concentration of biogases. Therefore, gas scrubbing and

cleaning is required when the levels of hydrogen sulfide in the gas are high. The

primary challenge associated with the use of biogas as a fuel is the need for gas

cleaning to ensure that the gas meets the quality requirements for the utilization

http://en.wikipedia.org/wiki/Reciprocating_engine

http://en.wikipedia.org/wiki/Microturbine


http://en.wikipedia.org/wiki/Hydrogen_sulfide


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equipment. Biogas cleaning is a capital-intensive, multistage operation that can

also carry high maintenance costs due to media replacements and/or power costs.

However, if the gas impurities are left untreated, they can increase the

maintenance requirements of the equipment fueled by the gas and thus reducing

equipment duration. Therefore, gas cleaning to reduce condensation, lower H2S

levels, and removal siloxanes is a prerequisite for effective gas utilization. Any

foam and sediments entrained in the gas stream are separated using a foam

separator in the digester gas piping, while for scrubbing H2S from biogas the most

commonly used methods include the use of iron sponge or chemical scrubbers

and the addition of ferric (Fe3+) salts to the feed. Finally, for removing seloxanes

there are two common types of systems (1) low-temperature drying systems and

(2) graphite molecular sieve scrubbers.

As has been stated, biogas can be used either for the production of heat only or

for the co-generation of heat and power. Alternatively, a stirling engine or gas

turbine, a micro gas turbine, high - and low - temperature fuel cells, or a

combination of a high - temperature fuel cell with a gas turbine can be used.

Biogas can also be used to produce steam by which an engine is driven, e.g., in

the Organic Rankine Cycle (ORC), the Cheng Cycle, the steam turbine, the steam

piston engine, or the steam screw engine (Deublin and Steinhouser, 2008).

Another very interesting technology for the utilization of biogas is the steam and

gas power station. Figure 14 shows the range of capacities for the power

generators which are available on the market as pilot plants or on an industrial

scale. The electrical efficiency indicates the ratio of electrical power to the total

energy content in the biogas.


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Figure 7: Capacity Range of engines in relation to their electrical efficiency

The generated current and heat can supply the anaerobic bioreactor itself, the

associated buildings, and neighboring industrial companies or houses. The surplus

energy can be fed into the public electricity network, and the heat into the network

for long - distance heat supply.

The acidogenic digestate is a stable organic material comprised mainly of

lignocellulosic compounds, but also of a variety of inorganic elements resulting

from the dead bacterial population. The material can be used as compost or to

make low grade building products e.g. fibreboard. The methanogenic digestate is

rich in nutrients and can be used as a fertilizer dependent on the quality of the

material being digested. Levels of potentially toxic elements should be chemically

assessed. This will be dependent upon the type and composition of the initial

substrate.


A typical mass balance of the anaerobic digestion process is shown in Figure 15.

From the diagram it can be stated that from 1 tonne of organic fraction of

Municipal Solid Waste (OFMSW) that is treated, 120 kg is the produced biogas,

423Kg is the digestate, 437kg is wastewater, while the remaining 10kg is the inert

material of the OFMSW that is removed and disposed to the landfill prior to the

biological treatment process. It must be mentioned that the mass balance


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considers a water recirculation of 370kg to the mixing tank, from the dewatering

process of the generated solid digestate. The recycled process water aims to

adjust/regulate the water to solid ratio of the feedstock, while at the same time

saving money and resources during the operation of the anaerobic treatment

plant.

Figure 8: Typical mass balance for an anaerobic digestion system

(Ostrem K., 2004) One tonne of waste produces between 80 and 130 m3 of biogas, depending on the

process, as has been reported by several large AD design firms treating MSW

(e.g. BTA, Valorga, WAASA, DRANCO, Linde, Kompogas).

The net energy output in anaerobic digestion systems using different raw materials

varies depending on transportation distance, means of transportation, conversion

techniques and needs for handling of raw materials and digested residues. For

Swedish conditions, from a life-cycle perspective, it appears that for transportation

distances up to 50 km, the energy needed for running the biogas systems typically

corresponds to 30-50 % of the energy content in the produced biogas. All raw

materials could be transported more than 150 km, some dry waste streams up to

700 km, before the energy balance turns negative. The higher the water content in

the raw material, the more sensitive the net energy output is to the transportation


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distance. There is great variability in data on biogas yield from different raw

materials, thus estimations about biogas yield strongly affect the net energy

output. Despite inherent uncertainties, the overall conclusion is that the net energy

input in the studied biogas systems normally significantly exceeds the energy

output in the form of produced biogas.

Energy input in various handling and transportation operations for bringing

different raw materials to the biogas plant and for transportation of digested

residues, and the biogas yield from various biomass resources. Values within

parentheses indicate interval found in the literature (EUBIA).

Table 13: Energy input and output from various biomass resources, (EUBIA)

A typical energy flow diagram of the product of anaerobic digestion is presented in

Figure 16 in which the methane content of biogas is 60%3, whereas the remainder

of the gas is predominately CO2, with trace elements of other gases, such as H2S,

NH3 and water vapor. According to Figure 16, 100m3 of biogas (60% CH4) acquire

an energy content of 560kWh which can give 336kWh of thermal and 224 of

electric energy, while the remaining 56.0kWh is attributed to energy losses. The

energy generated can be used within the plant for heat or electric supply purposes

or to sell the electric energy to the local electric network. Anaerobic digestion

plants large enough to produce electricity in a cogeneration unit can be self-

sufficient on the power generation from their produced biogas. The low

3 methane content of biogas ranges from 50% to as high as 75%, though most plants report values close to 60%.


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temperatures required for anaerobic digestion (less than 110°F), allow the heat to

be supplied entirely from the biogas as well.

Figure 9: Typical energy balance for an anaerobic digestion system (Ostrem

K., 2004)

1.2.9. Parameters effecting anaerobic digestion process

naerobic fermentation processes are affected by the changes in environmental

conditions; therefore, it is important to examine some of the important factors that

govern the anaerobic bioconversion process. These include organic loading rate,

biomass yield, substrate utilization rate, hydraulic retention time (HRT) and solids

retention time (SRT), start-up time, microbiology, environmental factors and

reactor configuration. The following sections elaborate on these factors.

1.2.9.1. Organic Loading Rate

In anaerobic process the loading parameters are expressed in terms of organic

loadings. More specifically, the organic loading rate of solid waste and organic

sludge is based on volatile solids (VS), while for wastewater it is expressed as

BOD or COD. Conventional environmental engineering practice has been to

express digester loadings on a weight to volume basis per unit time (kilograms of


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VS per day per cubic meter of volume - kg/d/m3). The stability of the anaerobic

fermentation and the biogas production rate are dependent upon organic loading

rates. In cases where organic loading rate is higher than normal, the digestion

process often becomes unbalanced due to the excessive production of volatile

acids to inhibitory concentrations. CO2 production under these conditions often

causes foaming of the digester and contributes to operating problems.

Maintenance of uniform or near uniform loading rates based on frequent or

continuous additions of substrate to the digester yields the most consistent

digester operation (Deublin and Steinhouser, 2008).

1.2.9.2. Biomass Yield

Biomass yield is a quantitative measure of cell growth in a system for a given

substrate which is represented by the yield coefficient (Y), given by the following

equation (Khanal, 2008).

Y = X/ S

where

X biomass concentration (mg VSS/L),

S substrate concentration (mg COD/L).

The biomass yield per mole of ATP totals 10.5 g volatile suspended solids for both

aerobic and anaerobic processes (Henze and Harremoes 1983). With respect to

the metabolic processes of microorganism, the total aerobic ATP generation is 38

mol, whereas for anaerobic digestion it is only 4 mol ATP/mol glucose. Therefore,

the biomass yield for the anaerobic treatment process is significantly lower

compared to the aerobic one. Anaerobic degradation of organic matter is

accomplished through a number of metabolic stages in a sequence by several

groups of microorganisms working synergistically. The yield coefficients for

different biological treatment processes and stages are presented in Table 14.

According to the table it can be seen that the yield coefficient of acid-producing

bacteria is significantly higher than that of methane-producing bacteria (Henze and

Harremoes, 1983), whereas in the aerobic treatment process for biodegradable


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COD irrespective of the type of substrates the yield coefficient is fairly constant

(van Haandel and Lettinga, 1994). In anaerobic digestion treatment process, the

yield coefficient depends not only on COD removed but also on the different

substrate conditions being metabolized as shown in Table 15. More specifically,

carbohydrate and protein compounds have relatively high yield coefficients, since

both microorganism groups, acidogens and methanogens, are involved in the

decomposition process of the above mentioned substrates to produce methane.

Therefore, the yield coefficients of the aforementioned compounds result from the

summation individual yield coefficient of acidogenic and methanogenic bacteria.

On the other hand, chemical compounds, such as acetate and hydrogen, have

lower yield coefficients, since only methanogenic bacteria are involved in the

metabolism of these substrates.

Table 14: Biomass yield coefficients for different biological treatment processes and stages (Young and McCarty,1969; Henze and Harremoes,

1983; van Haandel and Lettinga, 1994)

Process Yield Coefficient (Kg VSS/kg COD)

Acidogenesis 0.15

Methanogenesis 0.03

Overall 0.18

Anaerobic filter (mixed culture)

(carbohydrate + protein as substrate) 0.115-0.121

Anaerobic treatment process 0.05-0.15

Table 15: Biomass yield coefficients for different types of substrate (Pavlostathis and Giraldo, 1991)

Type of substrate Yield Coefficient (Y)

(Kg VSS/kg COD)

Carbohydrate 0.350

Proteins 0.250

Fats 0.038

Butyrate 0.058


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Propionate 0.037

Acetate 0.032

Hydrogen 0.038

The estimation of the attainable methane production depends on various

parameters among which the organic matter composition, the granulation of

waste, the proportions of the involved substrates, the level of microbial

degradability of the biomass, the relationship between nutrients (e.g. C/N ratio)

and the moisture and organic matter content. Additional factors which affect

methane yield are related to the anaerobic digestion method employed. Among

these factors are the number of stages, the temperature level (i.e. mesophilic,

thermophilic), the retention time of the substrate in the bioreactor, the type and

frequency of substrate agitation, and the quantity and frequency of the substrate

addition. These parameters must be analyzed in a laboratory test as well as in a

pilot scale to confirm the obtained results prior to the construction of a production

plant. The degradability of the substrates, the biogas yield, the maximum

recommendable volume load, the possible and practical substrate mixtures and

the changes of the concentrations of certain materials are important if large scale

anaerobic digestion plants are to be operated (continuous or batchwise mode) and

they can be determined through laboratory test (Reher, 2003). Before a large -

scale plant is constructed, the results from the laboratory test should be confirmed

in a pilot plant for the preliminary test of the fermentation process. According to

Reher (2003) and Tiehm and Neis (2002) a pilot plant should consists of a

hydrolyzer, a methane reactor, and a storage tank which should be individually

equipped with arrangements for maintaining moderate temperatures, and with

filling and cleaning devices. The recommended measurements evaluated and

monitored in the laboratory and/or pilot plant tests are the following (Deublin and

Steinhouser, 2008):

• Temperature

• pH value and redox potential

• dry matter, water content

• Content of organic dry matter (Loss on Ignition)


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• Degradability as total content of organic acids/acetic acid equivalent and

inhibitors

• Salt content

• Total content of N, P, K, Mg and S

• Availability of plant nutrients such as NO3-, NH4

+, P2O5, K2O, and Mg

• Granulation (maximum grain size), gross density

• Heavy metals (e.g. Pb, Cd, Cr, Cu, Ni, Zn, Hg)

• Content of short - chain fatty acids, principally acetic acid, propionic acid,

butyric acid, and iso - butyric acid

• C/N ratio.

The above mentioned measurements give important information on the biogas

yield, the level of nutrients, the extent of decomposition of the biomass which can

be provided during the fermentation, the fertilization value of the residue, and also

the preferable type, dimensions and mode of operation of the production plant.

1.2.9.3. Specific Biological Activity

Specific biological activity indicates the ability of microorganisms/biomass to utilize

and metabolize the substrate. According to Khanal (2008) specific biological

activity is usually reported as:

Specific substrate utilization rate = (kg CODremoval)/(kg VSS·day)

The anaerobic digestion process has a substrate utilization rate between 0.75–1.5

kg COD/kg VSS day, which is significantly higher than of composting (Henze and

Harremoes, 1983; Khanal, 2008). The reason of this difference is due to the fact

that oxygen transfer and diffusion limitation is not an issue in an anaerobic

digestion processes as it is in aerobic treatment plants. Additionally, when high

concentration of different substrates in close proximity through biomass

immobilization or granulation is maintained, a good balance of synergetic relation

between acidogens and methanogens can be obtained. Finally the improvement of

the understanding of the trace nutrient requirements of methanogens has

significantly increased the specific activity of anaerobic systems (Speece, 1983).


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1.2.9.4. Hydraulic Retention Time and Solids Retention Time

Hydraulic Retention Time (HRT) and Solids Retention Time (SRT) and are two

important design parameters in anaerobic digestion treatment processes. The

HRT is the ratio of the reactor volume to the flow rate of the influent substrate.

Therefore, it is the time that substrate spends in the bioreactor in contact with the

biomass (Cecchi et al., 2003). The time required to achieve a given degree of

treatment depends on the rate of microbial metabolism and subsequently on the

type and composition of input organic material. Waste containing readily available

biodegradable compounds such as sugar, require low HRT, whereas complex

waste, e.g. lignin organic compounds, is slowly degradable and needs longer HRT

for their decomposition.

SRT is the average residence time of solids into the reactor and it estimated by the

ratio between the content of total solids in the reactor and the solids flow rate

extracted from the reactor (Cecchi et al., 2003). Therefore, SRT controls the

biomass in the reactor to achieve a given degree of waste stabilization, whereas it

determines the permissible organic loading rate in the anaerobic process. If the

quantity of biomass extracted from the reactor is equal to the biomass produced in

the reactor, then the solids concentration in the reactor, as biomass, will be

constant in a given time and it can be said that the reactor is operating in steady-

state conditions. SRT is a measure of the biological system’s capability to achieve

specific effluent standards as well as to maintain a satisfactory biodegradation rate

of pollutants.

According to Speece (1996) the HRT is considered in process design especially

for complex and slowly degradable organic pollutants, whereas the SRT is a

design deciding factor for easily degradable organics. In case of methanogenic

bacteria, which are slow-growing microorganisms, special attention shall be given

to prevent their washout from the reactor in order to achieve a longer SRT.

Elevated HRTs require a bigger reactor volume thus increasing capital

expenditure. An early attempt to maintain a long SRT irrespective of HRT was the

use of the clarigester or anaerobic contact process, where the anaerobic sludge

was allowed to settle in the settling tank and was then returned back to the


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reactor. A wide variety of high-rate anaerobic reactors have been able to maintain

extremely high SRTs due to biomass immobilization or agglomeration. Such

systems operate under short HRTs without any fear of biomass washout. The

empirical HRTs for different anaerobic systems to achieve the same degree of

treatment are presented in Table 16.

Table 16: HRTs of anaerobic systems needed to achieve 80% COD removal efficiency at temperature >20 C (Van Haandel and Lettinga, 1994)

Anaerobic System HRT (h)

UASB 5.5

Fluidized/expanded bed 5.5

Anaerobic filter 20

Anaerobic pond a 144 (6 days) a : BOD removal efficiency.

1.2.9.5. Start-Up Time

Start-up is the initial operational period during which the process is brought to a

point where normal performances of the biological treatment system can be

achieved with continuous substrate feeding. Start-up time is important parameter

in anaerobic digestion processes due to the slow growth rate of anaerobic

microorganisms, especially methanogens, and their susceptibility to changes in

environmental factors. Anaerobic treatment systems often need quite a long start-

up time, which may weaken their competitiveness with aerobic treatment systems.

A start-up time between 2–4 months is generally obtained at a mesophilic

operational mode (35 C), whereas under thermophilic conditions (55 C) start-up

exceeds 12 months because of the high decay rate of biomass (Khanal, 2008).

The start-up time could be significantly reduced in case when the exact microbial

culture for the waste treated is used as a seed, thus leading to increased

generation time of the anaerobic microorganisms. To further reduce start-up time,

substrate loading rates and environmental factors such as nutrient availability, pH,


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temperature and redox potential should be maintained at levels favoured by

microbes during the start-up.

1.2.9.6. Microbiology

The microbiology of the anaerobic treatment system is much more complex than

composting, since digestion involves a sequential multistep process in which a

consortium of microorganisms working synergistically degrades the organic

matter. The stability of an anaerobic treatment plants constitutes a challenge due

to the sensitivity of anaerobic microorganisms, especially methanogenic bacteria,

to potential changes of the environmental factors such as pH, temperature, redox,

sufficiency of nutrients and trace elements. Special focus should be given to

anaerobic digestion operation to maintain suitable for the microorganism

conditions, since in case of system failure (e.g. unfavorable environmental

condition and/or biomass washout from the reactor) it may take significant time for

the system to return to a normal operating conditions due to the slow growth rate

of methanogenic bacteria.

1.2.9.7. Environmental Factors

It has been pointed out earlier that anaerobic processes are severely affected by

the changes in environmental conditions. Anaerobic treatment system is much

more susceptible than the aerobic one for the same degree to deviation from the

optimum environmental conditions. The successful operation of anaerobic

reactors, therefore, demands a meticulous control of environmental factors close

to the comfort of the microorganisms involved in the process. The effect of

environmental factors on treatment efficiency is usually evaluated by the methane

yield because methanogenesis is a rate-limiting step in anaerobic treatment of

wastewater. Hence, the major environmental factors are usually governed by the

methanogenesis. Brief descriptions of the important environmental factors are

outlined here.


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1.2.9.7.1. Temperature

Anaerobic processes, like other biological processes, strongly depend on

temperature. There are mainly two temperature ranges that provide optimum

digestion conditions for the production of methane – the mesophilic and

thermophilic ranges. The anaerobic conversion of organic matter has its highest

efficiency at a temperature 35–40 C for mesophilic conditions and at about 55 C

for the thermophilic conditions (van Haandel and Lettinga 1994). Anaerobic

processes, however, can still operate in a temperature range of 10–45 C without

major changes in the microbial ecosystem. Generally, anaerobic treatment

processes are more sensitive to temperature changes than the aerobic treatment

process. Temperature variations of ±3°C have minor effect on the fermentation

(Winter, 1985). In the thermophilic range (between 55 and 65°C), a constant

temperature level has to be maintained, since small deviations may cause a

drastic reduction of the degradation rates and thus of biogas production. Igoni et

al. (2008) and Tchobanoglous et al. (1991) proposed that the optimal temperature

ranges are the mesophilic, namely 30–38°C, and the thermophilic 44–57°C (Igoni

et al., 2008), respectively. It has been observed that higher temperatures in the

thermophilic range reduce the required retention time.

1.2.9.7.2. pH

The optimum operating pH depends upon the anaerobic fermentation stage and

subsequently on the associated bacteria namely acid-producing and methane-

producing bacteria. During digestion, the two processes of acidification and

methanogenesis require different pH levels for optimal process control. More

specifically, acidogenic bacteria prefer a pH between 5.5 and 6.5, while

methanogenic bacteria prefer a range of 7.8–8.2. In an environment where both

cultures coexist (e.g. one stage process), the optimal pH range is 6.8–7.4. A

favorable pH range for methanogenic bacteria is between 6 and 8 with an optimum

pH for the group as a whole near 7.0. In case where the process takes place in a

single bioreactor, methanogenesis is considered to be the rate-limiting step and it

is necessary to maintain the reactor pH close to neutral. Normally, acid and


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ammonia production vary only slightly due to the buffering effect of carbon

dioxide/bicarbonate (CO2/HCO3–) and ammonia/ammonium (NH3/NH4

+), which are

formed during fermentation, and the pH normally stays constant between 7 and 8.

1.2.9.7.3. Water Content

Bacteria consume the available organic substrate in dissolved form. Therefore, the

production biogas and the water content of the initial organic matter are

interdependent. When the water content is below 20% by weight, biogas

production is significantly limited, whereas increasing water content biogas

production is enhanced, reaching its optimum at 91%–98% water by weight

(Kaltwasser, 1980).

1.2.9.7.4. Oxidation–reduction potentials (ORP)

Methane bacteria are very sensitive to oxygen and have lower activity in the

presence of oxygen thus reducing biogas yield. According to Mudrak and Kunst

(1991) the anaerobic process shows a certain tolerance to limited, even

continuous, quantities of oxygen. The redox potential can be used as an indicator

of the process of methane fermentation, since methanogenic bacterial growth

requires a relatively low redox potential. Hungate (1966) found that -300 mV is the

minimum redox value, whereas Morris (1975) reported that the optimum ORP

value for the growth of anaerobic microorganisms in any medium, is between -200

to -350 mV at pH 7. Finally, according to Archer and Harris (1986) and Hungate

(1967) it is has been established that methanogens require an extremely reducing

environment, with redox potentials as low as 400 mV.

1.2.9.7.5. Nutrients and Trace Metals

All biological treatment methods require nutrients and trace elements during waste

processing. Nutrients and trace metals are not directly involved in waste

processing and stabilization but they are the essential components of existing

microbial cells growth and synthesis of new cells. Therefore, the presence of


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nutrients and trace metals provide the needed physicochemical conditions for the

optimum growth of microorganisms. If the digested organic material does not have

one or more of the important nutrients and trace elements, the waste degradability

is severely affected, since microbial cells are unable to grow at optimum rate and

to produce new cells.

1.2.9.7.6. Toxicity and Inhibition

Anaerobic microorganisms are inhibited by the substances present in the influent

waste stream and by the metabolic byproducts of microorganisms. Ammonia,

halogenated compounds, heavy metals and cyanide are examples of the former,

while ammonia, volatile fatty acids, and sulfide are examples of the latter.

Toxicants, components in the feed material causing adverse effects on bacterial

metabolism, are responsible for the occasional failure of anaerobic digesters. With

reference to investigations of Konzeli-Katsiri and Kartsonas (1986), Table 17 lists

the limit concentrations (mg L–1) for inhibition and toxicity of heavy metals in

anaerobic digestion.

Table 17: Inhibition of anaerobic digestion by heavy metals (Konzell-Katsiri and Kartsonas,1986)

Heavy Metal Inhibition (mg L–1) Toxicity (mg L–1)

Copper (Cu) 40-250 170-300

Cadmium (Cd) - 20-600

Zinc (Zn) 150-400 250-600

Nickel (Ni) 10-300 30-1000

Lead (Pb) 300-340 340

Chromium III (Cr) 120-300 200-500

Chromium VI (Cr) 100-110 200-420

1.2.9.7.7 Volatile Fatty Acids (VFAs)

Volatile fatty acids accumulation during process imbalance directly reflects a

kinetic uncoupling between acid producers and consumers (Switzenbaum et al.,


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1990). The volatile fatty acids concentration has been most suggested for

monitoring of anaerobic digester (Hill and Holmberg, 1988; Lahav et al., 2002;

Feitkenhauer et al., 2002; Mechichi and Sayadi, 2005). In a low buffered system,

pH, partial alkalinity and VFA measurements are useful for process monitoring,

whereas in highly buffered system only VFA is reliable for indicating process

imbalance (Murto et al., 2004).

1.2.9.8. Reactor Configuration

The configuration of the anaerobic digester is of paramount importance in

anaerobic fermentation processes. The relatively low biosynthesis rate of

methanogens in an anaerobic system demands special consideration for

bioreactor design. The selection of bioreactor types depends on the requirement of

a high SRT/HRT ratio, in order to prevent the washout of slow-growing and

sensitive methanogenic bacteria. Therefore, the anaerobic digestion performance

of the bioreactors is based on their capability to maintain a high SRT/HRT ratio

and thus to retain biomass. Another approach for reactor configuration selection is

based on required effluent quality. Because of relatively high half-saturation

constants for anaerobic microorganisms, continuous stirred tank reactors may not

be suitable, as immediate dilution of the waste leads to low concentrations of

organic matters, but still too high to meet the effluent discharge standards, which

are below the range of anaerobic degradation. Under such conditions, a staging or

plug flow type reactor would be more appropriate (Khanal, 2008).


The technologies used for the anaerobic digestion appear to be more ecological

than those of composting. The three categories of greenhouse effect, acidification

and heavy metals play an important role in the environmental impact assessment.

Carbon dioxide emission cannot be prevented, if biogenic matter is degraded. On

the other hand, methane is freed in nature as soon as biomass is piled up into

heaps. For an aerobic treatment after anaerobic digestion, there is the

disadvantage that the organic matter is well inoculated with anaerobic bacteria.


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Even if just a very small share of the organic matter is degraded during

composting after anaerobic digestion, methane emissions may be larger than

those caused by composting alone. As far as energy is concerned, digestion

plants are very good from an ecological point of view, mainly because they do not

need external fossil and nuclear energy. The production of renewable energy has

positive consequences on nearly all impact categories, because of savings in or

compensation for non-renewable energy.


It is difficult to discuss in detail the economics of deploying an anaerobic digestion

plant for biowaste, because of the many factors that affect the costs and the

variation in circumstances and costs between different countries. When comparing

systems costs, one must consider which of the following cost items are included in

the analysis (1) Predevelopment costs (Siting and permitting, Land acquisition,

Environmental impact assessment, Engineering planning and design,

Hydrogeological investigation), (2) Construction costs (Infrastructure e.g. access

roads, piping, utility connections, Cleaning and excavation, Buildings and

construction, Equipment e.g. tanks, machinery, electronics, Labor), (3) Operating

costs (Maintenance fees, Labor, Materials, Water and energy, Supervision and

training, Insurance, Overheads, Wastewater disposal, Solid residuals disposal,

Regulatory fees. Figure 17(a) presents the capital cost curves for European MSW

digesters, while Figure 17(b) presents their maintenance and operational costs

incorporating the above mentioned parameters (CIWMB, 2008).


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Figure 10: (a) Capital cost and (b) M&O costs curves for European MSW

digesters (CIWMB, 2008)

In general, when looking at the treatment cost per tonne of MSW for the large

facilities built in Europe, it is clear that over the last few years the trend is for a

reduction in overall treatment costs making anaerobic treatment systems more

competitive. However, economies of scale mean that the complex industrial

systems need to process many thousands of tonnes of MSW per year to have a

reasonable treatment cost per tonne. According to the Carbon Finance Unit of the

World Bank in 2008, the capital cost of an anaerobic digestion treatment system

with a capacity of 300 tonnes/day is around 15-55 million € (319€ per tonne capital

cost), while the operation and maintenance cost is 40-70 € per tonne.

Another source estimates the relevant treatment costs to 70 – 100 €/t (Neamt

Master Plan, 2008). Based on recent EU data, the financial cost of anaerobic

digestion in EU member states (including annualized capex, opex, maintenance

expenditures and other specific costs or revenues) varies between 48 - 107

€/tonne for the various end uses of biogas namely electricity, CHP, supply to the

grid or as vehicle fuel (EC, 2010). Finally according to the different uses of biogas

the report provided by ARCADIS & EUNOMIA (2010) suggests the following costs

for anaerobic digestion:

AD with Electricity Only: capital costs 375€ per tonne, operating costs

37.50€ per tonne for facilities with a capacity ranging from 20,000 to 30,000

tonnes with appropriate post-treatment of the produced digestate.


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AD with combined heat and power: capital cost of 478 € per tonne, and an

operating cost of 38.75 € per tonne

AD with Gas Upgrading for Use as Vehicle Fuel: capital cost of 440 € per

tonne and operating expenditures of 45.25 € per tonne

AD with Gas Upgrading for Use in Grid: capital cost of 275 € per tonne

whereas for operating expenditures no data were available.


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1.3. MECHANICAL BIOLOGICAL TREATMENT (MBT)

A mechanical biological treatment system is a waste processing facility that

combines a waste sorting facility with biological treatment methods e.g. anaerobic

digestion and/or composting. MBT plants are designed to process mixed

household waste as well as commercial and industrial waste. Therefore, MBT is

neither a single technology nor a complete solution, since it combines a wide

range of techniques and processing operations (mechanical and biological)

dictated by the market needs of the end products. Thus, MBT systems vary greatly

in their complexity and functionality. Figure 18 presents a process diagram of a

Mechanical Biological Treatment facility.

Figure 11: Mechanical Biological Treatment flow chart

1.3.1. Mechanical sorting component

The "mechanical" element is usually an automated mechanical sorting stage. This

either removes recyclable elements from a mixed waste stream (such as metals,

plastics, glass and paper) or processes them. MBTs typically involve a

combination of screens, magnetic separation, eddy current separation, optical

separation and air classification.


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The mechanical sorting processes recover a part of MSW as recyclable materials,

while another part formulates a combustible product known as ‘Refuse Derived

Fuel (RDF) which covers a wide range of materials sorted in such a manner in

order to obtain high calorific value. RDF can be incinerated in power stations,

pyrolysis and gasification systems, co-incinerated in other industrial combustion

processes for energy production.

1.3.2. Biological processing compartment

The "biological" element includes the biological treatment of the biodegradable

organic materials that has been sorted, since after the mechanical sorting stage.

Therefore, the biological processing compartment refers to the following methods:

Aerobic treatment (composting)

Anaerobic digestion

Biodrying

By applying composting, the organic materials are treated with aerobic

microorganisms. The microorganisms break down the organic compounds into

carbon dioxide and a stabilized solid end product (compost). More details on the

aerobic treatment of organic waste are given in section 1.1.

Anaerobic digestion breaks down the biodegradable organics to produce biogas

(mainly methane) and a stabilized solid end product which has similar

characteristics and potential applications with compost. The biogas can be used,

after cleaning, to generate electricity and heat. More information on the anaerobic

digestion of organic waste are given in section 1.2.

Biodrying of organic waste material involves the rapid heating of waste through the

action of aerobic microbes. During this partial composting stage the heat

generated by the microbes result in rapid drying of the waste. These systems are

often configured to produce a refuse-derived fuel where a dry, light material is

advantageous for later transport combustion.


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Some MBT systems incorporate both anaerobic digestion and composting

treatment methods. This may either take the form of a full anaerobic digestion

phase, followed by post - composting of the produced digestate. Alternatively a

partial anaerobic digestion phase can be induced on water that is percolated

through the initial substrate, dissolving the readily available organic matter, with

the remaining material being sent to a windrow composting facility.


A typical mass balance diagram of an MBT process with aerobic digestion is

shown in Figure 19. According to the figure, for 100 tn of processing MSW, 46kg is

assumed to be the biodegradable organic fraction which is mixed with additives

producing a final end compost which accounts to 18tn (39% of the biodegradable

waste treated). During the process of composting, 18tn are the mass losses due to

the leachate and emissions production, while 9.6tn is the residue that remains

after refining the mature compost.


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Figure 12: Schematic presentation of inputs and outputs of a typical

mechanical sorting component with aerobic digestion (Juniper, 2006)

Below a mass balance of an MBT with anaerobic digestion is presented. This is

the process whereby only a fraction of 50% to 65% of the total organic fraction is

actually digested, while the remaining 50 to 35% is bypassed and is not subjected

to anaerobic decomposition. The digested residue is then intensively mixed with

the non-digested organics. The dry matter concentration of 45% in the resulting

mixture of the two fractions allows for efficient aeration and rapid aerobic

decomposition. A plant treating 100,000 ton per year of residual solid waste is

recovering recyclables and producing burnable fractions. About 28,000 tonnes per

year of organics are diverted to digestion, to which also about 7,000 ton per year


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of non-digested dewatered sewage sludge is added. No wastewater is generated

at the plant.


mechanical sorting component with anaerobic digestion

A typical mass balance diagram of an MBT process with biodrying is shown in

Figure 21. According to the figure, for 1 tn of processing MSW, during the

biodrying process 250kg of gas products are produced, while the remaining

material is separated mechanically to 550kg of Solid Recovered Fuel (SRF), 35kg

metals and 165kg residues.


mechanical sorting component with biodrying


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The energy for reaching high temperatures and for drying during the aerobic

phase is mainly provided by the fraction that was not digested. The energy

balance for an MBT treatment process can be calculated through the energy

balances of the composting and the anaerobic digestion process.


The products of the Mechanical Biological Treatment technology are:

Recyclable materials such as metals, paper, plastics, glass etc.

Unusable materials (inert materials) safely disposed to sanitary landfill

Biogas (anaerobic digestion)

Organic stabilized end product

refuse derived fuel - RDF (High calorific fraction).

MBT systems can form an integral part of a region's waste treatment

infrastructure. These systems are typically integrated with curbside collection

schemes. In the event that a RDF is produced as a by-product then a combustion

facility would be required. Alternatively MBT practices can diminish the need for

home separation and curbside collection of recyclable elements of waste. This

gives the ability of local authorities and councils to reduce the use of waste

vehicles on the roads and keep recycling rates high.


The environmental impacts produced from the MBT can be drawn account being

taken of the respective environmental impacts of the composting and the

anaerobic digestion process.


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The treatment cost of a mechanical biological treatment system with a capacity of

150,000 tonnes per annum is around 45 €/tn (Neamt Master Plan, 2008).

According to ARCADIS & EUNOMIA (2010) a typical mechanical sorting

component with biodrying acquires a capital cost of €250 per tonne, with operating

costs of €21 per tonne before residue disposal.


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1.4. CASE STUDIES OF BIOLOGICAL TREATMENT SYSTEMS IN THE TARGET AREA

1.4.1. General

As proved by the results of the analyses of the composition of municipal solid

waste that took place in both Romania and Bulgaria by the SUROVINA working

team, the generated waste is characterized by high organic content. Therefore, the

potential for the application of biological methods in Romania and Bulgaria, as well

as the whole Balkan Region is very high. Specific case studies of biological

methods applied in Greece, Romania and Slovenia are described below. More

specifically, information is provided about indicative mechanical biological

treatment facilities in Greece and indicative composting facilities in Slovenia and

Romania.

In Bulgaria currently no composting, anaerobic digestion or MBT treatment plants

are operating and only home composting takes place in 25 municipalities including

5,500 households.

Greece is considered to rely heavily on MBT/mixed waste composting to deliver its

Landfill Directive obligations, as shown by the relevant infrastructure that has

already been developed (Kalamata, Ano Liosia, Chania, Heraklion and Kefalonia).

The composting unit in Kalamata was the first that was built and operated in

Greece from 1997 – 2002 with certain problems and difficulties mainly originating

from the fact that no schemes for the separate collection of organic waste were

applied. Nowadays, the idea of trying to put this unit in operation phase again is

also seriously discussed. Regarding the unit in Chania, according to 2008 data, 34

tones of biodegradable waste are composted every day (8,900 tones on annual

basis). In Athens, there is one MBT plant in Ano Liossia, covering approximately

20% of the whole waste produced in the area, and produces RDF and low quality

compost. During 2009 a biodrying facility started its operation in Heraklion in Crete

(75,000 tn/year) and a MBT facility in the island of Kefalonia (25,000 tn/year).


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Currently, eight (8) aerobic treatment (composting) facilities for bio-waste and two

(2) anaerobic treatment facilities are operating in Slovenia. Both anaerobic

digestion plants are processing organic waste including bio-waste. The anaerobic

digestion plant of Bioenerg has an installed electrical capacity of 1,460 kWe and

the one of Koto d.d. 526 kWe. Totally 13 anaerobic digestion plants are operating

in Slovenia treating various feedstocks. Apart from the composting and biogas

plants some mechanical – biological treatment units are also in operation.

Currently five composting facilities are operating in Romania. The existing

facilities are treating mainly green waste from parks and gardens and a small part

of household organics. Additionally, one composting facility in Region 3 South and

one composting facility in Region 2 South-East are under construction.

1.4.2. Mechanical Biological Treatment Plant in the West Attica Region, Greece

The Ano-Liosia Integrated Waste Management Scheme is situated in the Western

suburbs of Athens in Greece. The entire processing plant comprises of a landfill,

an industrial unit of incineration of hospital waste and an MBT scheme (Figure 22)

for waste. The latter includes a large composting facility. The MBT part for

mechanical recycling of waste is the largest one in Europe and one of the largest

in the world. It receives waste from the Attica region. Currently, the population of

Attica exceeds 4.5 million people. The plant was designed and constructed after

an international tender, which was procured by the Association of Communities

and Municipalities of the Attica Region (ACMAR). ACMAR is the Public Authority

responsible for the management (treatment, recycling and disposal) of Solid

Waste of about 95% of the population of the Attica Region. The construction of the

factory of Mechanical Recycling was funded by the European Union and by the

Greek government.

The MBT plant constructed by ENVITEC is located in Ano Liosia, next to the

sanitary landfill of West Attica and it occupies an area of 178,000m2. It has a


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treatment capacity of 1,200 tn of MSW and 50 tonnes of green waste in a daily

basis which equals to 330,000 tonnes/year. The plant recycles packaging material,

namely ferrous and aluminum materials and solid fuel (RDF: paper and plastic

waste) as well as compost, while the remaining residues are landfilled.

Figure 15: MBT plant in Ano Liosia

The MBT consists of the following components:

A. Entrance Facilities – Weighting of waste, Unit for the Reception of Waste

B. Unit of Mechanical Separation

C. The Composting Unit

D. The refinery unit

E. Curing Unit

F. Packaging Unit

G. Wastewater Treatment Unit

H. Unit for Treatment of Air Emissions from the Composting Unit

I. Unit for Treatment of Air Emissions from the Mechanical Separation Unit.


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1.4.3. Mechanical Biological Treatment Plant in Chania, Greece

The plant of Mechanical Separation and Composting located in Chania was

designed to treat municipal solid waste from a number of municipalities, which

amount to 70,000 tn/yr and 10,500 tn/yr of green waste. The recyclable waste is

separated from mixed waste and sorted into organic, plastic, paper, ferrous and

aluminum materials, while RDF and compost is produced. Overall 65% of this

quantity is recovered and put into the market, while the remaining 35% is disposed

in the nearby sanitary landfill for treatment residues.

The plant is designed to operate six hours/day, 260 days/yr and the installed

capacity is 2.3 MW. The total operational cost of the plant is estimated at 40€/ tn.

The revenues from the sale of the compost product and of the recovered materials

are approximately 15€ / tn. The net operation cost 25 - 30€/ tn.

Figure 16: MBT plant in Chania, Crete


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1.4.4. Mechanical Biological Treatment Plant in Kalamata, Greece

The MBT plant in Kalamata constitutes of a mechanical separation unit and a

composting plant. It processes the total quantity of municipal solid waste

generated in the municipality of Kalamata as well as a portion of the sewage

sludge produced in the Waste Water Treatment plant of the town. The design

capacity of the plant is 400 tn/week mixed MSW and 40 tn/week sewage sludge.

In peak periods, the plant has treated up to 30% more load (520 tn/week).

Figure 17: MBT plant in Kalamata, Peloponnese

1.4.5. Composting Plant for Solid Waste at the Landfill Site in Piatra Neamt, Romania

The primary objective of the Composting Plant for the Organic fraction of Solid

Waste in Piatra Neamt is to compost as much as possible of the municipal

biodegradable waste (organic fraction) aiming at reducing the needed landfill

capacity. The coarse material necessary for the enhancement of the composting

process include wood waste in form of wood chips from the industry in Piatra

Neamt. The capacity of the composting plant is 12,000 tn/yr of organic fraction

(biodegradable), with available space for a possible future extension of 5,000 tn/yr.

This relates to the collected biodegradable organic fraction of the MSW. Structure

fraction for the composting process to operate a structure material is necessary to

ensure C/N ratio, air distribution, etc. The necessary amount is approximately

equal to the organic fraction i.e. 13,000 tn/yr (structure fraction) and with an

extension of 5,000 tn/yr to 18,000 tn/yr. The total design capacity of the


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composting plant in Piatra Neamt is 25,000 tn/yr (12,000 tn biodegradable +

13,000 tn structure fraction).

Figure 18: Composting plant in Piatra Neamt, Romania

1.4.6. Composting Plant in Vrhnika, Slovenia

At the composting plant in Vrhnika about 10,000 tn of separately collected

household waste as well as green and garden waste from the region around

Ljubljana are processed. The plant comprises of an enclosed composting vessel

with wheel loader, turning negative aerating curing area and a positive aerated

compost storage. The material is placed in four closed boxes, turned weekly and

composted for a total of four weeks. For a better control of odour emissions, the

exhaust air from the curing area is treated via a biofilter. After about eight to twelve

weeks, the finished compost is screened and marketed. A further increase in

product quality is added by the storage of the finished compost on the pressure

aerated storage area where there are also aerobic conditions until is placed on the

market (mainly landscaping and horticulture). The operational cost for the

composting plant in Vrhnika is 18 €/tn of waste treated.


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Figure 19: Composting plant in Vrhnika, Slovenia

1.4.7 Composting Plant in Puconci, Slovenia

The composting plant in Puconci treats 4,000 tn/yr of biowaste and green waste.

The plant has an open windrow composting platform with negative aeration and

positive aerated curing area.

Figure 20: Composting plant in Puconci, Slovenia


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THERMAL TREATMENT TECHNOLOGIES

2.1. GENERAL

This section deals with the description of the alternative thermal practices for

municipal solid waste management. Thermal methods for waste management aim

at the reduction of the waste volume, the conversion of waste to harmless

materials and the utilization of the energy that is hidden within waste as heat,

steam, electrical energy or combustible material.

According to the New Waste Framework Directive 2008/98/EC, the waste

treatment methods are categorized as “Disposal” or “Recovery” and the thermal

management practices that are accompanied by significant energy recovery are

included in the “Recovery”. In addition, the pyramid of the priorities in the waste

management sector clearly shows that energy recovery is more desired option in

relation to the final disposal.

Figure 28: Pyramid of the priorities in the waste management sector

That is why more and more countries around the world develop and apply Waste-

to-Energy technologies in order to handle the constantly increasing generated

municipal waste. Technologically advanced countries in the domain of waste


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management are characterized by increased recycling rates and, at the same

time, operation of a high number of Waste-to-Energy facilities (around 420 in the

27 European Member-States).

At European level, there are great variations in the municipal solid waste

management practices applied in the 27 EU Member – States. It can be said that

on average, 40% of the generated municipal waste is landfilled, 40% is recycled

and 20% is incinerated.

Figure 29: Management practices for municipal waste in the EU countries

(Eurostat 2008)

There are EU countries where more than 90% of the generated municipal waste is

landfilled, while the rest 10% is recycled or energetically recovered, while in other

EU countries that are advanced in the waste management field only 10% of

municipal waste is disposed at landfills, 65% is recycled and the rest is subjected

to thermal treatment methods. Most specifically, in relation to the thermal waste

treatment the public opinion is against this alternative and there are EU countries

where no thermal management practices are applied for the management of the


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generated municipal waste (Bulgaria, Estonia, Cyprus, Latvia, Lithuania, Malta,

Romania, Greece), while in other EU countries thermal management practices

are applied at very limited degree (Slovenia (1%), Poland (1%), Ireland (3%).

European countries that are characterized as advanced in the field of solid waste

management have already achieved very high recycling rates and at the same

time use thermal methods for a large part of the generated municipal waste. More

specifically, the percentage of thermal waste treatment is 54% in Denmark, while

the relevant figure for Sweden, Holland, Luxembourg Belgium, Germany, France

(Autret et al. 2007), Austria and Portugal are 49%, 39%, 36%, 36%, 35%, 32%,

27% 19% respectively. It has also to be noted that the green cities in Europe

(Stockholm Hamburg, etc) have incorporated thermal treatment facilities in their

planning for effective solid waste management.

Thermal treatment of municipal solid waste includes all processes that result in the

conversion of the waste content in gas, liquid and solid products with release of

thermal energy.

The thermal waste management technologies can be categorized as follows:

Combustion - Incineratyion

Pyrolysis

Gasification

Plasma technology.

Additional innovative thermal waste management techniques combine

incineration, pyrolysis and gasification, which constitute the three basic thermal

treatment options for solid waste. The application units include typical

constructions of conventional methods. The main reasons justifying the rapid

expansion of new methods include the benefits from the applications. These

benefits can be ecological (environmentally friendly air emissions, low quantities of

inert solid residues), energetic (production of energy and less use of fossil fuels)

and economic (lower capital cost).


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The main targets of the application of thermal management practices in the field of

solid waste management are:

The minimization of the waste quantities that end up at landfills.

The conversion to inert materials that are less harmful for human health and

the environment.

The reduction of the environmental pollution and particularly the avoidance

of the generation and release of volatile substances, such as furans and

dioxins.

The utilization of the colorific value of waste for energy production (heat,

power, fuel).

At this point it should be noted a lot of effort has been made to develop models in

order to correlate of the net calorific value of the waste to be treated with different

characteristics, such as the elemental waste content. For the case of the municipal

solid waste, a large numbers have been proposed, but there is no model that

could be characterized as generally acceptable and reliable. Indicatively, the

following are mentioned:

Dulong: HHV = 81C +342,5 (H – O/8)+22,5 S – 6(9H –W)

Steuer: HHV = 81 (C – 3 x O/8) +57 x 3 x /8 +345 ( – /10) +25S – 6(9H+W)

Scheurer – Kestner: HHV = 81 (C – 3 x O/4) + 342,5H + 22,5 S+ 57x3x O/4 -

6(9H+W)

Chang: HHV=8561,11 C+179,72 H-63,89 S-111,17 O-91,11 Cl-66,94N

Wilson: HHV = 7831 Corg+35,932(H-O/8)+2212S -3545Cinorg +1187 O +578N

In general, it can be expressed that the calorific value of a material depends on the

content in the basic combustible elements, which are C and H and S at lower

degree. Moisture and ash are also main parameters for the potential of energy

utilization of a material. The moisture included in waste constitutes an obstacle for


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the easy thermal treatment, since it requires a significant amount of energy so as

to be removed allowing for waste to be combusted and provide the thermal load

included within the waste. On the other side, ash involves inorganic constituents

included in waste (metals, glass and other inert materials, such as soil), which

cannot act as energy sources (Kathiravale et al. 2003, Menikpura & Basnayake

2009, Rao et al. 2004, Minutillo et al. 2009, Friedl et al. 2005).

The thermal waste treatment facilities have to be combined with collection systems

of the generated waste, sanitary landfills, plants for recovery of materials from

waste, composting facilities, etc.

It has to be noted that Directive 2008/98/EC of the European Parliament and of the

Council of 19 November 2008 on waste and repealing certain Directives

(commonly known as Waste Framework Directive) clarifies when the incineration

of municipal solid waste is energy effective and, therefore, can be considered are

recovery process and not disposal one. More specifically, the Waste Framework

Directive considers incineration facilities for municipal solid waste as recovery

plants in the case that the energy efficiency is above or equal to:

0.60 for incineration installations in operation and permitted in accordance

with legislation before 1st January 2009

0.65 for incineration installations permitted after 31st December 2008.

The energy efficiency is calculated using the following formula:

Energy efficiency R1 = [Ep - (Ef + Ei)] / [0,97 × (Ew + Ef)], where:

Ep = annual energy produced as electricity or heat, expressed in GJ/year. It is

determined by multiplying heat produced for commercial use by the coefficient 1.1

and the energy in the form of electricity by the coefficient 2.6

Ef = energy input to the system from fuels leading to steam production on annual

basis, expressed in GJ/year

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32008L0098:EN:NOT


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Ew = annual energy contained in the treated waste calculated by the use of the net

calorific value of the waste, also expressed in GJ/year

Ei = energy imported excluding Ew and Ef on annual basis, expressed in GJ/year

0.97 = is a mathematical factor accounting for energy losses due to bottom ash

and radiation.

In the way, the planning of incineration facilities in the future is encourages to go

towards installations characterized by high energy efficiency. The assessment of

the energy efficiency of 231 Waste-To-Energy Plants that represent 70% of the

relevant total capacity at European level showed that 169 plants out them were

characterized by values of energy efficiency higher than 0.60, while 251 plants had

energy efficiency lower than the value of 0.60 or did not reply to the question in

relation to the energy efficiency. Consequently, 40% of the Waste-toEnergy Plants

that are operating within the European Union, Switzerland and Norway already

satisfy the criterion of the Waste Framework Directive so that their operation can

be considered as recover and not disposal (Stengler 2010).

It is noted that the formula referring to the energy efficiency does not constitute

performance indicator of the plant. On the basis of the Waste Framework

Directive, the aforementioned limits are not committing, since the climatological

conditions should also be taken into consideration, since they can influence the

energy efficiency of the plants. The limits on the energy efficiency of waste-to-

energy plants that are set by EU can be satisfied according to the Confederation of

European Waste-to-Energy Plants (CEWEP, www.cewep.eu), even in the case of

exclusive production of electrical energy. In order to confirm the aforementioned

estimation, a thermal waste treatment medium-size unit with capacity 300,000

Tonnes per year produces 25 MW of electrical power with an indicative

performance degree 26.5%, while the value of the energy efficiency is estimated

about R1 = 0,697 (www.wtert.gr).

The satisfaction of the energy efficiency target from a waste-to-energy plant does

not only depend on the energy included within the waste to be treated, but also on

other factors, such as the input to the system from fuels leading to steam

http://www.wtert.gr/


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production and the amount of the other energy forms that is introduced to the

installation. Therefore, it is not possible to determine the energy efficiency just

using data on the net calorific values of waste. Nevertheless, in the case that the

net calorific value is relatively high, then the energy efficiency target can be

achieved more easily.

Thermal waste management methods should be applied together with separation

at source of all materials that can be recycled in order to maximize material

recovery from waste. The advantages of thermal methods in waste treatment are

summarized as follows:

Reduction of the weight and volume of the treated waste: The final solid

residues have weight that varies from 3 to 20% in relation to the initial

weight of waste, depending on the technology that is used. Gasification and

pyrolysis result in lower quantities of solid residues comparing to

incineration.

Absence of pathogenic factors in the products:

The products of thermal treatment, due to the high temperatures that

are developed, are characterized from complete absence of

pathogenic factors.

Demand for limited areas:

The thermal treatment units are characterized by low demands for

land for their installation.

The pyrolysis and gasification processes require less space in

relation to incineration.

Utilization of the energy content of waste:

Through the thermal treatment technologies, the exploitation of the

energy content of waste is possible.

This energy can be either electric or thermal energy.

Reduction of the burden paused to the landfill sites and consequent

increase of their lifetime.

Extraction of the organic fraction of municipal waste from landfill sites, as

required by the relevant legislative framework (Directive 1999/31/EC).


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Indicative disadvantages of the application of thermal methods are the following:

Relatively high capital cost:

Higher than that of other technologies for the management of

municipal waste.

Significant part of the total capital cost, especially for the case of

incineration, is spent on antipollution measures.

Increased operation cost

In general, the thermal management techniques are characterized by

relatively high operation cost. The cost is reduced substantially as

the capacity of the plant increases.

Demand for high quantities of waste:

Especially for the case of incineration – combustion, a minimum

capacity is required so that the units are financially feasible.

Estimated minimum served population from incineration facilities is

100,000 inhabitants (around 50,000 tones of waste annually).

Gasification and pyrolysis can be applied for much lower waste

quantities (around 15,000 tones of waste per year)

Need for specialized personnel.

2.2. INCINERATION

2.2.1. General

Incineration, which is commonly referred as combustion, is the oxidization of the

chemical compounds with oxygen (O2) in order to transform the chemical energy

of solid waste organic matter into thermal energy. The incineration of carbon-

based materials can be implemented in an oxygen-rich environment (greater than

stoichiometric), typically at temperatures higher than 850oC. The incineration of

waste is one of the oldest thermal treatment technologies and the most commonly

used worldwide.


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The process produces a waste gas comprised primarily of carbon dioxide (CO2)

and water gas (H2O). Air emissions also include nitrogen oxides, sulphur dioxide,

etc. The most important factor during the process is the presence of oxygen.

During the full combustion there is oxygen in excess and, consequently, the

stoichiometric coefficient of oxygen in the combustion reaction is higher than the

value “1”. In theory, if the coefficient is equal to “1”, no carbon monoxide (CO) is

produced and the average gas temperature is 1,200°C. The reactions that are

then taking place are:

C + O2 CO2 + 393.77J

CxHy + (x+ y/4) O2 xCO2 + y/2 H2O

In the case of lack of oxygen, the reactions are characterized as incomplete

combustion ones, where the produced CO2 reacts with C that has not been

consumed yet and is converted to CO at higher temperatures.

C + CO2 +172.58J 2CO (3)

The scope of this thermal treatment method is the reduction of the volume of the

treated waste with simultaneous utilization of the contained energy. The recovered

energy can be used for:

• heating

• Steam production

• Electric energy production

In order to achieve the complete incineration of solid waste, a number of

preconditions have to be satisfied. These include the following:

• adequate fuel material and oxidation means at the combustion heart

• achievable of the ignition temperature

• suitable mixture proportion

• continuous removal of the gases that are produced during combustion

• continuous removal of the combustion residues

• maintenance of suitable temperature within the furnace


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• turbulent flow of gases

• adequate residence time of waste at the combustion area (Gidarakos,

2006).

The method could be applied for the treatment of mixed solid waste, as well as for

the treatment of pre-selected waste. It can reduce the volume of the municipal

solid waste treated by 90% and its weight by 75%. The incineration technology is

viable for the thermal treatment of high quantities of solid waste (more than

100,000 tn/year).

Figure 21: Incineration process flow chart


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Indicative incineration facilities operating at European level can be seen in the

photos below.

Picture 1: MSW incineration plant in Amsterdam

Picture 2: MSW incineration plant in Brescia

Picture 3: MSW incineration plant in Vienna


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Picture 4: Incineration Plant in Zorbau, Germany for municipal solid waste and

industrial waste

Picture 5: Incineration plant in Thun, Switzerland for

municipal solid waste and dewatered sludge

Figure 22: Diagrammatic configuration of incineration plant (with energy

recovery) in Paris

2.2.2. Types of incinerators

There are two main types of incinerators (combustion units). The facilities which

need minimum pre-treatment of waste before they are placed for the incineration


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process (mass-fired) and the facilities which operate with specific waste fractions

that are derived from pre-treatment of municipal solid waste (Refused-Derived

Fuel (RDF) and Solid Recovered Fuel (SRF).

Mass-burn incineration (Figure 32) is currently the most widely deployed thermal

treatment option, with almost 90% of incinerated waste being processed through

such facilities. As the name implies, waste is combusted with little or no sorting or

other pre-treatment.

Figure 23: Typical mass-fired waste incineration plant

(with energy production)

There are, of course, many dangers that this process may face, such as the

introduction of dangerous and high-volume objects. These dangers, though, can

be handled with careful monitoring of the whole process by the facility personnel or

by interrupting the process manually when/if needed. The energy contained in

waste also depends on the period of the year, climate and waste composition.


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The second type of facilities described above are those which use a mixture of

specific waste fractions from the whole municipal solid waste mixture that derive

from the pre-treatment of waste in MBT facilities (MBT: mechanical biological

treatment) and is comprised of materials, such as: organics, paper, textiles,

leather, rubber materials, etc. The facilities under this type are less (in terms of

number worldwide) than the mass-fired facilities because of the fact that a pre-

treatment facility for the production of (RDF/SRF) is needed.

Their advantages compared to the mass-fired units are:

1. Faster boiler response than mass burn.

2. Units can be designed for cofiring.

3. Higher thermal boiler efficiency because of the lower excess air

requirements.

4. Boiler and overall plant costs are generally lower than those referring to

mass burn units.

5. Potential for sewage sludge disposal.

6. RDF-fired units are easier in use

7. Less space is needed for their installation

8. Finally, the pre-treatment of municipal solid waste gives the chance to

remove materials such as PVC and metals that contribute to the production

of dangerous gases which are transferred along with the gases produced

from the combustion unit.


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Figure 24: Typical RDF-fired incineration facility

These facilities have the following disadvantages:

1. The need to build, own, and operate a prepared fuel system is required.

2. Because of the required processing facility, the overall facility horsepower

requirements may be higher than those for mass bum units.

3. Depending on the process, some combustible material may be lost in

processing.

The process target is the production of a final mixture with high calorific value. This

is why there are quality specifications which the produced RDF must come in line

with. More specifically:

The minimum of the calorific value should be equal to 4,000 kcal/g (16.744

MJ/kg)

The moisture content should not exceed 20%

The paper and plastic content should exceed 95% (dry weight).


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The process in this type of facilities takes place in special combustion chambers

with total capacity between 8 and 25 t/h (Vehlow 2006).

In Germany the total waste capacity of the first category was 21.5 million tons of

waste in 2009 in addition to the 3.3 million tons of waste of the second category. In

terms of economy the municipal solid waste incinerators are better in terms of

mean energy values (as today), while the second type of incinerators provide

better economic results in the case of high energy values when the pre-treatment

cost is relatively small (Fiege & Fendel 2010).

The incineration facilities represent the main municipal solid waste thermal

treatment method used today. In these units the lignite and cement industries in

which specific waste fractions are used are also included (Fiege & Fendel 2010).

In Table 18 the operational incineration facilities in the United States of America

can be seen:

Table 18: Operation incineration facilities in the USA

Type Number of Units Capacity,(tons/day) Capacity Million

(tons/year)

Mass-fired 65 78,489 24.3

RDF Incineration 15 22,022 6.9

The total population that the incineration facilities serve is estimated to 31 million.

This estimation is based on the assumption that each inhabitant generates 1.3

tons of municipal solid waste on annual basis.

Energy and environmental advantages of waste incineration By using 1 ton of municipal solid waste in a modern incineration facility

approximately 550 kWh are produced, while the use of at least 250 kg of carbon or

the use of 160 liters of oil is prevented. Furthermore, this technology is the only

solution against landfilling of non-recyclable waste, since landfilling produces CH4

which is an important greenhouse gas, where the 40% of methane produced is

being released to the atmosphere even in modern landfills. The methane that


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escapes has a dynamic (GHG - Greenhouse Gas Potential) 23 times greater than

that of the same CO2 volume.

Considering the electricity produced and the CH4 emissions minimization (due to

the minimization of waste being landfilled), the incineration facilities help the

reduction of «Greenhouse gases» by 1.1 to 1.3 tons CO2 per ton of municipal solid

waste being incinerated instead of being disposed at landfills.

Besides the energy advantages that this treatment method provides, thermal

treatment contributes to the minimization of greenhouse gases. This reduction for

just the case of the USA is estimated to 40 million tons CO2.

In 2004 the incineration facilities produced 13.5x109 kWh of electricity, an energy

amount which was more than any other existing source of renewable energy

(besides hydroelectric and geothermal facilities). For instance, wind energy

produced 5.3x109 kWh, while solar energy produced 0,87x109 kWh (Table 19).

Table 19: Electrical Energy production from Renewable Energy Sources in USA in 2002 (except hydroelectric) (DOE-EIA, Annual Energy Outlook 2002)

Energy Source Production in 109 kWh % Energy from RES

Earth (Geothermic) 13.52 28.0%

Waste* 13.50 28.0%

Biogas* 6.65 13.8%

Wood/Biomass 8.37 17.4%

Sun (Heat) 0.87 1.8%

Sun (Photovoltaic) 0.01 0.0%

Wind 5.3 11.0%

Total 48.22 100.0%

* http://www.eia.doe.gov/cneaf/solar.renewables/page/mswaste/msw.html

At European level incineration facilities provided the electricity network with 19 x

109 KWh in 2007, amount of energy which is capable of providing the necessary

electricity for the operation of 148 million lambs (15W) for a whole year. If these

http://www.eia.doe.gov/cneaf/solar.renewables/page/mswaste/msw.html


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lambs were set in a line, they would cover the distance from Brussels to Honolulu

(11,800 km).

There are various types of incinerators with various advantages and

disadvantages. The most widely used are: The moving grate, the fixed grate,

rotary-kiln, fluidized bed, etc.

(a) (b) (c)

Figure 25: Three types of incinerators: (a) fixed grate (left), (b) rotary kiln (middle), (c) fluidized bed (right) (Finbioenergy, 2006)

Moving grate incinerator

The grates are placed to the combustion chamber wall and they implement the

following operations:

Movement of the solid waste stream throughout the facility

Provision of air amount at quite steady rate

Material stirring at the main combustion zone

Transfer of the ash produced during the combustion process

The grates must be covered with materials with high tolerance to mechanical

movements, thermal and chemical reactions. Emphasis must be given to the

materials tolerance to S and Cl which are corrosive when combined with high

temperatures. The stages of the main process are:


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Drying: evaporation of the moisture contained within waste with the use of

fire, heated air, eradiation

Vaporization of most of the VOCs (volatile organic compounds) with

temperature rise.

Combustion: The heat needed is provided with the use of irradiation from

the flame and the flame chamber wall.

Gasification and burning: Because of the waste combustion, a great

number of substances turn into gas. The remaining C is fully oxidized, while

the gases from the combustion and gasification process are burnt.

Combustion completion: the end solid product is obtained at the end of the

grate.

Modern incinerators consist of turbines for the hot combustion gases that pass

through the heat exchange sections of the combustion chamber, to be turned into

electric energy or heat.

Rotary kiln Incinerators

This type of incinerator processes a variety of waste streams that other

technologies cannot. This design of incinerator has two chambers: a primary

chamber and a secondary chamber. The primary chamber in a rotary kiln

incinerator consists of an inclined refractory lined cylindrical tube. The movement

of the cylinder on its axis facilitates the movement of waste. In the primary

chamber, there is conversion of the solid fraction to gases, through volatilization,

destructive distillation and partial combustion reactions. The secondary chamber is

necessary to complete gas phase combustion reactions. The unit consists of a

system of gaseous emissions control.

The clinkers spill out at the end of the cylinder. A tall flue gas stack, fan, or steam

jet supplies the needed draft. Ash drops through the grate, but many particles are

carried along with the hot gases. The particles and any combustible gases may be

combusted in an "afterburner".

The chamber has to be covered by materials with tolerance to high temperatures,

while a continuous flow of waste is necessary. The temperature inside the


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chamber is between 800 and 1,400 C, while the effective combustion is achieved

through absolute control of the temperature and the waste movement inside the

facility. Generally, the higher the temperature, less time is needed for the waste to

remain in the combustion chamber.

Because of the fact that the gases produced inside the chamber remain for a short

period in order to achieve full combustion, a chamber for afterburning is placed.

The residues of the chamber are then led to the cooling system.

The agitation of waste inside the chamber depends on the time remaining,

because this is where the agitation takes place. The gases must be low in CO and

H/C considering that the chamber operates with excess of O2 (Gidarakos 2006).

The main parameters of this type of incinerators that should be considered

include:

The temperature of the rotary kiln which leads to the combustion of waste

The internal pressure of the chamber that must be negative in order to

avoid gaseous emissions and particles to the atmosphere

The provision rate for O2 and the waste flow rate so that the process

conditions are suitable.

Fluidized bed Incinerators

A strong airflow is forced through a sand bed. The air seeps through the sand until

a point is reached where the sand particles separate to let the air through and

mixing and churning occurs, thus a fluidized bed is created and fuel and waste can

now be introduced.

The sand with the pre-treated waste and/or fuel is kept suspended on pumped air

currents and takes on a fluid-like character. The bed is thereby violently mixed and

agitated keeping small inert particles and air in a fluid-like state. This allows all of

the mass of waste, fuel and sand to be fully circulated through the furnace.

Evaporation takes place due to the O2 provided, the mixing and the high

temperature.


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Temperature is the main operation parameter for this type of incinerators. It

depends on the waste treated, the gases produced, while the temperature varies

from 750°C to 850°C.

The O2 needed and the retention time are the most important parameters inside

the chamber. The determination of these parameters depends on the waste

provided for processing. The O2 concentration is controlled in order to achieve a

perfect combustion. With this incinerator, temperature variations inside the

chamber can be avoided and, as a result, the production of gases due to

incomplete combustion can also be avoided.

Fuels rich in ash and moisture can also be used for the production of energy. The

rate that fuel is turned to energy becomes even higher and, thus, the need for air

becomes less (55% compared to the common 100%) (Yassin et al. 2009).

Typical incineration facility

A typical incineration facility includes the following parts:

A Weighing System

This system is for weighing solid waste for the better control and recording of the

incoming waste streams and, thus, it is designed to be practical in order to

minimize the time that vehicles remain at this point.

A Reception Site

Due to the fact that waste does not arrive on continuous basis (contrary to the

feeding of the facility), the existence of waste reception and temporary storage site

is considered essential. The design of the site is made in a way that the following

are ensured:

• The unloading time is as little as possible

• All transferred waste is received


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• The homogeneity of the waste that will be used as feeding material is

achieved

• The smooth feeding of the facility is ensured.

Moreover, the design of the reception site should be based on the minimization of

the environmental impact. For instance, the solid waste should remain for a

maximum of two days in order to avoid the relevant odors, while the bottom of the

site has to be characterized by weathering to allow the leachates and washing

wastewater to go away.

Feeding System

The feeding system has to be adapted to the feeding rate and velocity of the

installation.

Combustion Hearths

The ignition of solid waste at incineration facilities is achieved through the use of

specific burner, which operates with secondary fuel. Basic parameters for the

appropriate operation of the combustion hearths are:

• Achievement of the minimum desired temperature

• Adequate combustion time

• Achievement of turbulence conditions / homogenous waste incineration.

Boiler

The boiler is the system with which the energy content of the fuel material (hot off-

gases) can be utilized in a suitable way through steam production (e.g. at

neighboring industrial facilities or for the heating of the neighboring urban areas.

Pressure, temperature and steam production rate are basic parameters for the

effective operation of the boiler. Its construction has external insulation in order for

the system not to lose temperature during the process. The materials used for its

construction must also be tolerant to high temperatures (and temperature

differences) between the inside and the outside of the facility.


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System for the removal of residues

Residues represent 20 - 40% of the weight of the initial waste and are categorized

as:

• Residues that go out of the grates: 20 - 35%

• Residues that go through the grates: 1 - 2%.

The residues are collected at hoppers where they are transferred with the use of

specific cooling system.

Emission control system

The role of the emission system control focuses on particles, HCl, HF, SO2,

dioxins and heavy metals and is discussed below (Niessen, 2002). After the

emissions pass through the boiler, the gaseous emissions pass through a cleaning

facility and, then, they are emitted to the atmosphere. In the cleaning systems a

large number of different technologies for the removal of flying particles, NOx,

x, etc. that are considered to be safe and secure can be applied.

The incineration process can be presented with a mass balance diagram for a

typical incineration facility. The waste streams percentages depend on the

composition of the incoming waste and the emissions control system that is used.

In particular for the production of energy from waste incineration it is estimated

that 1 ton of waste produces approximately 300kWh electricity and 600kWh of

thermal energy.

Air emissions, wastewater and solid waste are the result of the process of waste

incineration. A detailed analysis of the composition and properties is provided

below.


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2.2.3. Air emissions

The gases produced by incineration facilities contain N2 and excess O2, dust

particles, the typical products of combustion and other harmful substances,

depending on the composition of the incoming waste. The main ones are: SO2,

NOx HCl, HF, heavy metals and polycyclic H/C, which are the most dangerous

pollutants in the exhaust gases, such as dioxins and furans.

An average of 4,000 – 5,000 m³ per tonne of waste gas with a temperature of

1,000°C are generated from the incineration, which in the first phase of cleaning of

the produced gas drops sharply to 350°C.

The limit values of air emissions are listed in Tables 20, 21, 22 and 23.

Table 20: Daily average values of air emission limit values (Directive 2000/76/EC on the incineration of waste)

Total Dust 10 mg/m3

Gaseous and vaporous organic substances, expressed as

TOC 10 mg/m3

HCl 10 mg/m3

HF 1 mg/m3

SO2 50 mg/m3

NO & NO2, expressed as NO2, for existing incineration

plants with a nominal capacity exceeding 6 tonnes /hour or

new incineration plants

200 mg/m3

NO & NO2, expressed as NO2, for existing incineration

plants with a nominal capacity 400 mg/m3


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Table 21: Half-hourly average values of air emission limit values (Directive 2000/76/EC on the incineration of waste)

(100 %) (97 %)

Total Dust 30 mg/m3 10 mg/m3

Gaseous and vaporous organic substances,

expressed as TOC

20 mg/m3

10 mg/m3

HCl 60 mg/m3 10 mg/m3

HF 4 mg/m3 2 mg/m3

SO2 200 mg/m3 50 mg/m3

NO & NO2, expressed as NO2, for existing

incineration plants with a nominal capacity

exceeding 6 tonnes /hour or new incineration

plants

400 mg/m3 200 mg/m3

Table 22: Average values over the sample period of a minimum of 30 minutes and a maximum of 8 hours

(Directive 2000/76/EC on the incineration of waste)

Cadmium and its compounds (Cd) total

0,05 mg/m3 Thallium and its compounds (Tl)

Mercury and its compounds (Hg) 0,05 mg/m3

Antimony and its compounds (Sb)

Total

0,5 mg/m3

Arsenic and its compounds (As)

Lead and its compounds (Pb)

Chromium and its compounds (Cr)


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Cobalt and its compounds (Co)

Copper and its compounds (Cu)

Manganese and its compounds (Mn)

Nickel and its compounds (Ni)

Vanadium and its compounds (V)

Dioxins and Furans 0,1 ng/m3

Table 23: Limit values of CO concentrations (Directive 2000/76/EC on the incineration of waste)

Daily average value 50 mg/m3 of combustion gas

at least 95 % of all measurements

determined as 10-minute average

values

150 mg/m3 of combustion gas

all measurements determined as half-

hourly average values taken in any

24-hour period

100 mg/m3 of combustion gas

The concentration of CO in combustion gases (excluding the start and stop) does

not exceed the above limit values.

The competent authority may grant exemptions for incineration plants using

fluidised bed technology, provided that the authorized emission limit value for CO

equals to 100 mg/m3 hourly maximum.

Then, further reference is made to dioxins and furans, which are among the most

dangerous pollutants because characterized by high toxicity (Allsopp et al. 2001).


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Dioxins: They include two aromatic rings joined with a pair of individuals O.

The toxic effects of dioxins (Yang & Kim 2004) and furans had not been

understood worldwide until the late 80's. With the implementation of MACT

regulations, a "toxic equivalent» (TEQ-Toxic equivalent) of dioxin emissions from

Waste-to-Energy Facilities in the U.S.A. has decreased since 1987 1,000 times in

a value of less than 10 gr TEQ per year (Figure 35). On the other side and in direct

relation to the above, the main source of dioxins, as recorded by US EPA (Figure

36) is the uncontrolled burning of waste, which emits about 600 gr per year.

Figure 26: Dioxin emissions in USA (Themelis & Gregory 2002)

Figure 27: Dioxins emission in the USA (Deriziotis, 2004).


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In Germany in 1990 1/3 of dioxin emissions originated from incinerators, and in

2000 the relevant figure was less than 1% although the amount of MSW

incinerated more than doubled in relation to 1990. Overall the level of dioxins from

400gr in 1990 was limited to less than 0.5 gr in 2000.

Furans: They differ from the dioxins only in that the two aromatic rings are joined

by an atom of O.

A total of 75 compounds known as polychlorinated dibenzo-p-dioxins (PCDD) and

135 dibenzofurans (PCDF) are classified in this group of compounds.

Reference to exposure to dioxins is meant exposure to a mixture of PCDD and

PCDF, the toxicity of which is determined by the toxic equivalency factors (Toxic

Equivalent Factors, 1 - TEF), which are calculated in relation to the toxic effects of

2,3,7,8 TCDD, also known as the Seveso poison.

Dioxins and furans are produced in almost all processes of combustion in the gas

phase, while the exact mechanism of their formation remains unknown. It is known

that a temperature of formation is 300°C, in which two reactions are possible, the

formation and decomposition. The presence of chlorinated organic compounds in

the waste to be incinerated and the increased levels of O encourage their creation.

The operating conditions of the incinerator affect decisively the creation of dioxins

and not the composition of the waste and the quantity of PVC contained in them.

They can affect humans through respiration or absorption through the skin, in case

they are released to close proximity to the recipients. In other cases their

introduction in the body is caused by consumption of food and particularly fruit and

vegetables.

There is evidence for the contribution of dioxins and furans to processes of

carcinogenicity in humans, making it necessary to take measures to reduce their

concentrations in the air emissions.


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Primary measures

- Improvement of the incineration of waste and suspended particles (products of

incomplete combustion).

- Optimization of the requirement of O2.

- Improvement of thermal control systems to ensure control of the combustion air

- The use of improved grates.

- Adaptation of appropriate systems of grates to changes in the composition of

the waste (e.g. calorific value).

- Control of the temperature of crossing through the filter at a level lower than

that of the formation of dioxins (200°C).

Secondary Measures

- Improvement of the cleaning of steam boilers (continuous cleaning).

- Preliminary collection of particulate phase before cooling (high temperature

removal of particulate matter).

- Interference in the temperature of the electrostatic filter to reduce the formation

of dioxins.

- Improvement of the systems for the purification of gases by improving the

collection of particulate matter and pollutants.

- Removal of PPDD / PCDF by adsorption of active C.

Gas cleaning systems

The existing technologies on the management of air pollutants are summarized in

Table 24.


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Table 24: Existing technologies for the management and treatment of gaseous pollutants

Pollutant Abatement Technology

Suspended Solids

Cyclones

Electrostatic filters

(Wet - dry)

Bag Filters

Acid Gases

Dry Adsorption

Semi-Dry Adsorption

Wet Sparying

Nitrogen Oxides Selective non-catalytic reduction

Selective Catalytic Reduction

In order to achieve the removal of particulate and gaseous pollutants different

methods of cleaning are employed. These include deposition chambers, which

remove 40% of airborne particles, wetting screens (efficiency 95%), cyclones

(efficiency 60-80%), fluid absorption towers (efficiency 80-95%), electrostatic

precipitator (efficiency 99-99.5%) and bag filters (efficiency 99.9%).

.

Next, the main systems for determining the composition of the gas produced

during the incineration of waste are described (Figure 37).


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.

Figure 28: Cyclones (left), electrostatic precipitators (middle) & bagfilters (right)

Bag filters: The gases pass through porous materials, where the particulates are

trapped. Depending on the requirements, hardware filters are made of natural

fibers, plastic fibers, glass, minerals, etc. The dust which is concentrated in cells of

the filter is removed by vibration or shock or air in countercurrent (Figure 38).

Figure 29: Typical bagfilters


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Electrostatic precipitator (electric filters): The electrostatic precipitator (Figure 39)

consists of the cathode, which may be a simple thin wire and the anode, i.e. the

inner casing of the electric filter. Another device consists of a system of parallel

plates with a potential difference between them. Between cathode and anode

voltage develops at 30-80 kV. When particles enter the field of the cathode, they

become electrically charged and those negatively charged move towards the

positive pole (anode). The speed of the particles depends on their mass and the

Coulomb forces that are developed.

Figure 30: Typical Electrostatic precipitator

Cyclones: The cyclones are based on the development of centrifugal force in the

entry of gases in a symmetric space, which at the bottom is cone-shaped.

Particles due to centrifugal force and the rotational flow are driven to the wall and,

then, removed to the bottom. The cyclones are often deployed along with

electrostatic precipitators.

Besides the removal of suspended solids, the removal of other pollutants is often

necessary, e.g. acid gas, if their content is higher than the relevant acceptable

limits. Emphasis is given on the HCl, generated mainly from the combustion of

PVC, and oxides of nitrogen, sulfur, phosphorus. The only effective and

appropriate way is in this case the operation of towers of wet and dry absorption

(scrubbing). The liquid absorption towers are necessary in any case for

incinerating toxic and hazardous waste.


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The process of liquid absorption is based on absorption of gaseous pollutants

using a selected washing fluid (solvent). The effectiveness of the process depends

mainly on the available surface of the solvent, which controls the mass transfer

from gas to liquid phase. To this end, various techniques are employed, such as:

• Venture type scrubbers

• filling Towers

• disc Towers

• absorption-type film Towers (thin layer).

Most incinerators in central Europe are using the same technology of liquid

absorption. The process takes place in different units consisting of two phases, an

initial acid absorption phase and a second phase, neutral or slightly alkaline. The

configuration of the acid absorption is often spray or venturi type and in that phase

reduction of the temperature of flue gas from 180-200oC to 63-65oC is achieved.

For the second phase (neutral or slightly alkaline) filling towers are mainly used.

Commercially available absorption tower systems operate with or without

producing waste.

Such two-phase systems are quite effective in removing waste gases from the

incineration of halogen hydrides, HF, HCl, HBr, the Hg and SO2. With this

technology the initial concentrations of these compounds in the waste gases are

reduced well below the statutory limits.

The dry or semi-dry absorption towers (Figure 40) based on simple and low cost

technologies exist and are operational in many facilities in the world. In most

cases, the adsorbent medium is either injected directly into the flue gas duct or

through spray towers in dry or semi-dry form. The products of absorption are

removed, in a second phase through a membrane filter. The process of absorption

can be performed with various reagents (limestone, CaCO3, calcium oxide, CaO,

lime, Ca(OH)2, etc.).


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Figure 40: dry or semi-dry absorption towers (scrubbing)

Today, the technology of absorption towers using dry CaCO3 is gradually

abandoned, as the composition of air emissions produced by the treatment does

not comply with strict statutory limits (Dvorak et el. 2009).

2.2.4. Wastewater

Wastewater is generated by the use of water during the incineration process and

in particular:

• ash quenching (0.1 m3 H2O / t waste)

• gas cooling (2 m3 H2O / t waste)

• liquid absorption towers (2 m3 H2O / t waste)

• In some electrostatic precipitator to remove particulates from the collection

points.

The wastewater contains suspended particles, inorganic and organic in solution.

They are toxic and need treatment before discharge into drains. The most

common methods of treatment are the precipitation and then adjustment of the pH.

Wet gas cleaning system: The liquid comes in contact with the gases where

migration of substances from gases in the liquid phase takes place. The

absorption depends on surface transport, residence time and type of fluid. The

fluid system is developed so that it ensures the removal of ultrafine particles that


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are not easy to remove through the application of dry systems, e.g. filters. The

main wet cleaning systems are:

• flush Towers (scrubber)

• Ventouri scrubbers

• Rotating sprinklers.

The limit values of pollutant parameters for discharging wastewater from the

cleaning of exhaust gases are summarized in Table 25.

Table 25: Emission limit values for discharges of waste water from the cleaning of exhaust gases

(Directive 2000/76/EC on the incineration of waste)

Polluting Substances Emission limit values

expressed in mass

concentrations for unfiltered

samples

Total suspended solids as defined by

Directive 91/271/EC

95% /

30mg/L

100% / 45

mg/L

Mercury and its compounds (Hg) 0.03 mg/L

Cadmium and its compounds (Cd) 0.05 mg/L

Thallium and its compounds (Tl) 0.05 mg/L

Arsenic and its compounds (As) 0.15 mg/L

Lead and its compounds (Pb) 0.2 mg/L

Chromium and its compounds (Cr) 0.5 mg/L

Copper and its compounds (Cu) 0.5 mg/L

Nickel and its compounds (Ni) 0.5 mg/L

Zinc and its compounds (Zn) 1.5 mg/L

Dioxins and furans 0.3 ng/L


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2.2.5. Solid residues

The secondary solid residues that are generated during incineration can be

categorized as follows:

• Fly ash: The ash is composed of the lightest part of the ash, which drifted

from the exhaust and is collected by special filters. The ash has high

concentrations of heavy metals, soluble salts, organic and the higher

content of all residues of chlorinated organic compounds.

• Bottom ash: This is the residue that is collected at the bottom of the

furnace.

• Ash from the boilers

• dust filter cleaning

• solid residues from flue gas purification process (WASTESUM, 2006).

The solid residues stream must be treated before its final disposal, while a main

portion of their quantities could be recycled by applying specific processes.

If the bottom ash is not used, it may be released under the same conditions as

MSW without any problem.

Technologies for the inactivation of fly ash, which is considered hazardous waste,

are in development. Most common is the conversion to material useful for road

construction, structural applications, etc. The use of ash in road construction -

paving is common practice in Europe. The disposal in a landfill must take into

account the leachability of the different components. If a method of inactivation is

not implemented, it should be placed in hazardous waste disposal site.

For the treatment of filters dust various systems are used such as heat (high

temperature). The purpose of working at high temperatures is to melt the filter dust

and transform it into material that is glassy state, which may be allocated to

different uses or placed as inactive.


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In order to ensure the complete control of emissions, sampling and analysis is

required for the determination of the composition of the incoming waste, the

generated solids (residue - fly ash), the produced gas and wastewater generated

during the processing of waste gases.

Quantifying the environmental impact of the implementation of incineration

The method of combustion can cause a variety of environmental impacts taking

into account that there are emissions into the environment as gas, liquid and solid

pollutants. Table 26 summarizes all the amounts of solid waste, wastewater and

air emissions during operation of combustion plants.

Table 26: Summary of quantities of solid waste, wastewater and gases produced during the operation of an incineration plant

Solids (ash, metals, glass, other non-

combustible materials)

25-40% by weight of

waste

Gases: Dust, CO, CO2, H2O, NOx, SO2,

dioxins, furans)

4-5,000m3 gas /

tonne of waste

Wastewater: (suspended particles, organic-

inorganic breakdowns) ~ 4m3 water / tonne waste

The efficiency of the removal of hazardous components of waste treated in

incinerators must be at least 99.99% and is defined as:

DRE = Win - Wout / Win * 100%

Where DRE = efficiency of the incinerator

Win = rate of supply of a particular substance waste

Wout = rate of emission of that substance in the waste gas

The legislation for hazardous waste incinerators does not allow the production of

gas with a concentration of solid particles greater than 180 mg / dscm, for O2

content of 7%. To monitor compliance with that restriction in a variety of


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conditions, current conditions are extrapolated to the conditions of the legislation,

according to the formula below:

Pc = Pm * 14 / (21 - Y)

Where: Pc = corrected concentration of particulate matter (mg / dscm)

Pm = the measured concentration of particulate matter (mg / dscm)

Y = the measured concentration of O2 in the flue gas chimney (%)

The emission factors vary depending on the type of treatment of the produced

gases. Also, heat and electricity are considered to have separate emissions

(Gidarakos 2006, Niessen 2002).


According to a typical mass balance of the incineration process, for one tonne of

input MSW, 200-350 kg is assumed to be bottom ash (10% by volume and

approximately 20 to 35% by weight of the solid waste input), 35-45kg flue gas

cleaning residue including fly ash and 25-30 kg metals.

The rest of the input is converted into energy. The typical amount of net energy

that can be produced per tonne of domestic waste is about 0.7 MWh of electricity

and 2 MWh of district heating.


Produced heat and electricity may be exploited with a reciprocating engine or

microturbine often in a cogeneration arrangement in order to feed the incineration

system. Excess electricity can be sold to suppliers or put into the local grid.

Electricity produced by incineration systems is considered to be renewable energy.

http://en.wikipedia.org/wiki/Incineration#cite_note-autogenerated7-1%23cite_note-autogenerated7-1

http://en.wikipedia.org/wiki/Reciprocating_engine

http://en.wikipedia.org/wiki/Microturbine


http://en.wikipedia.org/wiki/Cogeneration


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The produced bottom ash may be used in construction of roads, embankments

and landfills, in accordance with the local legislation. At present, the ways to use

bottom ash and fly ash as additives in cement plants are under research. The

metals may be used in metal recycling industry.


The incineration of solid waste generates pollutant emissions (gases, ash, dust

and smoke), wastewater, slag and odors. The possible presence in the waste of

Cl, F, S, N and other elements could contribute to toxic or corrosive gases. The

wastewater that originates from the treatment of exhaust gases and quenching of

incinerator ash contains heavy metals and inorganic materials with increased

acidity or alkalinity and temperature, while its disposal is permitted only after

pertinent treatment and compliance with applicable regulations. Dioxins and furans

can be products of incomplete combustion and may be decomposed completely

by pyrolysis. The particulate emissions, acid gases (HCl, HF, SO2) and heavy

metals (Hg, Cd, Pb) are of significant importance. Novel incineration technologies

completely decompose dioxins and furans, neutralize toxicity and stabilize the

residue, which could be used in the construction sector. Released aerosols mainly

consist of ash absorbing other toxic pollutants; these reduce atmospheric visibility,

resulting in public complains, and minimization of aerosol emissions is a must.

If electricity and heat generated from incineration is used, the waste replaces

natural resources used for conventional production of energy. The production of

energy from renewable sources has positive consequences on nearly all

environmental impact categories, because of savings in or compensation for non-

renewable energy.


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According to the Carbon Finance Unit of the World Bank in 2008, the capital cost

of an incineration system with a capacity of 1,300 tonnes per day is 20-120 €/tone,

while the operation and maintenance cost is 55-80 €/tonne. For the case of

Romania, the Neamt Master Plan of the year 2008 considers that an average

value for the application of the relevant thermal technology is 120-140 €/t for a

capacity of 150,000 t/a. According to EU data the financial cost of incineration,

including the annualized capital, O&M expenditures as well as other specific costs

or revenues from sale of the energy, ranges between 58 to 104 €/tonne (EC,

2010).

2.2.10. Applicability in the target area

In general, thermal management practices are characterized by higher cost in

relation to other management solutions. This together with the existing economic

crisis can be a reason why Romania, Bulgaria and Greece have not yet decided to

apply any thermal waste management method exclusively for municipal waste.

Furthermore, in many cases the public opinion is not really ready to accept the

option of incineration being afraid mainly for the air emissions, but it is considered

that these fears and skepticisms will be gradually overcome. On the other hand, it

has to be noted that incineration is a well tested method which is widely applied in

all advanced in waste management issues European countries that at the same

time achieve high recycling rates. Furthermore, the modern incineration plants can

be located even at the centre of big cities like in Paris, Vienna, etc. provided that

their operation is monitored as required by the relevant Directive on the

incineration of waste (2000/76/EC). It is also important to ensure that incineration

units have high energy performance so that the relevant treatment can be

considered as recovery and not disposal. Finally, energy production through

incineration plants can contribute to reducing CO2 emissions.

http://en.wikipedia.org/wiki/Euro_sign



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Most specifically, in Romania the incineration practices are already applied but

there are still no incineration facilities just for treating municipal solid waste. The

relevant data for incineration units in Romania are summarized in Table 27.

Table 27: Synopsis on data on incineration units in Romania No. Name and address

of facility Capacity (tones/year)

Cost Waste streams incinerated

Public opinion

1. OLTCHIM- VALCEA

county

11.445 tones

/year. Co-

incineration of

the own waste

generated

It was

accepted by

the

population.

2. S.C.STEMAR

S.R.L.Vaslui -

Vaslui

966 tones

/year. Co-

incineration of

the own waste

generated

It was

accepted by

the

population.

3. VRANCART S.A.-

Adjud, Vrancea

County

17.199 tones

/year. Co-

incineration of

the own waste

generated

It was

accepted by

the

population.

At national level, the total capacity of Co-incineration of the own waste generated is of 29 610 tones/year.

4. CARPATCEMENT HOLDING SA – Sucursala Bicaz

419 432

tones/year

Co-

incineration

in cement

kilns

It was

accepted by

the

population.

5. SC LAFARGE

ROMCIM SA -

MEDGIDIA,

Constanta County,

203 000

tones/year

Co-

incineration

in cement

kilns

It was

accepted by

the

population.

6. SC REPA does It was


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CARPATCEMENT

HOLDING SA –

Bucuresti- Fieni,

Dambovita County

not have

information

about capacity

accepted by

the

population.

7. HOLCIM

(ROMANIA) SA

CIMENT –

CAMPULUNG,

Arges County

26 208

tones/year

Co-

incineration

in cement

kilns

It was

accepted by

the

population.

8. SC

CARPATCEMENT

HOLDING SA

BUCURESTI -

Deva- Cement

Factory

Chiscadaga,

Hunedoara County

131.400

tones/year

Co-

incineration

in cement

kilns

It was

accepted by

the

population.

9. Holcim (Romania)

SA - Ciment Alesd,

Bihor County

77.000

tones/year Co-

incineration

in cement

kilns

It was

accepted by

the

population.

10. LAFARGE CIMENT

(ROMANIA) S.A.

BUCURESTI –

Hoghiz, Brasov

County

20. 788

tones/year

Co-

incineration

in cement

kilns

It was

accepted by

the

population.

Total authorized capacity of co-incineration in Romania is of 907,438 tones/year.

Incinerators

11. SC

CHIMCOMPLEX SA

Borzesti –- Bacau

County

680 tones/year incineration

of their own

waste

It was

accepted by

the

population.

12. SC ANTIBIOTICE 432 tones/year incineration It was


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SA IASI - Iasi

County

of their own

waste

accepted by

the

population.

13. S.C.KOBER SRL –

Turturesti- Neamt

County

1.248

tones/year incineration

of their own

waste

It was

accepted by

the

population.

14. COMPANIA

NATIONALA,,

IMPRIMERIA

NATIONALA"

SA,BUCURESTI-

Bucuresti


of their own

waste

It was

accepted by

the

population.

15. SC CHIMESTER

BV.SA

Bucuresti


of their own

waste

It was

accepted by

the

population.

At national level, the total authorized capacity of incineration of facilities’ own

waste generated is 2,514 tonnes/year.

Incinerators for hazardous contaminated packaging waste

16. S.C. MONDECO

SRL, Suceava-

Suceava County

10.800 tones/year

hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

17. SC PROD IMPORT

CDC SRL ALTAN

TEPE,

COM.Stejaru-

TULCEA County

1.500

tones/year hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

18. Compania 2.628 hazardous It was


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Nationala

Administratia

Porturilor Maritime

Constanta SA -

Constanta County

tones/year medical

waste and

hazardous

industrial

waste

accepted by

the

population.

19. SC PRO AIR

CLEAN SA,

TIMISOARA-

Timisoara County

3.577 tones/year

hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

20. SC IF TEHNOLOGII

SRL CLUJ- Cluj

County

1.430


medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

21. SC IRIDEX GROUP

IMPORT EXPORT

SRL- Bucuresti

6.000 tones/year

hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

22. S.C. AVAND S.R.L.,

Iasi- Iasi County

11.300


medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

23. SC SUPERSTAR

COM SRL-

SUCEAVA County

2.010 tones/year

hazardous

medical

waste and

hazardous

It was

accepted by

the

population.


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industrial

waste

24. S.C. ECO FIRE

SISTEMS SRL-

CONSTANTA

County

10.080 tones/year

hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

25. SC GUARDIAN

SRL- Dolj County

4.620 tones/year

hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

26. ENVISAN NV

BELGIA-

SUSCURSALA

PITESTI- Arges

County

93.312 tones/year

hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

27. SC ECOBURN

SRL- PLOIESTI-

Prahova County

4.000 tones/year

hazardous

medical

waste and

hazardous

industrial

waste

It was

accepted by

the

population.

The Romanian total authorized capacity of waste incineration is 156,789

tonnes/year.

In Greece, no incineration techniques have been applied for the case of municipal

solid waste. In fact, there is only one incineration facility which operates in the

Attica Region in order to treat hospital waste. It should also be noted that less

hospital waste treated than its capacity allows. Referring to municipal solid waste,


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it has to be noted that the academic society organizes events in order to deal with

the skepticisms about the application of incineration (www.wtert.gr). Furthermore,

some peripheral waste management plans foresee the construction and operation

of incineration units, but have not yet been implemented. For instance, the

relevant scheme for Crete Region foresees the operation of an incineration unit

with capacity to treat 170,000 tonnes of treated municipal solid waste (SRF and

RDF). There are also supporters of the incineration option for the case of Attica

Region who have organized events in order to explain the reasons for doing so

and the relevant benefits.

In Bulgaria, there is total absence of thermal treatment facilities and no significant

progress is anticipated in the years to come due to the unwillingness to adopt such

practices yet, as well as the economic crisis. Unfortunately, landfilling is expected

to continue to be the main treatment method used for the management of

municipal solid waste.

In Slovenia there is only one incinerator for municipal solid waste. Furthermore,

there were two cement kilns having environmental permit. Within 2011 one cement

kiln lost the environemental permit for co-incineration of municipal solid waste, so

now there is only one cement kiln with the environmental permit for co-

incineration. Regarding the incinerator for municipal solid waste, there can be

incinerated only treated municipal solid waste. The capacity for the incinerator is

25.000t of treated municipal solid waste. It can be said that the public opinion does

not accept the installation and operation of incineration facilities until now.

http://www.wtert.gr/


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2.3. GASIFICATION

2.3.1. General

Gasification is a process of incomplete combustion of solid waste (under

stoichiometric conditions regulating the supply of oxidant). A variety of processes

take place, while the gas is formed at temperatures above 700°C and is rich in H2,

CH4, H2O, N2, CO, CO2 and small amounts of high H/Cs. The purpose of this

method is the maximum release of CO and H2. The mixture of CO and H2 is known

as synthesis gas (or syngas).

It is theoretically the next stage of pyrolysis. At this stage the residual coke is

oxidized at high temperatures (> 800oC). As a gasification agent steam, CO2, O2 or

air are used.

The main reactions taking place during the process of gasification are:

(1) Oxidation (exothermic)

C + O2 CO2

(2) Reaction of water evaporation (endothermic)

C + H2O CO + H2

(3) CO + H2O CO2 + H2 (exothermic)

(4) Boudouard Reaction (endothermic)

C + CO2 2CO

(5) Reaction of formation of CH4 (exothermic)

C + 2H2 CH4

The heat to keep the process going derives from the exothermic reactions, while

the combustion products are mainly produced by the endothermic reactions.

It is likely that other reactions take place at low temperatures where with the

addition of H2O CO2 is formed and at higher temperatures CO.


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The gasification reaction rate depends on temperature, porosity, internal structure

of the fuel source and diameter of the pores. Specifically, untreated waste is

harder to gasify than that from cracking. Similarly, the loose material is brittle

compared to the coherent solid material. Solid materials readily allow the passage

of air through the plating reactor.

Figure 31: Gasification process flow chart

The difference in the gasification of pyrolysis is that during gasification additional

fuel gas is fed for further conversion of organic residues into gaseous products

(Figure 42) (Gidarakos 2006, Girods et al. 2009, Klein 2002).


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Figure 32: Process for converting waste into energy (Vasudevan & Mathew 2007)

Syngas generally has a heating value of 250-300 Btu/scf, compared to natural gas

at approximately 1,000 BTU/scf. Typically, 70–85% of the carbon in the feedstock

is converted into the syngas. The ratio of carbon monoxide to hydrogen depends

in part upon the hydrogen and carbon content of the feedstock and the type of

gasifier used.

Typical gasification plant

The main types of gasification facilities are:

• Vertical fixed bed

• Horizontal fixed bed

• Fluidized bed

• Multiple foci

• Rotating furnace.

From all five of these types of facilities (Rezaiyan & Cheremisinoff 2000), the

vertical fixed bed facilities (Figure 43), the horizontal fixed bed (Figure 44) (Dalai et

al. 2009) and the fluidized bed (Groi et al. 2008) are more widespread.


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The vertical fixed bed plants have advantages, such as simplicity and low

investment costs, but are directly affected by fluctuations in the composition of

incoming waste (it is preferred to be homogeneous, e.g. the RDF in concentrated

form - pellets). The gas product of the plant is of low calorific value and

simultaneously small quantities of liquid and important quantities of solid products

are produced.

Based on the results of pilot applications for units operating at 650 - 820oC, it has

been proved that:

• The resulting solids have great adsorptive capacity and can be used in

tertiary treatment facilities and sewage water.

• The gas product can be used as fuel in engines burning oil at a ratio of 4:1,

the performance of the machine can reach 76% of the performance that it

would have if there was exclusive use of oil.

The gases resulting from the treatment of the gaseous product (high performance

cyclones) are comparable in composition to the gases produced by incineration

and in some cases contain less polluting load (Bebar et al. 2005).

Figure 43: Vertical steady bed gasification plants

Regarding facilities of horizontal fixed bed, they are the type widely used in


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commerce. The facility consists of two parts: (a) the main gasification chamber

and (b) the combustion chamber. The first stage carries the process of gasification

and gas produced is burned completely in the second chamber with excess air at

650 - 900°C. The exhaust gases of the combustion chamber are products through

complete combustion that have temperatures ranging from 650oC - 900oC and can

be exploited through the recovery of energy contained in them. The exhaust gases

are driven through heat recovery to produce steam or hot H2O. The low velocity

and turbulence in the first chamber minimize the entry of particles into the gas

stream and lead to lower particulate emissions than conventional combustion

chambers. Such units are commercially available from different manufacturers in

standard sizes capacity 200 – 1,700 kg / h.

Figure 44: Horizontal steady bed gasification plants

Finally, the fluidized bed plants are still at pilot level. With minor modifications, the

fluidized bed combustion with excess air can act as a gasification plant fluidized

bed with air flow below the stoichiometric ratio.

But other than the horizontal bed units, the other systems have not been

developed at full-scale and additional research is required towards this direction.

The produced gas can be utilized in various ways, including:


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Combustion to produce steam. The advantages compared to incineration, is

that the gases are cleaned before combustion, thus enabling operation at

higher pressure boiler and superheater of steam at higher temperatures, to

achieve improved performance and electricity, which can approach 30%.

Power internal combustion engine which drives electrical generator. The

electrical energy can exceed 40% but requires a very thorough cleaning of

gas before feeding the machine.

Movement of Steam turbine and combined cycle. And this method, which

also requires a good cleaning before the gas supply, can result in yields of

40% in electricity.

Feeding into the city gas network. Requires good cleaning and stable

quality.

Provision of gas to the industry, such as cement for direct combustion in

burner. In this case a significantly reduced cleaning is required.

Supply of the gas to an industry where it is used for Steam generation. The

cleaning requirements are a function of boiler operating conditions (Ahmed

& Gupta 2009, Belgiorno et al. 2003, Bjorklunda et al. 2001, Brothier et al.

2007, Ganana et al. 2006, He et al. 2009).

An indicative waste gasification plant is shown in Picture 6.

Picture 6: MSW gasification plant in Chiba (Japan)


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Emissions from gasification units

The end products of gasification are:

• gas rich in CO, H2, CO2 and saturated H / C (mainly CH4) that can be used

as fuel (Scheidl et al. 1991)

• Solid waste material consisting of C and aggregates.

Table 28 summarizes all types of solid waste, wastewater and off-gases generated

during the operation of a gasification unit.

Table 28: Summary of solid waste, wastewater and air emissions generated during the operation of a gasification unit

Solids pure C embedded in various inert materials

Gases CO, H2, saturated H / C

The gasification plant can operate with either supply of air or supply of pure O2. In

the case that there is supply of air, because of the presence of atmospheric

nitrogen, the calorific value of the gas product is relatively low and is around 5.6

MJ/m3. A typical composition is: 10% CO2, 20% CO, 15% H2, 2% CH4, 53% N2.

If the supply is pure O2, the standard composition is: 14% CO2, 50% CO, 30% H2,

4% CH4, 1% CxHy, 1% N2 and energy content between 10 and 11.2 MJ / m3.

Based on the principle of the gasification processes (this also accounts for the

case of pyrolysis) there are limited air emissions comparing with the

implementation of the incineration process due to the less air used (US

Department of Energy 2000, Radian International LLC 2000). In each case,

regarding the permissible levels of emissions generated during gasification, they

are identical with all techniques of thermal processing of solid waste and what has

already been described about the limits of the combustion – incineration process.


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In the following figure, a schematic flow diagram of the Gasification Plant in

Caribbean of the ITI Energy Limited is presented.


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Figure 45: ITI gasification plant flow diagram


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2.3.2. Feedstock

Gasification enables the capture — in an environmentally beneficial manner — of

the remaining “value” present in a variety of low-grade hydrocarbon materials

(“feedstocks”) that would otherwise have minimal or even negative economic

value. Gasifiers can be designed to run on a single material or a blend of

feedstocks:

Solids: All types of coal and petroleum coke (a low value byproduct of

refining) and biomass, such as wood waste, agricultural waste, and

household waste.

Liquids: Liquid refinery residuals (including asphalts, bitumen and other oil

sands residues) and wastewater from chemical plants and refineries.

Gas: Natural gas or refinery/chemical off-gas.

2.3.3. Gasifier

The core of the gasification system is the gasifier, a pressurized vessel where the

feed material reacts with oxygen (or air) and steam at high temperatures. There

are several basic gasifier designs, distinguished by the use of wet or dry feed, the

use of air or oxygen, the reactor’s flow direction (up-flow, downflow, or circulating),

and the gas cooling process. Currently, gasifiers are capable of handling up to

3,000 tonnes/day of feedstock throughput and this will increase in the near future.

After being ground into very small particles — or fed directly (if a gas or liquid) —

the feedstock is injected into the gasifier, along with a controlled amount of air or

oxygen and steam. Temperatures in a gasifier range from 1,400-2,800 degrees

Fahrenheit. The heat and pressure inside the gasifier break apart the chemical

bonds of the feedstock, forming syngas. The syngas consists primarily of H2 and

CO and, depending upon the specific gasification technology, smaller quantities of

CH4, CO2, H2S and water vapor.


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2.3.4. Oxygen plant

Most gasification systems use almost pure oxygen (as opposed to air) to help

facilitate the reaction in the gasifier. This oxygen (95–99% purity) is generated in a

plant using proven cryogenic technology. The oxygen is then fed into the gasifier

through separate co-feed ports in the feed injector.

2.3.5. Gas Clean-Up

The raw syngas produced in the gasifier contains trace levels of impurities that

have to be removed prior to its ultimate use. After the gas is cooled, the trace

minerals, particulates, sulfur, mercury and unconverted carbon are removed to

very low levels using commercially proven cleaning processes common to the

chemical and refining industries.

For feeds (such as coal) containing mercury, more than 95% of the mercury can

be removed from the syngas using relatively small and commercially available

activated carbon beds (WASTESUM, 2006).


A typical mass balance of the gasification process is shown in Figure 46. On the

basis of the diagram it can be stated that 1 tonne of treated feedstock leads to

680-810 kg produced syngas, 170-300 kg carbon char and ash that can be

recycled or disposed of at a landfill, while the remaining 20 kg is the residue from

the flue gas treatment that must be sent to a hazardous waste landfill.


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Figure 46: Schematic presentation of inputs and outputs of a typical gasification process

In the following figure (Figure 47), the energy and mass balances of the

Gasification Plant in Caribbean of the ITI Energy Limited are presented.


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Figure 47: Mass and energy balance of the ITI gasification plant


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Assuming 100% chemical energy in the fuel feedstock, the typical gasifier converts

this fuel to 70-80% chemical energy in the gaseous phase, 15-20% heat and some

heat loss and unconverted fuel, which depends on the type of gasifier and the fuel.


Syngas can be combusted to produce electric power and steam or used as a

building block for a variety of chemicals and fuels. Most solid and liquid feed

gasifiers produce a glass-like byproduct called slag, which is non-hazardous and

can be used in roadbed construction or in roofing materials. Also, in most

gasification plants, more than 99% of the sulfur is removed and recovered either

as elemental sulfur or sulfuric acid.

Hydrogen and carbon monoxide, the major components of syngas, are the basic

building blocks of a number of other products, such as chemicals and fertilizers. In

addition, a gasification plant can be designed to produce more than one product at

a time (co-production or “polygeneration”), such as the production of electricity,

steam, and chemicals (e.g. methanol or ammonia). This polygeneration flexibility

allows a facility to increase its efficiency and improve the economics of its

operations (Rezaiyan & Cheremisinoff, 2005; Klein, 2002; Radian International

LLC, 2000; Belgiorno et al., 2003).


The environmental impacts of the use of gasification systems are generally much

milder than incineration. They focus on air emissions and solid residues, as in all

thermal technologies. At high temperatures used in gasification, toxic metals

including cadmium and mercury, acid gases including hydrochloric acid and

ozone-forming nitrogen oxides could be released. Also, dioxins and furans may be

generated in the cooling process following the burning of ordinary paper and

plastic in case that the operation of the unit is not made and controlled properly.

Using municipal solid waste for fuel releases into the atmosphere the carbon


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which is in the paper, cardboard, food waste, yard waste and other biological

materials, plus the carbon in plastic products and containers made from

petroleum. The gasification of petroleum-based plastics adds to greenhouse gases

in the same way as burning fossil fuels, such as coal, oil or natural gas.

Gasification may reduce solid waste volume by 85 to 92%. In addition, the use of

gasification processes reduce methane emissions produced from the disposal to

landfill sites, while being a waste to energy treatment method, it enables the

displacement of CO2 that would have been emitted if the electricity had been

generated from fossil fuels.



of a gasification system with a capacity of 900 tonnes per day is 10-115 €/tonne,

while the operation and maintenance cost is 55-100 €/tonne. The relevant cost is

approximately 130 Euro/t on the basis of the estimations of Neamt Master Plan.


The application potential of gasification and plasma gasification is also considered

high, since these methods have recently proved that they are effective and

flexible, since they can also be used for the treatment of other waste streams (e.g.

sludge, hospital waste, etc.) apart from municipal waste. That is why the

gasification practices are considered as suitable alternative especially in the case

of isolated areas, such as islands. The relevant cost is similar to that of other

thermal management practices, higher than that of biological options, the relevant

land demand is limited and the energy yield is also considered of vital importance.

The experience from the operation of such plants is less than that from




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incineration units. Without doubt, the existing economic crisis in the whole Balkan

Region is a significant obstacle for such attempts today. An example on the

application of this technology is Slovenia, a country with less economic problems

comparing to Romania, Greece and Bulgaria, is provided below.

EfW Gasification CHP Plant in Celje, Slovenia

The Energy from Waste (EfW) Gasification plant in Celje, Slovenia constructed by

KIV constitutes part of an integrated waste management scheme for the city of

Celje and the surrounding districts.

Picture 7: Celje Waste to Energy CHP Plant

The waste treatment facilities comprise kerbside recycling, Eco Islands, a picking

station with baling machinery, MBT and the EfW. The remaining MSW goes

through a MBT facility, where loose RDF output from the residual waste becomes

the fuel supplying the KIV EfW plant. Additionally belt pressed sewage sludge is

mixed into the RDF just prior to being fed into the KIV capacity gasifier.

The waste treatment plants (MBT + EfW) are designed to cope with the waste

from up to 240,000 people across 24 Municipalities. The plant has been designed

to divert the waste away from landfill. It is a town / city sized solution, only

requiring short waste shipments thereby minimising carbon footprint.


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The mixed RDF (80%) and Sewage Sludge (20%) has a combined net calorific

value (CV) of 13.6MJ/kg. With this CV the 18MWth capacity gasifier plant is

capable of handling up to 37,000 tpa, based on guaranteed operational hours of

7,800. 15MWth of high pressure superheated steam produces 2.1MWe of power

as it passes through a steam turbo alternator. The plant is ‘heat led’ and feeds the

recovered energy of up to 13MWth into the existing District Heating scheme as hot

water at 110 C. If the scheme was ‘electricity led’, it would produce 3.8MWe of

gross power.

Figure 48: Flow diagram of the EfW plant in Celje


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Figure 49: Process scheme of the EfW plant in Celje

Figure 50: Energy and mass balance of the EfW plant in Celje


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2.4. PYROLYSIS

2.4.1. General

Pyrolysis is the method during which natural and chemical decomposition of the

thermally unstable organic substances included in waste is taking place under

temperature in the area of 400 - 800°C, with the absence of air or O2. The great

difference of pyrolysis comparing with incineration and gasification is that it is a

highly endothermic process and requires outer energy source so as to take place.

In fact, it is difficult to have conditions of complete absence of O2, so in practice

pyrolytic systems are operating with oxygen quantities less than the stoichiometric

ones.

The reactions that are taking place are initially decomposition ones, when organic

constituents characterized by low volatility are converted into other more volatile

substances:

CxHy CcHd + CmHn

Furthermore, in the primary stages of the pyrolysis stage condensation reactions,

hydrogen removal reactions and reactions forming rings are taking place that lead

to the formation of a solid residue containing carbon from organic substances of

low volatility:

CxHy CpHq + H2 + coke

Then, other reactions of the organic pollutants occur. In the case of O2 existence,

CO and CO2 are formed or the interaction with 2 is possible. The produced

coke can be gasified into 2 and CO2.

The pyrolysis products can be liquid, solid or gaseous. The exact amounts depend

on the nature of the waste to be treated, the heating conditions, the temperature


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and the treatment duration (contact time) (Institution of Mechanical Engineers,

2007; Gidarakos, 2006).

The main advantages of pyrolysis comparing to incineration include the following:

The decomposition temperature is much lower than that of incineration.

The decomposition is taking place in reducing atmosphere and not oxidizing

one, like in the case of incineration. The requirement for lower O2 quantity

also results in limited air emissions.

The content of ash in C is much higher than in the incineration.

Metals included in waste are not oxidized during pyrolysis and, therefore,

can be exploited more easily.

The combustion of the pyrolysis gas does not produce ash and the cleaning

process of the off-gasses is easier.

The initial waste volume is reduced at higher degree comparing with

incineration.

The main disadvantages of pyrolysis include:

The biggest problem of pyrolysis is that the waste to be treated has to be

cut down in small pieces sorted prior to the pyrolysis process and this can

substantially increase the cost for the installation and operation of pyrolysis

units.

The pyrolysis products have certain problems and in no case they can be

disposed at the environment as they are.

The systems for the cleaning of the generated gases and wastewater are

characterized by high cost.

At present, the application of the method at full scale is very limited.

The pyrolysis method has several different variations, one of which is thermolysis.


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A typical mass balance of the pyrolysis process is shown in Figure 51. On the

basis of this diagram it can be stated that 1 tonne of treated carbon-based

materials of Municipal Solid Waste (MSW) produces 380 kg syngas, 220 kg

wastewater, 240kg char, while the remaining 150kg are other residues (metals,

inert, salt).

Figure 51: Pyrolysis process flow chart

Considering the complexity of biomass composition, pyrolysis and the absence of

the thermodynamic parameter, it is difficult to determine the conversion of energy.

In general it can be considered:

Q = CP T + QP,

where Q is absorption of heat during pyrolysis process, kJ·kg-1,

CP is specific heat capacity of substance, kJ· (kg·K)-1,

T is change of temperature,

QP is enthalpies of reactions during the process, kJ·kg-1.

The proposed energy balance model equation is described below: For 1,000kg

feedstock, 320kg biomass fuel is needed. The Lower Heating Value (LHV) of the

biomass and biomass fuel is 3,900 kcal·kg-1 and the LHV of the process products

is as follows:


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• The output of the gas is 250m3, Lower Heating Value (LHV) is 15.18MJ·m-3

• The output of the charcoal is 300kg and its Lower Heating Value (LHV) is

7,100 kcal·kg-1 (WASTESUM, 2006).


The pyrolysis gas produced from the pyrolysis process can be utilized in boilers,

gas turbines or internal combustion engines in order to generate electricity, while

part of the pyrolysis ash can be used for manufacturing brick materials.


The environmental impacts of the pyrolysis process focus on air emissions and

solid residues, as in all thermal technologies. Due to the high temperatures used in

pyrolysis, toxic metals including cadmium and mercury, acid gases including

hydrochloric acid and ozone-forming nitrogen oxides can be released. On the

other hand, pyrolysis enables fossil fuel substitution by the MSW and, in addition,

slow pyrolysis may stabilize a portion of the C in these effects of biochar remain

for 10 years after initial application. Furthermore, the methane emissions produced

from the disposal of MSW to landfill sites are reduced.



of a gasification system with a capacity of 70-270 tonnes per day is 30-60 €/tonne,

while the operation and maintenance cost is 55-100 €/tonne. In general, the

application of the pyrolysis process can be considered viable for smaller waste

quantities in relation to incineration.




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Pyrolysis is one of the innovative thermal waste management methods with limited

application in full scale. The majority of the existing pyrolysis units in operation are

pilot ones. This is the main reason why it is not expected soon to have this option

available in the Balkan Region within the near future. Nevertheless, it is expected

that there will also be some developments in this field later, perhaps next decade.

The relevant cost is considered as preventive factor for the development of such

systems for Romania, Bulgaria, Greece and Slovenia for the time being.


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2.5. PLASMA GASIFICATION TECHNOLOGY

2.5.1. General

Plasma refers to every gas of which at least a percentage of its atoms or

molecules is partially or totally ionized. In a plasma state of matter, the free

electrons occur at reasonably high concentrations and the charges of electrons

are balanced by positive ions. As a result, plasma is quasi-neutral. It is generated

from electric discharges, e.g. from the passage of current (continuous, alternate or

high frequency) through the gas and from the use of the dissipation of resistive

energy in order to make the gas sufficiently hot. Plasma is characterized as the

fourth state of matter and differs from the ideal gases, because it is characterized

by ‘collective phenomena’. ‘Collective phenomena’ originate from the wide range

of Coulomb forces. As a result, the charged particles do not interact only with

neighboring particles through collisions, but they also bear the influence of an

average electromagnetic field, which is generated by the rest charges. In a large

number of phenomena, collisions do not play important role, as ‘collective

phenomena’ take place much faster than the characteristic collision time (Blachos,

2000).

Plasma Technology can be used as a tool for green chemistry and waste

management (Mollah et al., 2000). Thermal plasmas have the potential to play an

important role in a variety of chemical processes. They are characterized by high

electron density and low electron energy. Compared to most gases even at

elevated temperatures and pressures, the chemical reactivity and quenching rates

that are characteristic of these plasmas is far greater. Plasma technology is very

drastic due to the presence of highly reactive atomic and ionic species and the

achievement of higher temperatures in comparison with other thermal methods. In

fact, the extremely high temperatures (several thousands degrees in Celsius

scale) occur only in the core of the plasma, while the temperature decreases

substantially in the marginal zones (Gomez et al., 2009).


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Five distinct categories of processes are used as the basis for the plasma systems

catering for waste management (Juniper, 2006). These are:

Plasma pyrolysis (Huang & Tang, 2007; Sheng et al., 2008)

Plasma combustion (also called plasma incineration or plasma oxidation)

Plasma vitrification

Plasma gasification in two different variants (Malkow, 2004)

Plasma polishing using plasma to clean off-gases

Plasma gasification is the most common plasma process. It is an advanced

gasification process which is performed in an oxygen-starved environment to

decompose organic solid waste into its basic molecular structure. Plasma

gasification does not combust the waste as incinerators do. It converts the organic

waste into a fuel gas that still contains all the chemical and heat energy from the

waste. Also, it converts the inorganic waste into an inert vitrified glass (Moustakas

et al., 2005; Moustakas et al., 2008).

Mixed solid waste is shredded and fed into a reactor where an electric discharge

similar to a lightning (the plasma) converts the organic fraction into synthesis gas

and the inorganic fraction into molten slag. Typically temperatures are greater than

7,000°F achieving complete conversion of carbon-based materials, including tars,

oils, and char, to syngas composed primarily of H2 and CO, while the inorganic

materials are converted to a solid, vitreous slag. The syngas can be utilized in

boilers, gas turbines, or internal combustion engines to generate electricity while

the slag is inert and can be used as gravel.

Figure 52: Plasma gasification process flow chart

Waste

Plasma Energy

Usable Inert Slag

Synthesis Gas CO, H2, CO2, N2

Controlled Air Feed


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Picture 8: Molten slag pouring from plasma waste gasification reactor

(Pyrogenesis Inc, Montreal, Canada)

The advantages of the process include: Good environmental performance,

production of about 400 KWh net of electricity per tonne of waste treated, no by-

products going to landfill.

Picture 9: Final inert slag residue can be used in construction applications

The disadvantages of the process include: Relatively high cost, high level of

maintenance and skilled labor required for operations.


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Electricity is fed to a torch, which has two electrodes, creating an arc. Inert gas is

passed through the arc, heating the process gas to internal temperatures as high

as 25,000 degrees Fahrenheit. The following diagram illustrates how the plasma

torch operates.

Figure 53: Plasma torch operation

The temperature a few feet from the torch can be as high as 5,000-8,000oC. Due

to these high temperatures, the input waste is completely destroyed and broken

down into its basic elemental components. At these high temperatures all metals

become molten and flow out the bottom of the reactor. Inorganics, such as silica,

soil, concrete, glass, gravel, etc. are vitrified into glass and flow out the bottom of

the reactor. There is no ash remaining to go back to a landfill and the produced

vitrified residue called slag is the only material that can end up at landfills if no

suitable markets (e.g. as construction material) are found for that.

The plasma technology is flexible, since it can be used for the thermal treatment of

a variety of waste streams. The only variable is the amount of energy that it takes

to destroy the waste. Consequently, no sorting of waste is necessary and any type

of waste, except nuclear waste, can be processed.

The plasma reactor operates at a slightly negative pressure, meaning that the feed

system is simplified, because the gas does not want to escape. The gas has to be

pulled from the reactor by the suction of the compressor. Because of the size and

the negative pressure, the feed system can handle bundles of material up to 1


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meter in size. This means that sizeable waste can be fed directly into the reactor

and pre-processing of the waste is not needed. Also, the performance of the

plasma gasifier is not affected by the moisture of the waste (during incineration,

the moisture of waste consumes energy to vaporize and can impact the capacity

and economics of the process) (WASTESUM, 2006).

An indicative list of initiatives to apply plasma technology in the field of waste

treatment is given in the table below.

Table 29: Commercial Plasma Waste Processing Facilities (Circeo, 2007)

Location Waste Capacity (TPD) Start Date

Mihama-Mikata,

JP

MSW/WWTP

Sludge

28 2002

Utashinai, JP MSW/ASR 300 2002

Kinuura, JP MSW Ash 50 1995

Kakogawa, JP MSW Ash 30 2003

Shimonoseki, JP MSW Ash 41 2002

Imizu, JP MSW Ash 12 2002

Maizuru, JP MSW Ash 6 2003

Iizuka, JP Industrial 10 2004

Osaka, JP PCBs 4 2006

Taipei, TW Medical &

Batteries

4 2005

Bordeaux, FR MSW ash 10 1998

Morcenx, FR Asbestos 22 2001

Bergen, NO Tannery 15 2001

Landskrona, SW Fly ash 200 1983

Jonquiere, Canada Aluminum dross 50 1991

Ottawa, Canada MSW 85 2007

(demonstration)

Anniston, AL Catalytic

converters 24 1985

Honolulu, HI Medical 1 2001

Hawthorne, NV Munitions 10 2006


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Alpoca, WV Ammunition 10 2003


In general, the mass and energy balances have similarities with the respective

ones referring to gasification. A typical energy balance assumes that from one

tonne of waste treated more than 400 KWh net electricity is produced.


There are a number of applications for the plasma gasification syngas. For

example, it can be utilized as fuel source to produce electric power (e.g. in a

simplified steam-cycle configuration consisting of a conventional boiler/steam

generator with steam turbine) or in a gas engine, configured to accept lower heat

value gas. The gas can be used in a gas turbine, both in simple cycle and in

combined cycle operations. It can also be used as a feedstock for chemical

processes, e.g. the production of methanol.

The use of lower heat value plasma gasification syngas as a fuel source for gas

engines has been successfully demonstrated with syngas generated from various

feedstocks, including the gasification of biomass. Other applications for the

utilization of the plasma gasification syngas are as follows: separation of hydrogen

from the syngas, which can provide an excellent source of hydrogen for use with

fuel cells, using the syngas as a feedstock for the production of liquid fuels, such

as ethanol.

Applications for the glassy product include roadbed/fill construction and concrete

aggregate. Any reclaimed valuable metal could be sold to metal dealers and

processors. Metal alloy is bought and sold based on a commodity-based pricing

system.


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Plasma gasification uses an external heat source to gasify the waste. Almost all of

the carbon is converted to fuel gas. Plasma gasification is the closest technology

available to pure gasification. Because of the temperatures and drastic conditions

involved all the tars, char and dioxins are broken down. The exit gas from the

reactor is cleaner and there is no ash at the bottom of the reactor, while there are

no by-products that end up to landfills provided that there are available markets for

the produced slag. On the other hand, the use of plasma gasification processes

reduce methane emissions produced from the disposal to landfill sites, while as a

waste to energy treatment method, enables the displacement of CO2 that would

have been emitted had the electricity been generated from fossil fuels.



of a plasma gasification system with a capacity of 900 tonnes per day is 40-60

€/tonne, while the operation and maintenance cost is 55-100 €/tonne.

Nevertheless, most sources estimate that the cost is a little bit higher than other

thermal methods due to the use of electrical energy.

2.5.6. Applicability in the target region

The application potential of gasification and plasma gasification is also considered

high, since these methods have recently proved that they are effective and

flexible, since they can also be used for the treatment of other waste streams (e.g.

sludge, hospital waste, etc.) apart from municipal waste. That is why the

gasification practices are considered as suitable alternative especially in the case

of isolated areas, such as islands. The relevant cost is similar to that of other




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thermal management practices, higher than that of biological options, the relevant

land demand is limited and the energy yield is also considered of vital importance.

The experience from the operation of such plants is less than that from

incineration units.

The first attempt to apply gasification process in the target region and more

specifically in Greece was made he National Technical University of Athens, with a

unit that was installed in Mykonos in order to treat all types of waste generated on

the island with emphasis on municipal solid waste. The unit had been initially

designed and developed in the framework of the LIFE project entitled:

“Development of a demonstration plasma gasification / vitrification unit for the

treatment of hazardous wastes” and later was modified in order to cater for the

treatment of municipal solid waste, too. The scope was to investigate the use of

this innovative technique in an isolated area like an island in order to provide a

solution to the overall management of waste. General views of the whole

demonstration facility are available below:

Picture 10: General view of the demonstration gasification / vitrification unit


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Picture 11: Another general view of the demonstration gasification /

vitrification unit

The primary waste feeding system consists of a hopper intended for feed of solid

material having maximum moisture content of 50% and a maximum particle size of

2.5 cm. The screw conveyor solid feeder has a maximum capacity of about 85

kg/h of waste and the feeding capacity varies depending on the feed waste bulk

density. The feed rate is adjustable by varying the speed of the screw conveyor.

Waste is manually loaded into the hopper connected to the screw conveyor. The

feed rate is continuous and very steady, compared to a hydraulic feeder.

Waste is fed from a hopper through a screw feeder to the top of the furnace and

dropping down is passing through the very hot and free of oxygen region between

the two electrodes.

Picture 12: Feeding system


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The furnace is comprised of a crucible, with approximately 130 litters capacity. It

also includes a start-up natural gas burner for preheating and idle operation, a port

for gasification air injection, a water-cooling mechanism for the graphite

electrodes, an external surface water-cooling for the furnace walls and a tapping

hole for periodical or continuous slag removal. During the operation of the plasma

unit, the bottom part of the furnace contains the molten slag, while the upper

section of it contains the process gases and is lined with a suitable high-

temperature refractory. The required gasification air fed to the furnace is supplied

by a compressed air system. Adjusting the valves on the compressed air line can

control the flow rate.

Picture 13: Gasification / vitrification furnace

Figure 54: Plasma Gasification / Vitrification Process

Synthesis Gas Waste

Molten Slag

Iron Heel

+ -

Untreated Waste


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In the pilot unit the furnace in which waste gasification is taking place is preheated

at 600-800°C by burning propane in its interior. After preheating, two cylindrical

graphite electrodes are inserted in the furnace and their ends are approached to a

close distance. Two graphite electrodes are used to supply an electrical arc to the

furnace. The current flows from the anode (+) to the molten bath and from the bath

to the cathode (-). The cathode is grounded at zero (0) potential.

Graphite electrodes with male/female threads are used. The electrode dimensions

were 7.6 cm in diameter and 106.7 in length. Electrodes are installed with the

female end down, in order to avoid dust accumulation in the threads. Two

electrodes were screwed together on each side (anode and cathode) and are

mounted on flexible joints, which allow them to be moved over the slag pool and

improve mixing. The mechanism also permits the electrodes’ extension into the

furnace to be adjusted during operation (Carabin & Holcroft, 2005; Carabin et al.,

2004; Gagnon & Carabin, 2006).

.

The DC power supply for the electrodes has a maximum power output of 200 KVA

(Plasma arc power supply, input: 600 VAC-3 -60HZ, 3 X 200A fuses).

Then, a high voltage is applied between them producing an electrical arc which is

raising locally the temperature up to values as high as 5,000°C and creating a

plasma atmosphere. Air is not permitted to enter the furnace. Under these

conditions it is ensured that from the volatile part of the waste syngas is produced

consisting mainly of H2, CO, CO2 and H2O and containing in very low proportions

H2S and HCl, but without significant presence of NOx. A camera is installed in

front of a window on the top of the furnace, connected with a laptop, by which we

can watch or make video recording of the electrical arc and the decomposition of

the organic matter taking place in the interior of the furnace.

The slag could be tapped out periodically from the tap hole located on the front

side of the crucible, close to the bottom of the furnace. The slag was either poured

in a slag mold to form ingots or quenched in a water tank to produce granulated

slag.


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The inorganic part of the waste used is melted, drops to the bottom of the furnace

and from time to time is removed through a hole in the lower part of the furnace, is

collected to a fire resistant pan and is taken to the laboratory for analysis and

investigation of its toxicity.

The hot cyclone was designed to remove dust in the synthesis gas. The produced

gases, while entering the cyclone, are put in circular movement and the centrifugal

force makes particulate matter contained in the gases to be removed to a high

degree.

Picture 14: Cyclone Picture 15: Secondary Combustion Chamber

The result of its operation is the oxidation of the components of the furnace gases.

The secondary combustion chamber was designed to combust H2 and CO in the

synthesis gas. In order to combust CO and H2 into CO2 and H2O, air is added into

the secondary combustion chamber. Propane burners are used to maintain the

chamber temperature at 1,100oC. The operator can check local regulations to

determine the required temperature in secondary combustion chamber. This

temperature is required to fully combust CO and H2 in a region where no

hazardous by-products are created. In normal operation, the gas residence time in

the secondary combustion chamber is about two seconds. A single blower


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provides the combustion air for the burners and the combustion air for the

synthesis gas.

It is located at the outlet of the secondary combustion chamber. Its role is to cool

the combustion gases quickly to approximately 75oC so as to minimize any

production of dioxins, furans or other organic compounds. The shock-like cooling

avoids the formation of the aforementioned compounds from elementary

molecules in the synthesis gas due to the de novo Synthesis back reactions

(Calaminus & Stahlberg, 1998). These reactions are known to occur in waste heat

boilers where a slow cooling in the range from 400oC to 250oC of flue gases with

chlorine compounds, non combusted organic molecules and catalysts such as

dust will result in dioxin formation. The quench vessel uses two atomizing nozzles

to quench the gas from the secondary combustion chamber. These nozzles are

capable of providing 2 litters per minute of flow. Regulating the amount of the

quenching water can control the gas temperature exiting the vessel.

Picture 16: Quench Vessel Picture 17: Scrubber

It removes water-soluble components of the off-gas including hydrochloric acid

and most oxides of sulphur, prior to discharge. Since the synthesis gas may

contain acid gases (such as HCl or SO2), a packed tower type wet scrubber uses

caustic soda to neutralize the acid gas from the quench vessel. The pH of the

scrubbing solution is controlled at 9.0. The scrubber liquor is re-circulated through


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a wet bagfilter in order to remove suspended particles. The bagfilter is a cartridge

unit having series of cylindrical filters that are cleaned periodically by an automatic

sequence using pulses of compressed gas.

The pilot unit has a maximum hourly capacity of only 50 kg of waste and the

quantity of the syngas produced is too low for a gas engine to convert it in

electrical energy; therefore, the syngas has to be released in the atmosphere but

in a safe way. Hence, CO and H2 have to be transformed to CO2 and H2O and for

this purpose a Secondary Combustion Chamber (SCC) has been added in the

installation, which is maintained at high temperature (around 700-800°C) by

combusting propane with air and in which CO and H2 are burnt to CO2 and H2O.

The SCC in our installation is situated after the furnace and between the two units

is interceded a cyclone to remove the solid particles. After the SCC the flue gases

are objected to quenching by coming in contact with a big quantity of cold water

and this takes place in a pipe where flue gases and cooling water are moving

opposite each other. After quenching, the flue gases are passing for cleaning

through a scrubber with NaOH solution, then through a filter and finally before they

are released to the atmosphere via a stack are cooled in a heat exchanger to

condense and recirculate the maximum quantity of water vapors. The results of

the pilot application were positive and encouraging for future applications using

this technology. It is hoped that a full scale unit will operate soon in Mykonos and

other Greek islands using gasification or plasma gasification technology. However,

it is true that the existing severe economic crisis in Greece will cause significant

delays is these management plans.

No other similar applications have been made in Romania, Bulgaria or Slovenia.


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REFERENCES

Ahmed, I. & Gupta, A.K., Evolution of syngas from cardboard gasification, Applied

Energy 86 (2009) 1732–1740.

Alexander, R. (2000). Compost marketing trends in the U.S. Biocycle, 41 (7), 64–

66.

Alexander, M., (1977). Introduction to Soil Microbiology, John Wiley & Sons, New

York.

Allsopp, M., Costner, P. & Johnston, P. (2001). Incineration and human health,

State of knowledge of the impacts of waste incinerators on human health,

ISBN: 90-73361-69,9, Greenpeace Research Laboratories, University of

Exeter, UK.

ARCADIS & EUNOMIA (2010) Assessment of the options to improve the

management of biowaste in the European Union. In Final Report (Directorate-

General Environment, ed) European Commission 32.

Archer, D. B., and Harris, J. E. 1986.Methanogenic bacteria and methane

production in various habitats. In Anaerobic Bacteria in Habitats Other Than

Man, edited by E. M. Barnes and G. C. Mead, pp. 185– 223. Blackwell

Scientific Publications.

Bebar, L., Stehlik, P., Havlen, L. & Oral, J., Analysis of using gasification and

incineration for thermal processing of wastes, Applied Thermal Engineering 25

(2005) 1045-1055.

Belgiorno, V., De Feo, G., Rocca, C. D. & Napoli, R.M.A. (2003). Energy from

gasification of solid wastes, Waste Management 23, 1-15

Bernal, M. P., Paredes, C., Sánchez-Monedero, M. A. and Cegarra, J., (1998a).

Maturity and stability parameters of composts prepared with a wide range of

organic wastes, Bioresource Technology 63 91–99.

Bidlingmaier, W. (1996). Odour emissions from composting plants. The Science of

Composting, Part 1 (eds. de Bertoldi, M., Sequi, P., Lemmes, B., & Papi, T.),

Blackie Academic and Professional, London, UK.

Bishop, P. L. and Godfrey, C., (1983). Nitrogen transformation during sewage

composting, Biocycle 24 34–39.


NTUA Page 184

Bjorklunda, A., Melainab, M. & Keoleianb, G., Hydrogen as a transportation fuel

produced from thermal gasification of municipal solid waste: an examination of

two integrated technologies, International Journal of Hydrogen Energy 26

(2001) 1209–1221.

Blahos, L. (2000). Plasma Physics, the Fourth State of Matter, Giolas Editions, 1–

12.

Brothier, M., Gramondi, P., Poletiko, C., Michon, U., Labrot, M. & Hacala, A.,

Biofuel and hydrogen production from biomass gasification by use of

thermal plasma, High Temperature Material Processes 11 (2007) (2) 231-

244.

Bustamante, M. A., Moral, R., Paredes, C., Vargas-García, M. C., Suárez-Estrella,

F. and Moreno, J., (2008). Evolution of the pathogen content during co-

composting of winery and distillery wastes, Bioresource Technology 99 7299–

7306.

Carry, C. W., Stahl, J. F., Hansen, B. E. and Friess, P. L., (1990). Sludge

management and disposal practices of the county sanitation districts of Los

Angeles (USA), Water Science and Technology 22(12) 23-32.

CCME (Canadian Council of Ministers of the Environment), (2005). Guidelines for

Compost Quality. http://www.ccme.ca/assets/pdf/compostgdlns_1340_e.pdf

Cecchi F., Traverso P., Pavan P., Bolzonella D. and Innocenti L. (2003)

Characteristics of the OFMSW and behaviour of the anaerobic digestion

process, In: Mata-Alvarez J. (Ed.) Biomethanization of the organic fraction of

municipal solid waste, pp.141-179 IWA Publishing

Chiumenti, A., Chiumenti, R., Diaz, L.F., Savage, G.M., Eggerth, L.L., & Goldstein

N. (2005). Modern Composting Technologies. The J.G. Press, Inc. Emmaus,

Pennsylvania, USA.

Circeo, L. (2007). Plasma Arc Gasification of Municipal Solid Waste, EPA Region

4 Clean and Sustainable Energy Conference Embassy Suites Hotel at

Centennial Olympic Park, Atlanta, GA

CIWMB (2008) Current Anaerobic Digestion Technologies Used for Treatment of

Municipal Organic Solid Waste

Cornell University (2010). Composting trends and technologies,

compost.css.cornell.edu

http://www.trackg.com/R4CleanEnergy/Presentation-slides/Tuesday-tech-Ben%20Taube/Lou%20Circeo-Plasma%20Arc%20Gasification%20of%20Solid%20Waste.ppt


NTUA Page 185

Cooperband, L. R., (2000). Composting: art and science of organic waste

conversion to a valuable soil resource, Laboratory Medicine 31 283–289.

Crowe, M., Nolan, K., Collins, C., Carty, G., Donlon, B., Kristoffersen, M., Brogger,

M., Carlsbaek, M., Hummelshoj, R.M. and Thomsen, C.L. (2002). Part 3:

Technology and market issues. Copenhagen: EEA

Dalai, A.K., Batta N., Eswaramoorthi I. & Schoenau G.J., Gasification of refuse

derived fuel in a fixed bed reactor for syngas production, Waste Management

29 (2009) 252–258.

de Bertoldi, M., Vallini, G. and Pera, A., (1983). The biology of composting: A

review, Waste Management and Research 1(2) 157-176.

Deriziotis P.,Substance and perceptions of environmental impacts of dioxin

emissions. M.S. thesis, May 2004, Columbia University (data by U.S. EPA).

Deublin D., & Steinhouser A. (2008). Biogas from Waste and Renewable

Resources. An Introduction, Wiley-Vch.

Diaz, L. F., Savage, G. M. and Golueke, C. G., (2002). Composting of Municipal

Solid Wastes. in: G. Tchobanoglous and F. Kreith (Eds.), Handbook of Solid

Waste Management, McGraw Hill USA, New York, pp. 11-70.

Diaz, L. F. and Savage, G. M., (2007). Bioremediation. in: L. F. Diaz, M. de

Bertoldi, B. W. and S. E. (Eds.), Compost Science and Technology, Elsevier,

Amsterdam, pp. 159-176.

Directive 2008/98/EC of the European Parliament and of the Council of 19

November 2008 on waste and repealing certain Directives.

Directive 2000/76/EC of the European Parliament and of the Council of 4

December 2000 on the incineration of waste DoE (Department of the Environment), (1996). Code of Practice for Agricultural

Use of Sewage Sludge, HMSO, London, UK.

Dvorak, R., Parizek, T., Bebar L. & Stehl k P., Incineration and gasification

technologies completed with up-to-date off-gas cleaning system for meeting

environmental limits, Clean Techn Environ Policy 11 (2009), 95–105.

Dziejowski, J.E., & Kazanowska, J. (2002). Heat production during thermophilic

decomposition of municipal wastes in the dano-system composting plant.

Microbiology of Composting (eds. Insam, H., Riddech, N., & Klammer, S.),

Springer-Verlag, Berlin, Germany.

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32008L0098:EN:NOT


NTUA Page 186

EA (Environment Agency), (2001). Technical Guidance on Composting Operations

(Draft), Environment Agency UK, Bristol.

Eggerth, L.L., Diaz, L.F., & Gurkewitz, S. (1989). Market analysis for multi-

compost products. Biocycle, 30 (5), 29–34.

Engeli, H., Edelmann, W., Fuchs, J. and Rottermann, K., (1993). Survival of plant

pathogens and weed seeds during anaerobic digestion, Water Science and

Technology 27(2) 69-76.

Epstein, E., Willson, G.B., Burge, W.D., Mullen, D.C., & Enkiri, N.K. (1976). A

forced aeration system for composting wastewater sludge. J. Water Pollut.

Control Fed., 48 (4), 688.

Epstein, E., (1997). The science of composting, Technomic Publishing, Lancaster,

Pennsylvania, USA.

European Commission, (2006), Integrated Pollution Prevention and Control

Reference Document on the Best Available Techniques for Waste

Incineration.

European Commission, (2010), On future steps in bio-waste management in the

European Union. Commission staff working document. SEC(2010) 577 final

Feitkenhauer, H., Sachs, J. V. and Meyer, U. (2002), On-line titration of volatile

fatty acids for the process control of anaerobic digestion plants. Water

Research, 36, 212-218.

Fermor, T. R., Wood, D. A. and Lynch, J. M., (1989). Microbiological processes in

compost, International Symposium on Compost Production and Use, San

Michele All’Adige, Italy, pp. 282–300.

Friege, H. & Fender A., Competition between different methods for recovering

energy: from waste leading to overcapacities, ISWA World Congress 2010,

Hamburg, 15th – 18th November 2010.

Finstein, M. S., Miller, F. C. and Strom, P. F., (1986). Waste treatment composting

as a controlled system, Biotechnology 8 396–398.

Franklin Associates, Ltd., Cal Recovery Systems, Inc., & NUS Corporation (1990).

Market development study for compost, prepared for the U.S. Environmental

Protection Agency, EPA ContractNo. 68-01-7310.


NTUA Page 187

Friedl, A., Padouvas, E., Rotter, H. & Varmuza K., Prediction of heating values of

biomass fuel from elemental composition, Analytica Chimica Acta 544 (2005)

191-198.

Gajalakshmi, S. and Abbasi, S. A., (2008). Solid waste management by

composting: State of the art, Critical Reviews in Environmental Science and

Technology 38(5) 311-400.

Ganana, J., Turegano, J.P., Calama, G, Roman S. & Al-Kassir, A., Plant for the

production of activated carbon and electric power from the gases originated in

gasification processes, Fuel Processing Technology 87 (2006) 117 – 122.

Gaur, A. C., (2000). Bulky organic manures and crop residues. in: H. L. S. Tandon

(Ed.), Fertilizers, organic manures, recyclable wastes and biofertilizers,

Fertilizer Development and Consultation Organization, New Delhi, India.

Gidarakos E. (2006). Hazardous Waste, Management, Treatment, Disposal, Zigos

Editions, Thessaloniki

Gidarakos, E (2007). Municipal solid waste management, Lecture Notes,

Treatment technologies of toxic and hazardous wastes. Technical University

of Crete.

Girods, P., Dufour, A., Rogaume, Y., Rogaume, C. & Zoulalian, A., Comparison of

gasification and pyrolysis of thermal pre-treated wood board waste, Journal

of Analytical and Applied Pyrolysis, 85 (2009) 171–183.

Golueke, C.G., & McGauhey, P.H. (1953). Reclamation of municipal refuse by

composting. Sanitary Engineering Research Laboratory, University of

California, Berkeley, USA, Tech. Bulletin No. 9.

Golueke, C. G., (1972). ”Composting” a study of process and its principles, Rodate

press Emmaus, Pensylvania.

Gomez, E., Rani, D.A., Cheeseman, C.R., Deegan, D., Wise, M. & Boccaccini,

A.R. (2009). Thermal plasma technology for the treatment of wastes: A critical

review, Journal of Hazardous Materials, 161, 2-3, 614-626

GTZ, (2000). Utilisation of organic waste in (peri-) urban centres. GFA-UMWELT &

IGW. Eschborn, Germany.

Gro , B., Eder, C., Grziwa P., Horst, J. & Kimmerle, K., Energy recovery from

sewage sludge by means of fluidised bed gasification, Waste Management 28

(2008) 1819–1826.


NTUA Page 188

Harrison, E.Z., Olmstead, D., & Bonhotal, J. (2003). What’s behind a compost

label or seal? Biocycle, 44 (9), 28–30.

Hassen, A., Belguith, K., Jedidi, A., Cherif, A., Cherif, M. and Boudabous, A.,

(2001). Microbial characterization during composting of municipal solid waste,

Bioresource Technology 80 217-225.

Haug, R.T. (1993). The Practical Handbook of Compost Engineering. Lewis

Publishers, Boca Raton, Florida, USA.

He, M., Xiao, B., Liu, S., Guo, X., Luo, S., Xu, Z., Feng,Y., & Hu, Z., Hydrogen-rich

gas from catalytic steam gasification of municipal solid waste (MSW):

Influence of steam to MSW ratios and weight hourly space velocity on gas

production and composition, International Journal of hydrogen energy 34

(2009) 2174-2183.

Henze, M., and Harremoes, P. (1983). Anaerobic treatment of wastewater in fixed

film reactors—a literature review. Water Sci. Technol. 15:1–101.)

Hill, D. T. and Holmberg, R. D. (1988), Long chain volatile fatty acid relationships

in anaerobic digestion of swine waste. Biological Wastes, 23, (3), 195-214

Hogg, D., Barth, J. and Faviono, E., (2002). Comparison of Compost Standards

within the EU, North America, and Australasia, The Waste and Resources

Action Programme, Banbury, Oxon, UK.

Hogg, D., Lister, D., Barth, J., Faviono, E. and Amlinger, F., (2009). Frameworks

for use of compost in agriculture in Europe, Eunomia Research and

Consulting.

Huang, H. & Tang, L. (2007). Treatment of organic waste using thermal plasma

pyrolysis technology, Energy Conversion and Management, 48, 1331–1337

Hungate, R. E. (1967). A roll tube method for cultivation of strict anaerobes. In

Methods in Microbiology, edited by J. R. Norris and D. W. Ribbons, pp. 117–

132. Academic Press, New York, USA.Kaltwasser, B. J., Biogas: Regenerative

Energieerzeugung durch anaerobe Fermentation organischer Abfalle in

Biogasanlagen. Wiesbaden 1980: Bauverlag.

Hungate, R. E., (1966) The Rumen and Its Microbes. New York, Academic.

Igoni A.H., M.J. Ayotamuno, C.L. Eze, S.O.T. Ogaji, S.D. (2008), Designs of

anaerobic digesters for producing biogas from municipal solid-waste, Applied

Energy 85 (2008) 430–438


NTUA Page 189

Institution of Mechanical Engineers. (2007). Energy from waste, A wasted

opportunity?, United Kingdom

Ipek, U., Obek, E., Akca, L., Arslan, E. I., Hasar, H., Dogru, M. and Baykara, O.,

(2002). Determination of degradation of radioactivity and its kinetics in aerobic

composting, Bioresource Technology 84 283–286.

IWMI (International Water Management Institution), (2003). Co-composting of

faecal sludge and municipal organic waste - A Literature and State-of-

Knowledge Review. http://www.eawag.ch/forschung/sandec/publikationen

/ewm/dl/ cocomp_review.pdf.

Jacobs (2008) Development of a Policy Framework for the Tertiary Treatment of

Commercial and Industrial Wastes: Technical Appendices, Report for

SNIFFER / SEPA, March 2008.

Jordan, C. A. et al, (2008).Gasification of Biomass and Municipal Solid Waste for

Power Production in the Caribbean. ITI Energy Limited, Innovation

Technology Center, United Kingdom

Jördening, H.-J., Winter, J. (Eds.), 2005. Environmental Biotechnology. Concepts

and Applications. Wiley-VCH Verlag GmbH and Co., KGaA, Weinheim.

Juniper Consultancy Services Limited. (2006). Independent Waste technology

Reports, Bathurst house, Bisley GL6 7NH, England

Kaltwasser B. J. (1980) Biogas: Regenerative Energieerzeugung durch anaerobe

Fermentation organischer Abfälle in Biogasanlagen. Bauverlag GmbH,

Wiesbaden, Berlin.

Kathiravale, S., Yunus, M.N.M., Spian, K., Samsuddin, A.H. & Ramhan, R.A.,

Modeling the heating value of Municipal Solid Waste, Fuel 82 (2003) 1119-

1125.

Kern, M., Mayer, M., Wiemer, K., Systematik und Vergleich von Anlagen zur

anaeroben Abfallbehandlung, in: Biologische Abfallbehandlung III (Wiemer, K.,

Kern, M., eds.), pp. 409–437. Witzenhausen (1996): M.I.C. Baeza.

Khanal SK. Bioenergy generation from residues of biofuel industries. In: Khanal

SK, editor. Anaerobic biotechnology for energy production: principles and

applications. Iowa: John Wiley & Sons; 2008..

Klein, A., Gasification: An alternative process for energy recovery and disposal of

Municipal Solid Wastes, MS Thesis, Columbia University, 2002.

http://www.eawag.ch/forschung/sandec/publikationen%20/ewm/dl/%20cocomp_review.pdf

http://www.eawag.ch/forschung/sandec/publikationen%20/ewm/dl/%20cocomp_review.pdf


NTUA Page 190

Konzeli-Katsiri, A., Kartsonas, N., Inhibition of anaerobic digestion by heavy

metals, in: Anaerobic Digestion of Sewage Sludge and Organic Agricultural

Wastes (Bruce, A. M., Konzeli-Katsiri, A., Newman, P. J., eds.), pp. 104–119.

London (1986): Elsevier Applied Science

Lahav, O., Morgan, B. E. and Loewenthal, R. E. (2002), Rapid, Simple, and

Accurate Method for Measurement of VFA and Carbonate Alkalinity in

Anaerobic Reactors. Environmental Science & Technology, 36, (12), 2736-

2741.

Mahony, T., O'Flaherty V. (2002). Feasibility Study for Centralised Anaerobic

Digestion for the Treatment of Various Wastes and Wastewaters in Sensitive

Catchment Areas. Ireland, Environmental Protection Agency.

Malkow, T. (2004). Novel and innovative pyrolysis and gasification technologies for

energy efficient and environmentally sound MSW disposal, Waste

Management 24, 53-79.

Mavropoulos A. (2008). Municipal solid waste treatment technologies. Municipal

solid waste Association in Crete

Mechichi, T. and Sayadi, S. (2005), Evaluating process imbalance of anaerobic

digestion of olive mill wastewaters. Process Biochemistry, 40, 139-145.

Mena, E., Garrido, A., Hernández, T. and García, C., (2003). Bioremediation of

sewage sludge by composting, Communications in Soil Science and Plant

Analysis 34(7-8) 957-971.

Menikpura, S.N.M. & Basnayake, B.F.A., New applications of “Hess Law” and

comparisons with models for determining calorific values of municipal solid

wastes in the Sri Lankan context, Renewable Energy 34 (2009) 1587-1594.

Miller, F. C., (1992). Composting as a process based on the control of ecologically

selective factors. in: F. Blaine-Metting (Ed.), Soil Microbial Ecology:

Applications in Agriculture Environment Management, Marcel Dekker Inc.,

New York.

Minutillo, M., Perna, A. & Bona, D.D., Modelling and performance analysis of an

integrated plasma gasification combined cycle (IPGCC) power plant, Energy

Conversion and Management, 50 (2009) 2837-2842.


NTUA Page 191

Misra, R. V., Roy, R. N. and Hiraoka, H., (2003). On-farm composting methods.

Land and Water Discussion Paper 2, Food and Agriculture Organization of

The United Nations, Rome.

Mollah, M.Y.A., Schennach, R., Patscheider, J. Promreuk, S. & Cocke, D.L.

(2000). Plasma chemistry as a tool for green chemistry, environmental

analysis and waste management, Journal of Hazardous Materials B 79 301-

320

Morris, J. G. (1975). The physiology of obligate anaerobiosis. Adv. Microb.

Physiol. 12:169–246.

Moustakas, K., Fatta, F., Malamis, S., Haralambous, K.-J. & Loizidou M., (2005).

Demonstration plasma gasification/vitrification system for effective

hazardous waste treatment, Journal of Hazardous Materials B123 120-126

Moustakas, K., Xydis, G., Malamis, S., Haralambous, K.-J. & Loizidou M. (2008).

Analysis of results from the operation of a pilot gasification / vitrification unit

for optimizing its performance, Journal of Hazardous Materials, 151, 473-

480

Mudrack, K., Kunst, S. (1991) Biologie der Abwasserreinigung.: Gustav Fischer

Stuttgart.

Murto, M., Björnsson, L. and Mattiasson, B. (2004), Impact of food industrial waste

on anaerobic co-digestion of sewage sludge and pig manure. Journal of

Environmental Management, 70, (2), 101-107

Neves D.S.F. et al, (2007), Application of a dynamic model to the simulation of the

composting process, Department of Environment and Planning, University of

Aveiro, 3810 – 193 Aveiro, Portugal

Niessen, W. (2002). Combustion and Incineration Processes, Marcel Dekker Inc.

O’Leary, P. R. and Walsh, P. W., (1995). Decision Maker’s Guide to Solid Waste

Management, USEPA. http://www.bvsde.paho.org/bvsars/i/fulltext/decision

/decision.pdf.

Obeng, L.A. and Wright, F.W. (1987). The Co-composting of Domestic Solid and

Human Wastes. UNDP/World Bank Integrated Re-source Recovery Series

GLO/80/004, GLO 80/0/007, Report No. 7. World Bank Technical Paper (ISSN

0253-7494) No.57.

http://www.bvsde.paho.org/bvsars/i/fulltext/decision


NTUA Page 192

Ostrem K., (2004) Greening Waste: Anaerobic Digestion for Treating the Organic

Fraction of Municipal Solid Waste, Columbia University

Pagans, E. L., Font, X. and Sánchez, A., (2005). Biofiltration for ammonia removal

from composting exhaust gases, Chemical Engineering Journal 113(2-3) 105-

110.

Pavlostathis, S. G. and Giraldo-Gomez, E., (1991). Kinetics of anaerobic

treatment: A critical review, Critical Reviews in Environmental Control 21(5-6)

411-490.

Radian International LLC (2000). A Comparison of gasification and incineration of

hazardous wastes, DVN 99.803931.02, Austin, Texas

Rambøll, Ficthner,PM, Interdevelopment, (2008), Master Plan 2008-2038 for

integrated solid waste system in Neamt County, Bucharest

Rao, M.S, Singh, S.P., Sodha, M.S., Dubey, A.K. & Shyam, M., Stoichiometric,

mass energy and exergy balance analysis of post-consumer residues,

Biomass and Bioenergy 27 (2004) 155-171.

Reddy, K. R., Kaleel, R., Overcash, M. R. and Westerman, P. W., (1979). A

nonpoint source model for land areas receiving animal wastes: Ammonia

volatilization, Transactions of the ASAE 22 1398–1405.

Recycle & Composting Equipment Pty Ltd (2010). Windrow compost turners

http://composting.com.au/compost_turning/sittler_turners.html

Rezaiyan, J. & Cheremisinoff N. (2005). Gasification Technologies, A Primer for

Engineers and Scientists, Taylor & Francis Group, LLC

Rilling, N., Anaerobe Trockenfermentation f r Bioabfall, Ber. Abwassertechn. Ver.

1994a, 44, 985–1002.

Rilling, N., Untersuchungen zur Vergorung organischer Sonderafalle, in: Anaerobe

Behandlung von festen und fl ssigen R ckstnden, Dechema Monographien,

Bd. 130 (Maerkl, H., Stegmann, R., eds.), pp. 185–205. Weinheim 1994: VCH.

Regional Information Service Centre for South East Asia on Appropriate

Technology (RISE-AT) (1998), Review of current status of Anaerobic

Digestion Technology for treatment of MSW.

Sanchez-Monedero, M. A., Roig, A., Parades, C. and Bernal, M. P., (2001).

Nitrogen transformation during organic waste composting by the Rutgers

http://composting.com.au/compost_turning/sittler_turners.html


NTUA Page 193

system and its effect on pH, EC and maturity of the composting mixtures,

Bioresource Technology 78 301–308.

Scheidl, K., Boos, R., Prey T. & Wurst, F., High temperature gasification (HTG)

pilot plant studies with different waste materials: formation of PCDD/F and

older organic pollutants, Chemosphere, 23, (8-10) (1991) 1507-1514.

Schlegelmilch, M., Streese, J., Biedermann, W., Herold, T., & Stegmann, R.

(2005). Odour control at biowaste composting facilities. Waste Manage., 25

(9), 917–927.

Senn, C.L. (1974). Role of composting in waste utilization. Compost Sci., 15 (4),

24–28.

Shammas, N. K. and Wang, L. K., (2009). Biosolids composting. in: L. K. Wang, N.

C. Pereira, Y. T. Hung and N. K. Shammas (Eds.), Handbook of

Environmental Engineering: Biosolids Treatment Process, vol. 8, Humana

Press, pp. 669–714.

Sheng, H., Wang, R., Xu, Y., Li, Y. & Tian, J. (2008). AC plasma arc system for

pyrolysis of medical waste and POPs: Paper #77 Air and Waste

Management Association - 27th Annual International Conference on

Thermal Treatment Technologies 2, 605-612

Speece, R. E. (1996). Anaerobic Biotechnology for Industrial Wastewater

Treatments. Archae Press,Nashvillee, TN, USA.

Speece, R. E. (1983). Anaerobic biotechnology for industrial wastewater

treatment. Environmental Science Technology, 17, 416A 27A.

Stengler, E., The impact of energy recovery status for efficient Waste-To-Energy

plants, ISWA World Congress, Hamburg, 15th - 18th November 2010.

Switzenbaum, M. S., Giraldo-Gomez, E., & Hickey, R. F. (1990). Monitoring of the

anaerobic methane fermentation process. Enzyme and Microbial Technology,

12(10), 722-730.

Symons, G.E., and A.M. Buswell. (1933). The methane fermentation of

carbohydrates. J. Am. Chem. Soc. 55:2028–2036

Tchobanoglous G, Burton FL. (1991), Waste-water Engineering: treatment

disposal and reuse. 3rd ed. New York: McGraw-Hill; p.1334.


NTUA Page 194

Themelis N.J. and Gregory A., Mercury Emissions from High Temperature

Sources in the NY/NJ Hudson Raritan Basin, Proceeding of NAWTEC 10 ,

May 2002, American Society of Mechanical Engineers, 205-215.

Tiquia, S. M., Richard, T. L. and Honeyman, M. S., (2002). Carbon, nutrient and

mass loss during composting, Nutrient Cycling in Agroecosystem 62 15–24.

Tiquia, S. M., Tam, N. F. Y. and Hodgkiss, I. J., (1996). Microbial activities during

composting of spent pig-manure sawdust litter at different moisture contents,

Bioresource Technology 55 201–206.

Toffey, W. E., (1990). Large-scale sewage sludge composting: A case for

maintaining a diversified program, Water Science and Technology 22(12) 107-

116.

Trautmann, N. and Krasny, M., (1998). Composting in the classroom. Scientific

inquiry for high school students, Kendall/Hunt Publishing Co., Dubuque, IA.

U.S. Department of Energy, National Energy Technology Laboratory, A

Comparison of Gasification and Incineration of Hazardous Wastes, Final

Report (2000).

USEPA (United States Environmental Protection Agency), (2003). Environmental

regulations and technology. Control of pathogens and vector attraction in

sewage sludge, EPA/625/R-92/013.

Van Haandel, A. C., and Lettinga, G. (1994). Anaerobic Sewage Treatment: A

Practical Guide for Regions with a Hot Climate. John Wiley & Sons,

Chichester, England.

Vasudevan, R. & Mathew, G., Waste Management, Indian Scenario, ASSOCHAM

Environment Summit, 2007Waste to Wealth, Innovations and Emerging

opportunities, 24 October, 2007 Hotel Le Meridien, New Delhi, INDIA.

Vehlow J. (2006), State of the art of incineration technologies, Proceedings Venice

2006: Biomass and waste to energy symposium.

Verdonck, O., (1988). Composts from organic waste materials as substitutes for

the usual horticultural substrates, Biological Wastes 26(4) 325-330.

Verma, S (2002) Anaerobic digestion of biodegradable organics in municipal solid

waste, Columbia University


NTUA Page 195

Wastesum (2006). Management and Valorisation of Solid Domestic Waste for the

Small Urban Communities in Morocco, LIFE-3rd Countries, 2007-2009,

European Commission

Winter, J., Mikrobiologische Grundlagen der anaeroben Schlammfaulung, gwf

Wasser Abwasser (1985), 126, 51–56.

World Bank Carbon Finance Unit (2008), Municipal Solid Waste Treatment

Technologies and Carbon Finance, Thailand.

Wylie, J.S. (1957). Progress report on high-rate composting studies. Engineering

Bulletin, Proc. 12th Industrial Waste Conf., Series No. 94.

Yang H.-C. & Kim, J.-H., Characteristics of dioxins and metals emission from

radwaste plasma arc melter system, Chemosphere 57 (2004) 421–428.

Yassin, L., Lettieri, P., Simons, S.J.R. & Germana A., Techno-economic

performance of energy-from-waste fluidized bed combustion and gasification

processes in the UK context, Chemical Engineering Journal 146 (2009) (3)

315-327.

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