Small Scale AD for CCN Phase1 Report

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Lettinga Associates Foundation

for environmental protection

and resource conservation

Development of decentralised anaerobic digestion

systems for application in the UK

Phase 1 Final report

Client: Community Composting Network (CCN)Cath Kibbler ([email protected])

Date: 30 March 2009

Project number: 08-486Lettinga Associates FoundationPO Box 500

6700 AM Wageningen

The Netherlands

Tel: +31 317 482023

Fax: +31 317 482108

http://www.leaf-water.org Final reportI. Bisschops, H. Spanjers &

E. Schuman30-03-2009


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CCN Micro AD Phase 1 Report March 2009 2

Table of contents

Summary.................................................................................................................................................4 1. Introduction...................................................................................................................................6

1.1 Goal of the project .................................................................................................................... 61.2 Existing situation.......................................................................................................................61.3 System criteria .......................................................................................................................... 7

1.3.1 Initial scope set by CCN Steering Group .......................................................................... 71.3.2 Refined criteria .................................................................................................................. 8

2. Existing legislation........................................................................................................................92.1 Introduction............................................................................................................................... 92.2 Environmental Permitting / Waste Management Licensing......................................................9

2.2.1 Environmental Permitting Rules (Wales, England) ........................................................... 92.2.2 Waste Management Licensing (Scotland) ...................................................................... 10

2.3 Planning regulations ............................................................................................................... 102.4 Animal By-Product Regulation (ABP-regulation).................................................................... 102.5 Sewage and sewage sludge regulations................................................................................13

2.5.1 Sewage............................................................................................................................13 2.5.2 Sewage sludge................................................................................................................13

2.6

Gas safety regulations............................................................................................................14

2.6.1 Gas Safety.......................................................................................................................142.6.2 Explosive Atmospheres...................................................................................................14

3. Anaerobic digestion of (solid) waste .......................................................................................... 163.1 The anaerobic degradation processes ................................................................................... 163.2 Biogas.....................................................................................................................................17 3.3 Reactor conditions..................................................................................................................18

3.3.1 The right micro organisms...............................................................................................183.3.2 Nutrients .......................................................................................................................... 183.3.3 Temperature.................................................................................................................... 193.3.4 pH .................................................................................................................................... 20

3.4 General aspects of digester technology ................................................................................. 213.4.1 Retention time ................................................................................................................. 213.4.2 Load.................................................................................................................................22 3.4.3 Dry matter content and mixing ........................................................................................ 22

3.5 Digester configurations...........................................................................................................233.5.1 Fully mixed continuous systems......................................................................................233.5.2 Plug flow systems............................................................................................................243.5.3 Batch systems ................................................................................................................. 243.5.4 Accumulation systems..................................................................................................... 243.5.5 Dry digestion....................................................................................................................253.5.6 Practical considerations ..................................................................................................25

3.6 Potential problems..................................................................................................................253.6.1 Acidification ..................................................................................................................... 253.6.2 Scum layer.......................................................................................................................253.6.3 Foaming...........................................................................................................................26 3.6.4 Sediment layer.................................................................................................................264. Existing small scale digesters ....................................................................................................27

4.1 Fixed dome (China-type) ........................................................................................................ 274.1.1 Basic principle ................................................................................................................. 274.1.2 Polyethylene dome..........................................................................................................29

4.2 Floating dome (India-type)......................................................................................................294.2.1 General concept .............................................................................................................. 294.2.2 ARTI Compact Biogas Plant............................................................................................ 304.2.3 Water jacket plant............................................................................................................31

4.3 Bag design (Taiwan)...............................................................................................................31


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4.4 Prefabricated digesters...........................................................................................................334.5 Pilot scale self-built digesters .................................................................................................34

4.5.1 Simple digester with floating drum gas storage .............................................................. 344.5.2 Inclined barrel digester ....................................................................................................354.5.3 Biorealis digester.............................................................................................................354.5.4 Inner tube digester .......................................................................................................... 35

4.6 Research digesters.................................................................................................................364.6.1 Wheelie bin digester........................................................................................................364.6.2 University of Southampton and Greenfinch Ltd .............................................................. 37

4.7 Small digesters currently applied in practice in Western Europe...........................................374.7.1 Black water treatment......................................................................................................374.7.2 Portagester ....................................................................................................................38

4.8 Auxiliary equipment ................................................................................................................ 384.8.1 Reducing input particle size ............................................................................................ 384.8.2 Hygienisation...................................................................................................................39 4.8.3 Heating of the digester ....................................................................................................394.8.4 Pumps ............................................................................................................................. 414.8.5 Biogas treatment, storage and use ................................................................................. 414.8.6 Digestate storage and use ..............................................................................................43

5. Characteristics and digestibility of possible substrates..............................................................445.1 Food waste ............................................................................................................................. 455.2 Green waste ........................................................................................................................... 455.3 Animal manure........................................................................................................................ 455.4 Domestic wastewater (sewage)..............................................................................................465.5 Glycerine.................................................................................................................................47

6. Treatment scenarios...................................................................................................................486.1 Community Centre and Growing project ................................................................................ 486.2 All on-site ................................................................................................................................ 486.3 City farm and Urban Community centre................................................................................. 496.4 Locally centralised processing of food waste ......................................................................... 50

7. Discussion .................................................................................................................................. 517.1 Legislation...............................................................................................................................51 7.2

Substrate pre-treatment..........................................................................................................51

7.3 Digester configuration.............................................................................................................517.4 Biogas production and use ..................................................................................................... 527.5 Treatment and use of digestate..............................................................................................52

8. Proposed small scale anaerobic digester system......................................................................538.1 Digester dimensions ............................................................................................................... 548.2 Plant layout ............................................................................................................................. 548.3 System components - Slurry ..................................................................................................548.4 System components - Biogas.................................................................................................558.5 Transport of material between components ........................................................................... 56

9. References................................................................................................................................. 58


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Summary

The overarching goal of the project as defined by the Community Composting Network (CCN) is to

enable the CCN members to utilise anaerobic digestion as a technology in the decentralised

treatment and utilisation of bio-wastes. The project is divided in two phases. The first phase

considered the gathering of background information for the design and implementation of a small

scale digester for application by CCN members.

In this report the anaerobic digestion process and the possible digester configuration are discussed in

general, and various existing decentralised anaerobic digestion systems and the necessary auxiliary

equipment are described. Also the legislative aspects surrounding anaerobic digestion are included,

as well as descriptions of the possible substrates. In phase 2, based on the existing information asmall digester will be developed, that will be suitable for CCN members and will comply with the

regulations. The following criteria were defined: the digester volume should be relatively small (1 m3),

the digester should be robust, its effluent should be suitable for reuse and the digester should be easy

to built (do-it-yourself construction). With regard to the substrates for digestion, the focus will be on

kitchen waste, but also animal manure and green waste can be considered. Faeces and urine could

be used as substrates as well, but not in diluted form. Sewage and sewage sludge are in principle not

seen as suitable substrates, but are included in the discussion for completeness.

Several regulations apply for digesters. First, the Environmental Permitting Rules or Waste Managing

Licensing (Scotland) should be considered. When treating wastes these permits are required. In

specific cases exemptions can be made. An important regulation when using kitchen waste or any

other animal by-product containing substrates for a biogas plant is the Animal By-Product Regulation

(ABPR). This regulation states that catering waste (including kitchen waste) should be additionally

treated for the removal of pathogens: heating of material is required with a certain minimum

temperature and time period. Next to this other regulations may be applicable, for example withregard to biogas storage and handling.

The existing small scale digesters show large differences in configuration. Digesters of the fixed dome

type made of bricks and concrete are the most common type applied in developing countries, but

would not be suitable as such for application within CCN. Floating dome digesters made from existingplastic containers seem to provide a more feasible design, and also the so-called plastic bag

digesters appear interesting. However, these digester types are made for use in (sub)tropical

developing countries and some adaptation would be needed to make them suitable for a Western

European context. Existing small digesters designed for colder climates are often automated high-

tech applications, relying on monitoring and control equipment of the digestion process. Also the other

components of that kind of systems are usually automated. For the purposes of CCN some

monitoring and control is necessary, for example for the pasteurisation step. The required level of

automation, monitoring and control for the entire system is difficult to predict, as this largely dependson the experience gained during the first pilot test and the preferences of the CCN members. All of

the information gathered on existing systems will be useful in developing a custom design for CCN.

Several treatment scenarios are foreseen by CCN-members with regard to fractions and volumes of

kitchen waste, animal manure, faeces and urine, garden waste and use of digestate and biogas. Thedigester may be used in a community centre and city farm, or for treatment of all organic waste

streams of one rural household. Also the centralised processing of food waste is seen as an

interesting option. All of these scenarios include processing of kitchen waste, and therefore the APBR

has to be considered. In the scenarios different applications of biogas are foreseen, including heatingof buildings and use as vehicle fuel. From a small scale digester of 1 m 3 the amount of gas that can

be obtained is limited. During the design stage in phase 2 it will become clear how much gas can be

expected from a certain substrate mix, and the possibilities for gas use will become clearer. Sanitised

digestate, likely generated in small amounts, could perhaps be used directly, or after a short storage

in the open. Composting of digestate might be interesting as well.


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This report and the Phase 2 report are both available for download from the CCN website at:

http://www.communitycompost.org/index.php/projects/mad

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0

Unported License. Permissions beyond the scope of this license may be available by contacting

[email protected] or CCN, 67 Alexandra Road, Sheffield, S2 3EE, UK. To view a copy of

this licence, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to Creative

Commons, 171 Second Street, Suite 300, San Francisco, California 94105, USA.

Wed love to hear from any organisations or individuals who have used this report, passed it on, or

who have found it interesting/informative. Please get in contact with us by visitinghttp://www.communitycompost.org


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To-date the development of the sector has relied predominantly on composting garden waste.

Around 80% of sites compost garden waste exclusively - mainly from households but also from local

authorities parks and gardens and allotments and community gardens. Most sites (89%) use an

open composting process (windrows, bays or boxes). Around 13% of sites accept garden and food

waste (mostly meat excluded). In the past a small number of schemes composted some kitchen

waste with garden waste but ceased this practice with the introduction of the Animal By-Products

Regulations (ABPR). More recently a number of funded inner city and urban based schemes have

introduced ABPR compliant kitchen waste collection and composting schemes. Food waste

composting is likely to be a developing area for the sector and is expected to increase over the next

few years

Animal manure is frequently used in conjunction with green waste in rural projects and city farms,

where it is readily available on site or locally.

The current situation with regard to faeces and urine is not well understood as past surveys have not

asked for this information. However its known that a number of members have or have had

composting toilets at their projects.

CCN is aware of only one anaerobic system being used by a Member which was a pilot plant. Usually

the substrates are processed aerobically. Since its required that general food waste collected and

processed has to be treated in accordance with APB regulation, then anaerobic digestion has become

a more rational option.

1.3 System criteria

1.3.1 Initial scope set by CCN Steering Group

The system criteria were defined based on the scope agreed on by the Steering Group on the 17th

November 2008. This scope was:

The system should be suitable for Community Composting Network members as defined by:

- Capacity between 0.5 - 10m3

approx.

- To process 5 to 100 tonnes of material per annum.

There is a strong preference for this system to be modular so that in either twin or parallel processstreams can be created. Its perceived that this will ensure a more robust system overall (in the case

of maintenance down-time or failure of microbes to thrive). Its envisaged that this arrangement will

also allow continuous processing. Mobile systems can also be considered and are favoured by some

SG members.

The systems priority waste for processing is food waste but it should also preferably be capable of

processing other waste materials safely and within UK regulations: (in order of priority) sewage or

sewage sludge, animal manure, and dog & cat faeces. Its not anticipated that garden waste be

processed unless adding it to the feedstock mixture increases the productivity, reliability and

robustness of the system overall. These waste materials will either be collected in the immediatevicinity from the local community and delivered to the AD site, or else arise within the site where a

system is located (nb: these are two very different scenarios under the UKs ABPR regulations).

The system should produce a sanitised NPK effluent or digestate suitable for use in horticulture.

The system should produce biogas safely and include the means to store and should include

recommendations and options on how to use this. Consideration should also be given to using waste

heat for example to heat water or to raise the temperature of an enclosed horticultural growing space.

Digesters must meet British regulations:


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In order of priority (which could be based on our current incomplete level of understanding):

- Animal By-Products Regulations - for both on-site situations and where material is carried on

and/or offsite.

- Environmental Permitting Rules - these are currently under review, SEPA Waste Management

Licensing for Scotland.

- Gas handling & safety (currently CORGI)(and perhaps ATEX rules or regulations if applicable)

- Sewage and Sewage Sludge Regulations

- Planning Regulations (this is perceived as a potential problem)

1.3.2 Refined criteria

After discussing the different aspects related to the development of the small scale digester for CCN,

on 15th

December 2008 the previously defined scope (see previous section) was narrowed down by

LeAF and CCN to facilitate the system selection. The digester volume was set at 1 m3, with a

preference for modularity (using multiple digesters for larger applications), instead of a varying volume

for larger waste streams. Robustness and ease of building are important design features. The effluent

must be suitable for reuse, either as it is or separated in solid and liquid fraction. Preferably, the

complete digester system should be a do-it-yourself construction that can be built of readily available

materials without the need of professional help.

At the start of the project, it was foreseen that different representative scenarios would have to be

looked at, in order to determine the quantities of suitable inputs that would have to be treated. The

digester volume would then be based on those quantities. However, as the Community Composters

are used to having to work within constraints, the amount of waste collected can be made to fit the

system. Keeping the emphasis on the system rather than on the scenarios meant that the variables

could be reduced. This was the basis for deciding on a fixed volume, instead of having to design the

system to fit the quantities.

With regard to the substrates for digestion, the focus will be on kitchen waste. Animal manure is an

interesting substrate as well, but might not be available in all situations. Sewage and sewage sludge

will in principle not be considered for treatment in this small scale system. Garden waste can be

included, but as not all garden waste is suitable for digestion and sorting would be required,

composting might be the preferred treatment.

During the revision by CCN of the interim report several practical criteria came forward with regard to

the digester construction. As construction materials, steel and plastic drums are preferred over bricks

and concrete. This is because in the UK it is simpler and cheaper to get a plastic tank than to build an

(underground) brick tank. Labour costs for a brick or concrete construction would be very high. Above

ground systems are preferred over underground systems, again because of costs, but also because

of foreseen inconveniences with access for operation, maintenance and emptying. These issues are

especially important for the test build, later for the fully developed system underground construction

could maybe be considered.


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2. Existing legislation

2.1 Introduction

This chapter gives a brief overview of the legislation on waste processing and reuse/disposal when

using anaerobic digestion systems as a process unit. Several regulations apply or could apply in thiscase. Therefore, in Table 1 the regulations are given per sub-topic of using a small scale anaerobic

digestion system. Five topics are listed in this table in relation to the application of anaerobic digestion

systems. From the table it becomes clear e.g. that the for the use of kitchen waste as a substrate the

Animal By Product Regulation is applicable. The regulations will be described in the following

sections.

Table 1. Regulations with regard to the use of anaerobic systems divided into sub-topics.

Sub-topic Regulation or agreement that can be relevant

Installation of the system (general) Planning Regulations

Environmental Permitting Rules (England and Wales) or the Waste

Management Licensing (Scotland)

Use of kitchen waste as feed Animal By-Product Regulation (ABP-regulation)

Use of sewage as feed Sludge (Use in Agriculture) Regulations

Code of Practice for Agricultural Use of Sewage Sludge

Safe Sludge Matrix

The Urban Waste Water Treatment Regulations

Gas handling Regulations on Gas Safety

2.2 Environmental Permitting / Waste Management Licensing

The installation of a small biogas system can involve the requirement of a license or permit. For

Wales and England this is regulated in the Environmental Permitting Rules and for Scotland in theWaste Management Licensing.

2.2.1 Environmental Permitting Rules (Wales, England)

The new Environmental Permitting Rules (EP Rules) have been introduced in April 2008 in England

and Wales. An Environmental Permit (EP) is required for a waste operation or an installation. Some

exemptions are also in the Environmental Permitting Rules. In the Environmental Permitting

Guidance, Waste Framework Directive (Defra, 2008b) it is stated that households do not require

permits for carrying out waste operations involving only household waste or managing of waste withintheir own property (as is understood from the guidance). Disposal of waste by householders is not

allowed when it is likely that it causes harm to the environment or human health. Exemption can only

be done when any applicable EU directive allows it (Defra, 2008c). Digestion residue is classified as

waste and is subject to existing waste regulations (Environmental Agency, 2008).

Currently a review is being made on the exemptions of Environmental Permit Rules, including

anaerobic digestion. The proposed environmental permitting exemption is applicable when the total

quantity of waste that is treated or stored does not exceed 50 tonnes at any one time; that the wastes

can include catering, manure or green waste; that the gas is collected and burned; the total aggregate

net rated thermal input of the appliances that the gas is burned in is less than 1.5 megawatts; that it

results in a stable sanitised material that can be applied to land for agriculture, soil structure or

nutrient benefits.


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In case the anaerobic system is considered an exemption, no permit is required. Still, registration of

the installation is needed.

2.2.2 Waste Management Licensing (Scotland)

The SEPA positions statement on licensing of anaerobic digestion plants states that a waste

management license will be required when the anaerobic digestion process involves the recycling ordisposal of animal waste, including catering waste, either separately or in conjunction with manure

and silage effluents, and the capacity is less than 10 t/d. Also, using waste (e.g. green waste) mixed

with slurry and silage effluent fed to the anaerobic system involves the requirement of a permit. The

last situation applies when the waste arose on a farm or is brought to a farm (SEPA, 2008).

Unless only manure, slurries and silage effluent are used, the output of the AD plants is considered

waste. For the application of waste on the land, an exemption in the Waste Management Regulations

is made stating that the digestate and liquor from AD systems using animal and vegetable waste are

allowed to be used on agricultural land when there is a benefit for agriculture or ecological

improvement (SEPA, 2008).

2.3 Planning regulations

An anaerobic digestion system should also fulfill the requirements of the planning regulations. Themain responsibility of the planning management rests with the local planning authorities. Planning of

an area or district is a local matter and specific for each district. Therefore, the requirement of a

planning permit in case of building a biogas system has to be investigated specifically.

2.4 Animal By-Product Regulation (ABP-regulation)

The European Animal By-Product Regulation (ABP-regulation) is of interest when using animal by-

products or kitchen waste as a substrate for the anaerobic digestion.

In the regulation of the European Commission on animal by-products (ABP) not intended for human

consumption (in the text referred to as EC 1774/2002), restrictions on the use of food waste for

production of biogas are made. The objective of the ABP-regulation is to make sure that all meat andother products of animal origin used for anaerobic digestion meets treatment standards. In this way

sufficient pathogen removal is ensured, so that the treated material can be applied on the land (Defra,

2008a). The ABP-regulation will be discussed in more detail in this section. The following information

about ABP-regulation is gathered from guidance on the treatment in approved composting or biogas

plants of animal by-products and catering waste, Defra, 2008a, unless indicated differently.

In the APB-regulation 3 categories of animal by-products are distinguished:

1. Category 1 animal by-products.

Products with this label include animal by-products which are Specified Risk Material and catering

waste from means of international transport. Also parts of animals infected with TSE for example, are

in this category. Wastes of category 1 are not allowed to be used as substrates in biogas production

under any circumstances.

2. Category 2 animal by-products

This category consists of high risk animal by-products. They cannot be used as substrate in biogas

plants except where they have first been rendered in an approved rendering plant to the EU pressure-

rendering standard (133C/3bar/20 minutes). Digestate of rendered mammalian products cannot be

applied on agricultural land (but it can be used on non-agricultural land). Also manure and digestive

tract content are category 2 products. However, these products can be used without processing as a

raw material in an approved biogas plant (EC 1774/2002).


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3. Category 3 products

Wastes in this category are catering waste and other low risk animal by-products and can be

subdivided as follows:

- Category 3 animal by-products (low risk animal products)

- Category 3 catering waste

Category 3 animal by-products are low risk products, for example parts of animals which fit human

consumption. These products can be used as feed for anaerobic digestion but must be treated inaccordance with the EU Regulation. For anaerobic digestion this means that the system must include

a pasteurization phase. The EU standard treatment is 70oC for at least 1 hour with a maximum

feedstock particle size of 12 mm. Further, certain hygiene and plant management requirements are to

be met according the EU regulation. Moreover, the biogas plant should be approved to a competent

authority (EC 1774/2002).

Category 3 catering waste can also be used as a feedstock for anaerobic digestion under certain

regulations. Catering waste is defined as all waste food including used cooking oil origination in

restaurants, catering facilities and kitchens, including central kitchens and household kitchens (EC

1774/2002).

With regard to the treatment standards of catering waste (which are specified on a national level in

the UK) an important group is distinguished: the meat-excluded catering waste. Meat-excluded

catering waste means that measures were taken at source to ensure that meat was not included in

the catering waste. The meat fraction of the catering waste must be separately collected at source

and the meat and non-meat fraction must be never mixed.

Treatment of category 3 catering waste

For the category 3 catering waste national treatment standards are in force. These only apply when

the catering waste is the only animal by-product being used (except for manure, digestive tractcontent, milk or colostrums). For a biogas system it means that the waste has to be treated at a

minimum of 57C for 5 hours with a maximum particle size of 50 mm or at 70C for 1 hour. This is

also shown in Table 2.

Table 2. Treatment possibilities of category 3 catering waste when using it for anaerobic digestion.Minimum temperature Minimum time at minimum temperature Max particle size

57C 5 hours 50 mm

70C 1 hour 60 mm

This part of the treatment is a hygienisation step.

Besides the treatment standards above,

The biogas plants must either

1. treat only meat-excluded catering waste; or

2. following treatment, store the material for a minimum of 18 days. Storages may include anaerobic

digestion. (Defra, 2008a)

Premises should be permitted or licensed (or is exempt) by the relevant governmental agency of GB:

Environment Agency (England and Wales) / SEPA (Scottish Environmental Protection Agency,

Scotland).

The regulation at national level for category 3 catering waste does not apply for domestic composting

in case the catering waste will be applied only to land of the premise on which it is originating. In this

case, the catering waste is generated, composted and used at the same premise. This regulation is

established to enable households to compost their own kitchen waste. The ABPR Guidance

document describes this exemption as follows (quote):


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Regulation 16 of the Animal By-Products Regulations 2005 states that the composting requirements

of the Regulations do not apply to the composting of category 3 catering waste on the premises on

which it originates provided that (a) the decomposed material is only applied to land at those

premises; (b) no ruminant animals or pigs are kept at the premises; and (c) if poultry is kept at the

premises the material is composted in a secure container which prevents the poultry having access to

it during decomposition.

This means that there is an exemption only for situations where the catering waste is generated,

composted and then used all on the same premises i.e. once brought onto a site in the form of food,

the waste material does not then leave the site. It is not acceptable to collect waste from a number of

premises and then compost it and use it on a single premises.

Whether this exemption is also valid when applying on-site anaerobic digestion for processing kitchen

waste, is not clearly specified in the ABPR guidance. CCN has specified that this is the case as long

as the waste is created on site and the digestate is used on site.

Waste not regulated by the ABP-regulation

The use ofgreen waste is not affected by the Animal by product regulation. Green waste refers to

waste from a garden or park. Waste food from premises on which meat or products of animal origin

are not handled is also not affected by the animal by product regulation. For example, a green grocer

or vegetable market. This also means that waste from vegan kitchens (where per definition no

products of animal origin are handled) are also not affected by the Animal By-product Regulation.

Then however, it should be taken care of that indeed no products of animal origin are being used.

When green waste is mixed with catering waste it must be considered as catering waste and treated

accordingly. Therefore a mixture of grass cuttings and kitchen vegetable peelings would be

considered catering waste and should be treated as category 3 catering waste.

Additionally, in an anaerobic system where animal by-products are mixed with another feedstock suchas green waste (but these feedstocks are separately collected), the animal by-products are subject to

the treatment standards but the green waste is not. According to the EU treatment standard, a plant

mixing different feedstocks would need to address this in the Hazard Analysis and Critical Control

Points (HACCP) plan to ensure that each substrate is treated in accordance with the regulations.

HACCP

The Hazard Analysis and Critical Control Points (HACCP) approach identifies and evaluates the

hazards of a system or process. The focus of HACCP is to control the hazards as close as possible to

their source. The HACCP plan is used for approving and inspecting anaerobic systems. The following

steps are identified in a HACCP approach (Defra, 2008a):

- Conduct a hazard analysis

- Determine the Critical Control Points

- Establish critical limits

- Establish a system to monitor control of each critical control point

- Establish the corrective action to be taken when monitoring indicates that a particular Critical

Control Point is not under control

- Document and record all procedures, corrective actions and verification results

- Establish procedures for verification, audit and review to confirm that HACCP is workingeffectively.

In the HACCP approach the anaerobic system must be validated. In the pre-validation phase

evidence and data is provided to demonstrate that the system con comply with the requirements of

the applicable regulations. During the site validation, the operator will need to submit a HACCP plan

and has to demonstrate that the system can be operated on their particular site in a way which

complies with the requirements of the regulations eg. samples of the product should be tested for

Samonella.


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Requirements for approved biogas premises are given in EU regulations as well as national

regulations. These regulations are mentioned Defra (2008a). It is stated e.g. that there should be

some form of physical separation between the clean area for storing the digestion residues and the

unclean area where waste (substrates) are received. The unclean area (reception area) should be

easy to clean and disinfect. For very small facilities, bins or other covered, leak proof and lockable

containers can be used as reception area. Premises should operate on a one-way flow basis, thus

material should flow from the dirty end to the clean end. Treated material should not be contaminated

by untreated materials.

Access by livestock

The national regulations make it an offence to bring any catering waste onto any premises where any

farmed animal or any other ruminant, pig or poultry is kept unless it has been treated in an approved

biogas or composting plant. No approvals will be issued for composting or biogas plants which are

located on livestock premises or on other premises where ruminants, pigs or poultry are kept. CCN

added that poultry can be kept on the same premises but only as long as they are not allowed in the

feedstock or digestate.Digestion residue

The digestion residues of an anaerobic digestion system fed with catering waste or compost

consisting of catering waste, cannot be applied on pasture land. This is land that is intended to be

used for grazing or cropping for feedingstuffs following the application of compost of digestion

residues within two months (for pigs) and three weeks (for other farmed animals). So the land must

not be grazed during these periods nor cropping for feeding. Cutting hay or silage production is

permitted during these periods (Defra, 2008a).

Conclusion

If it is chosen for to use kitchen waste (originating from non-vegan kitchens), as substrate for

anaerobic digestion systems, the ABP-regulations imply that additional treatment of the kitchen wasteis required for pathogen removal and that certain treatment standards should be met. Further, the

anaerobic system has to be approved. The digestion residue might be applied on the land, in

accordance with the conditions above.

2.5 Sewage and sewage sludge regulations

In the area of sewage and sewage sludge several regulations are relevant.

2.5.1 Sewage

Treatment of sewage is regulated in the Urban Waste Water Treatment Regulations, which is different

for England and Wales and Scotland.

2.5.2 Sewage sludge

Use of sewage sludge is a highly regulated process. The use of sewage sludge on agricultural land

has to fit the Sludge (Use in Agriculture) Regulations and in the Code of Practice for Agricultural Useof Sewage Sludge.

In 1998 an agreement was made between the stakeholders in the UK (Water UK and the British

Retail Consortium) in respect to applying sewage sludge on agricultural land.

Its outcomes are or will be incorporated in the legislation and are as follows (ADAS, 2001):

1) Untreated sewage sludge can not be applied on agricultural land.


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2) Conventionally treated sludge can be only applied on the grazed grassland if it is deep injected

into the soil. No grazing or harvesting within 3 weeks of application is allowed. Conventionally

treated sludge can also be applied to agricultural land which is used to grow vegetables provided

that at least 12 months has elapsed between application of the sludge and harvest of the crop.

When the crop is a salad, the harvest interval must be at least 30 months (since a salad may be

eaten raw).

Conventionally treated sludge has been subjected to treatment methods which destroy at least

99% of the pathogens. Defined treatment processes are among others anaerobic digestion.

3) Enhanced treated sludge can be applied on land where crops as fruit, salad, vegetables andhorticulture take place. The harvest interval of 10 months should be applied. In case of animal

feed crops no harvesting interval is set in the agreement.

Enhanced treatment methods reduce the amount of pathogens with a 6 log and the sludge will be

free from Salmonella (ADAS, 2001).

More details about this subject are in the Safe Sludge Matrix (ADAS, 2001).

2.6 Gas safety regulations

2.6.1 Gas Safety

The Gas Safety is regulated in the Gas Safety Regulations. The Health and Safety Executive (HSE)

of the UK warns in general for an unprofessional dealing with gasses. Anyone who works on gas

appliances as part of their business must be registered. For owners of private property there is no

duty to maintain or regularly check their gas appliances by a CORGI-registered installer but it is

strongly advised by HSE. For safety reasons it is advised to never use a gas appliance if it is not

working properly. (HSE, 2008)

As mentioned in section 2.2 the proposed exemption of anaerobic digestion system for environmental

permitting is only applicable when the biogas is burned and the total input of all the gas appliances is

less than 1.5 megawatts (consultation draft, 20080725).

In all cases it is noteworthy that biogas contains methane which is highly flammable. Care should

always be taken when storing or using the biogas, especially when the gas is brought in contact with

air. Fire risks and the occurrence of explosive atmospheres should be dealt with. Presence of harmfuland/or smell-intensive gasses (e.g. hydrogen sulphide, nitrous oxide and ammonia) should be

prevented e.g. with a biogas treatment device.

2.6.2 Explosive Atmospheres

Because methane is highly flammable, it can generate, in the presence of oxygen, explosive

atmospheres. This can happen e.g. when biogas is leaking from its storage place to the air. An

explosive atmosphere is a mixture of dangerous substances with air, under atmospheric conditions, in

the form of gases, vapours, mist or dust in which, after ignition has occurred, combustion spreads to

the entire unburned mixture (ATEX, 2008). In biogas plants, different explosive risks can be identified,depending on the exact configuration of the plant. Some spaces will have a constant, long-term or

frequent dangerous potential risk of forming explosive atmosphere, for example the gasholder or the

combustion chamber of a gas flare. The digester itself can also be seen like this, in case air is leaking

into it. In other spaces occasionally an explosive atmosphere can occur, for example close to the gas

flare (Deublein, 2008).

ATEX is the name for a legal framework for controlling explosive atmospheres and the standards of

equipment and protective systems used in them. Two EU directives, the directive 99/92/EC (ATEX137) and Directive 94/9/EC (ATEX 95) are the basis of this framework. In Great Britain the


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requirements of Directive 99/92/EC were put into effect through regulations in the Dangerous

Substances and Explosive Atmospheres Regulations 2002 (DSEAR). The directive 94/9/EC is in GB

put into effect through the DTIs Equipment and Protective Systems Intended for Use in Potentially

Explosive Atmospheres Regulations 1996 (SI 1996/192) (ATEX, 2008).

DSEAR

Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) has the aim to

protect people from risks to their safety from fires, explosions and similar events in the workplace(including members of the public who may be put at risk by work activity) by putting duties on

employers and the self-employed. Dangerous substances include the flammable gasses. From thewebsite of DSEAR, it becomes clear that this regulation requires from employers to take measures to

control possible risks or remove risks, prepare plans to deal with accidents, inform employees, avoid

ignition sources.

DSEAR applies whenever:

- there is work being carried out by an employer (or self employed person);

- a dangerous substance is present (or is liable to be present) at the workplace;

- the dangerous substance could be a risk to the safety of people as a result of fires, explosions or

similar energetic events (DSEAR, 2008).

DSEAR applies to workplaces where dangerous substances are present, used, or produced.

Workplaces are any premises or parts of premises used for work. It thus applies for domestic

premises, if people are at work there (DSEAR, 2008).

Where the risk cannot be eliminated, DSEAR requires control and mitigation measures. Measures

that are listed on the website of DSEAR are (DSEAR, 2008):

Control measures:

- reduce the quantity of dangerous substances to a minimum;

- avoid or minimise releases of dangerous substances;

- control releases of dangerous substances at source;

- prevent the formation of a dangerous atmosphere;

- collect, contain and remove any releases to a safe place (for example, through ventilation);

- avoid ignition sources;- avoid adverse conditions (for example, exceeding the limits of temperature or control settings)

that could lead to danger;

- keep incompatible substances apart.

Mitigation measures:

- reducing the number of employees exposed to the risk;

- providing plant that is explosion resistant;

- providing explosion suppression or explosion relief equipment;

- taking measures to control or minimise the spread of fires or explosions;

- providing suitable personal protective equipment.

EPS regulations

Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres (EPS)

Regulations apply to all equipment intended for use in explosive atmospheres, whether electrical or

mechanical, and also to protective systems (ATEX, 2008). Products not covered by these EPS

regulations are amongst others equipment intended for use in domestic and non-commercial

environments where potentially explosive atmospheres may only rarely be created, solely as a result

of the accidental leakage of fuel gas (DTI, 2002). Because the biogas plant involves spaces where

explosive atmospheres can occur, electronic devices, and specific devices as the gasholder and the

gas transport system likely have to comply with regulations (certified equipment). It should be

considered when selecting the appropriate devices (Deublein, 2008). It is unclear whether there are

differences with regard to the applicable regulations, for a domestic plant and a commercial plant.


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3. Anaerobic digestion of (solid) waste

3.1 The anaerobic degradation processes

Different biological degradation processes exist in nature, and many of them are used for the

treatment of solid wastes and wastewaters. These processes can be divided into two groups: aerobicprocesses (using oxygen) and anaerobic processes (occurring in the absence of oxygen). Anaerobic

digestion is often portrayed as simple, but in fact it is complex: it is not one process, but encompasses

a group of different interlinked anaerobic processes. The following scheme (Figure 1) gives examples

of the different biological degradation processes:

Organic material:

- dead plant material

- dead animals

- manure

Organic material:

- kitchen waste

- horticulture waste

Artificial environment

- compost heap

Natural environment

- Forest soil

Aerobic (with O2)

CO2 and heat

humus

Organic material Organic material

Natural environment:

- Under water

- Inside intestines

Anaerobic (without O2)

CH4 and CO2

bog, manure digestate

Artificial environment:

- Digester

Organic material:

- dead plant material

- dead animals

- manure

Organic material:

- kitchen waste

- horticulture waste

Artificial environment

- compost heap

Natural environment

- Forest soil

Aerobic (with O2)

CO2 and heat

humus

Organic material Organic material

Natural environment:

- Under water

- Inside intestines

Anaerobic (without O2)

CH4 and CO2

bog, manure digestate

Artificial environment:

- Digester

Figure 1. Examples of natural degradation processes and some applications (adapted from Wulf, 2005).

An important difference between the two types of processes is the formation of an energy carrier,

methane, as the end product of anaerobic degradation, and the liberation of energy as heat in the

aerobic processes. During anaerobic digestion, organic material is transformed into methane (CH 4)

and carbon dioxide (CO2) in several different process steps. Different micro organisms are active in

the different steps, and the processes influence each other. The processes and the correlations

between them are shown in Figure 2.

Figure 2. Different process steps of anaerobic digestion


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The different steps shown in Figure 2 are:

1. Hydrolysis: transformation of solid organic material in soluble organic material

2. Acidogenesis (1st part of the acidification): formation of different shorter fatty acids and alcohols

3. Acetogenesis (2nd part of the acidification): formation of acetic acid

4. Methanogenesis (methane formation)

a. from acetic acid

b. from hydrogen and carbon dioxidec. from C1-compounds, such as methanol and formate

When a digester is fed with a complex mixture of substrates, which is the case when processing food

waste, the degradation of the different components (ingredients) will start at different points in the

scheme presented in Figure 2. Degradation of solids will start at step 1, whereas degradation of

volatile fatty acids that are already present in the feed will start at step 3. The system can be brought

off balance by disturbing the process flow, for example due to improper feeding or operational

problems.

To obtain a properly functioning anaerobic digestion it is important that the basic processes are well-

balanced. The different processes are in fact dependent on each other: they produce each others

substrates and consume each others products. Anaerobic digestion proceeds well when the products

of each step are transformed immediately in the next step. Furthermore, the combination of processes

can not proceed faster than the slowest process involved, and when one of the processes is disturbed

the overall digestion is negatively impacted. Generally, the hydrolysis is the slowest process, that is:

the rate-limiting step.

3.2 Biogas

The end product of a well established anaerobic digestion process chain is biogas, a mixture of

mainly methane (CH4, 50-70%) and carbon dioxide (CO2, 30-50%). Other gases such as hydrogen

sulphide (H2S) and ammonia (NH3) can also be present, usually in much lower amounts. Traces of

hydrogen (H2), oxygen (O2) and nitrogen gas (N2) can also be found. However, when oxygen and

nitrogen are present this can also indicate leakage. The composition and characteristics of biogas as

compared to natural gas are shown in Table 3 and Table 4.

Table 3. Comparison of the composition and parameters for natural gas and biogas (Persson et al. 2006).

Parameter Unit North Sea Natural Gas Biogas

Methane vol% 87 53-70

Carbon dioxide vol% 1.2 30-47

Nitrogen vol% 0.3 0.2

Oxygen vol% 0 0

Hydrogen sulphide ppm 1-2 0-10000

Ammonia ppm 0 135


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Table 4. Comparison of characteristics of natural gas and biogas (Wellinger and Lindberg 2001).

Parameter Unit North Sea Natural Gas Biogas *

Max. ignition velocity m/s 0.39 0.25

Theoretical air requirement m air/ m gas 9.53 5.71

Max. CO2-conc. in stack gas vol% 11.9 17.8

Dew point C 59 60-160

* 60% CH4, 38% CO2, 2% Other

Hydrogen sulphide can cause corrosion problems when burning the biogas, reducing the lifetime of

gas engines and other equipment. It is usually removed if above 500 ppm. Removal of compounds

from biogas is often called scrubbing. As well as hydrogen sulphide, water vapour and CO2 are also

often removed to increase the gas quality.

Biogas can be used for cooking, lighting, or electricity production. In developing countries cooking is

the main purpose. For small scale applications in developing countries the gas is normally used

untreated (unscrubbed), and special biogas stoves and lamps exist. Typically, 1 m3

of unscrubbed(=raw) biogas will allow for 2 hours of cooking or 1.5 kWh electrical output. A quantity of 2.5 m 3

unscrubbed biogas equals 1 kg of LP gas (AGAMA 2007).

3.3 Reactor conditions

As was shown in the previous sections, anaerobic digestion is a complex process, and the different

process steps should be well-balanced. Micro organisms carry out for these processes, which means

that a digester should count with a stable and well-functioning microbial biomass of the right

characteristics: the processes are interlinked so for a stable overall process all the needed different

micro organisms must be present, and in the right quantities. Several factors directly influence the

functioning of micro organisms and make it possible that the desired processes take place:

3.3.1 The right micro organisms

The micro organisms responsible for hydrolysis and acidification (also called acidifiers) are completely

different from the methane producing organisms (also called methanogens). Each group of microorganisms has their own requirements and properties. For example, many acidifiers can survive in the

presence of oxygen, and even use it, whereas it is toxic already in very low concentrations for the

methane producers. In a digester the conditions should be such that the requirements of all bacterial

population groups are taken into account. This can be achieved by feeding it with the right mix of

substrates and good operation and maintenance of the system.

3.3.2 Nutrients

Every living organism, including micro-organisms, needs a different mix of nutrients for cellmaintenance and growth. The most important nutrients that should be present (in the right amounts)

are: nitrogen, phosphorous, sulphur, calcium and magnesium, but also trace elements such as cobalt,nickel, manganese and iron.

Certain chemical wastewaters, for example, contain just a few organic compounds and almost nothing

else, and in those cases of serious nutrient deficiency addition of the missing nutrients is crucial for

obtaining a well functioning digesting process. Sometimes it is only one of the crucial trace elements

that is lacking, and often this is found out only after all other possible reasons for sub-optimal

digestion have been eliminated. Chemicals addition is costly, and therefore it is not standard practice

to always add the complete mix, just in case one of the compounds is present in sub-optimal


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amounts. Also, in too high amounts nutrients and trace elements can become toxic. Complex and

varied substrates such as kitchen waste should normally contain all necessary nutrients and trace

elements.

3.3.3 Temperature

Temperature is an important parameter for the anaerobic digestion process. Chemical reactionsproceed faster with increasing temperature and slower with decreasing temperature. Each 10C

temperature increase causes an increase in reaction rate of 2-3 times. The increase is limited

because the properties of compounds change with increasing temperature, but large rate increases

can be obtained before the processes are halted.

This relation between temperature and reaction rate is also valid for biological processes, because

they are in fact chemical processes that take place inside living organisms (or they are controlled by

those organisms). But because the micro organisms themselves are also sensitive to temperature,

the possibilities of increasing rates by increasing the temperature are limited, more than in the case of

purely chemical reactions.

Different groups of micro organisms have adapted to different temperature ranges to grow in, and

each group has its own optimum temperature. Within the range of a group each reaction has itsoptimum temperature with maximum activity. Above or below the optimum temperature the reaction

rate decreases. At temperatures just a few degrees above the optimum decay starts for most

bacteria, although some are relatively tolerant. At lower temperatures normally the activity goes down

but the organisms do not die. For anaerobic digestion bacteria are divided in three groups, each with

their own temperature optimum:

- 0-20C: psychrophilic micro organisms

- 20-40C: mesophilic micro-organisms- above 40C: thermophilic micro-organisms. Thermophilic digestion takes place at a maximum of

55-60C. However, bacteria exist that can grow at temperatures of 120C.

Figure 3 is a graphic representation of the temperature ranges. This graph is meant to give a general

idea of the differences between the three main groups, without going into detail. For each micro-organism the rates are different, which makes it impossible to include numbers on the y-axis.

0 20 40 60

Temperature (C)

Rate(growthspeed)

Psychrophilic

Mesophilic

Thermophilic

Figure 3. General indication of temperature ranges for bacterial growth


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Within each temperature group we can see that the rate increases with increasing temperature, and

when passing the maximum there is a rapid decline. As stated before, at lower temperatures micro-

organisms are merely less active, but at temperatures above their optimum they will die quite rapidly.

In general, the maximum rates of psychrophilic organisms are lower than the maximum rates of

mesophilic organisms, which are generally lower than for the thermophilic organisms. It can not be

generalised for example how much higher the thermophilic rate is when compared to mesophilic

conditions, the graph in Figure 3 should not be taken too literally.

The mentioned groups of micro-organisms are very different. Therefore a digester can not beswitched from one operating temperature range to the other without loss of activity. When a drastic

temperature change is applied, other micro organisms will start growing to take over the process.

3.3.4 pH

The pH is a measure for the acidity. Its scale runs from 0 (very acidic), via 7 (neutral), to 14 (very

basic). Figure 4 shows the pH scale with examples of common substances at different degrees of

acidity. All organisms are more or less sensitive to pH changes and have a pH-optimum at which they

function best.

Figure 4. pH scale with examples (Lower 2006)

In general three pH-ranges are distinguished for microbial activity:

-


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3.4 General aspects of digester technology

3.4.1 Retention time

In digester technology two types of retention times are distinguished: the hydraulic retention time

(HRT) and the solids retention time (SRT, also called sludge retention time). The HRT is the period of

time that a substrate is retained in the digester, whereas the SRT refers to the retention period of theactive biomass (=micro-organisms). In fully mixed systems with no retention of the active biomass the

HRT and the SRT are the same. Other types of systems are designed to retain the active biomass in

the system for as long as possible, which makes the SRT is much longer than the HRT. The result ofthat strategy is an increase of the active biomass concentration in the reactor and thus an increase of

the efficiency of the reactor volume. With a higher microbial activity the substrate can be converted in

a shorter time, making it possible to lower the HRT.

The allowable HRT is an important consideration, because it is the direct relation between the

digester volume and the substrate amount that can be processed: the retention time determines the

needed digester volume for a given substrate amount, and for a fixed digester volume it determines

the amount that can be processed in a certain time. For example: to digest an amount of 1 m3 per day

of a substrate that needs 20 days retention time, a volume of 20 m3

is needed.

For well-functioning digesters it is mostly the substrate characteristics that determine the retention

time. As explained before, the different processes each have a certain rate, and as a result the

different components of a complex substrate each need a different period of time to be fully degraded.The degradation of hemicellulose, fats and proteins can take up to several days, whereas simple

sugars and volatile fatty acids are transformed in a few hours. Figure 5 shows the biogas production

in time from a substrate when administered as a single feed. First the hydrolysis needs to start (phase

1), then the easily degradable compounds are formed, giving a fast biogas production increase

(phase 2). When only difficult to degrade compounds are left, the gas production decreases (phase 3)

and continuous at a low rate until all substrate is transformed (phase 4).

Figure 5. Biogas production from a single substrate feed (adapted from Gronauer et al. 2008)

Figure 6 gives an example of how the cumulative gas production increases at increasing retention

time in an anaerobic digester, and how the biogas production rate changes in time. Based on the

cumulative biogas production obtained from a substrate, a suitable retention time can be chosen.


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Figure 6. Cumulative biogas production and retention time (adapted from Gronauer et al. 2008)

The design of a digester is based on a certain substrate amount per day and a certain HRT for that

particular substrate (or substrate mix). This means that when the input is suddenly changed

significantly, the retention time should be changed accordingly. Normally a safety factor is used forthe design, allowing for small changes in the feed composition to be accommodated. However, whenafter a substrate change the resulting retention time is too short the process balance will be disturbed,

negatively affecting biogas production and effluent stability, or, in a worst case, leading to a complete

loss of activity (see also sections 3.1 and 3.6.1). A too long retention time is usually not a problem for

the process balance, but means that the digester volume is not used efficiently.

3.4.2 Load

The load is the amount of substrate provided per time per digester volume. An important

consideration when choosing the retention time is the load that can be applied without loosing the

balance between the different processes. The cumulative biogas production from a substrate is

determined by laboratory testing or derived from available information, but it is very unlikely that the

conditions in the digester will exactly match those tested in the laboratory or applied in other cases.

For example: a laboratory test with an easily degradable substrate might indicate that a very short

HRT is possible, but the amount of methane producing bacteria is then usually very high and normally

optimal conditions were applied. In a digester with fewer methanogens the short retention time would

lead to acidification of the digester content (see sections 3.3.4 and 3.6.1).

3.4.3 Dry matter content and mixing

The dry matter content is an important factor in choosing a certain digester type. Completely mixed

digesters can work with a maximum dry matter content of 8-10%, whereas plug flow digesters can

function with levels 15-20% (sometimes higher). Also other technical factors exist: pumping of

substrate is normally possible up to 12% dry matter. The characteristics of the available substrate

determine the choice for a digester type, and for existing digesters the dry matter content should beadapted to a suitable level. Mixing of the digester content is important because of several reasons:

- The fresh substrate is brought in contact with digested material, making/keeping the process

going by enhancing the contact between the substrate and the micro organisms.

- The temperature in the digester is equalised

- Floating layers or sediment layers can be (partly) prevented or removed

- Biogas bubbles entrapped in the material are liberated


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Installing a mixing mechanism is always an investment, and the mixing itself requires energy input.

Very basic digester systems used in developing countries are normally not mixed, except for natural

mixing provided by biogas bubbles flowing up, passing through the material. This makes the system

technically very simple, but reduces the biogas production efficiency.

3.5 Digester configurations

When choosing a certain digester configuration some important basic aspects should be considered

as a starting point. The scheme in Figure 7 shows the four most important aspects.

Moisture content of the substrate

orWet Dry

Temperature range

or or Psychrophilic Thermophilic

Feeding

orContinuous Discontinuous

Number of process steps

or Multiple stepsOne step

Mesophilic

Moisture content of the substrate

orWet Dry

Temperature range

or or Psychrophilic Thermophilic

Feeding

orContinuous Discontinuous

Number of process steps

or Multiple stepsOne step

Mesophilic

Figure 7. Examples of important factors in choosing a digester design

Anaerobic digester technology is divided in two main categories: wet digestion and dry digestion.

Wet digestion is applied for both wastewaters and slurries. Different technologies and reactor

configurations exist for both categories. A main difference between the approach for wastewater and

the approach for slurries is that wastewater digesters generally do not need mechanical mixing.

Mixing is brought about by pumping and carefully choosing the influent inlet mechanism. Well-knownsystems are the Upflow Anaerobic Sludge Blanket reactor (UASB) and other upflow reactors. Pure

wastewater treatment technologies are not further considered in this report.

3.5.1 Fully mixed continuous systems

A fully mixed continuous system is regularly fed with a certain amount of substrate (determined by the

retention time) while the same amount of digestate is removed from the digester. As a result, the

volume of digester liquid is kept constant. Small digesters might be fed once or twice daily, whereas

larger systems are usually fed much more often, for example every hour. Figure 8 shows a CSTR.

Figure 8. Schematic representation of fully mixed digester system (Arogo Ogejo et al. 2007)


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Strictly speaking, the feeding is not continuous, and the hydraulic retention time is in fact an average

retention time. The feed (flow and composition) should be optimised to make the biogas production as

high and as constant as possible. The gas production from each feed (as shown in Figure 5) should

overlap the gas production from the previous feed, obtaining an average continuous gas production

that lies a bit below the maximum in that graph. In the case of fully mixed systems the SRT is by

definition equal to the HRT, because the biomass and the substrate are always fully mixed. Usually

these reactors need long retention times in the order of several weeks or even months, as the active

biomass is part of the digestate that is removed from the reactor.

3.5.2 Plug flow systems

This reactor type is suitable for substrates with a higher solids concentration. These reactors

generally are fairly large horizontal tubes or narrow channels, in which feeding is done by introducing

material into the digester at one end and taking out material at the other end. Plug flow digesters are

not always mixed, and when mixing is done it is only in the vertical direction. As a result the digester

contains material in different stages of the digestion process, and the composition of the content is

totally different at the beginning of the digester and at the end. Usually part of the effluent is mixed

with the influent to provide active biomass and buffering capacity form the start of the process.

Figure 9. Scheme of plug flow digester system without effluent recycling (Arogo Ogejo et al. 2007).

Non-mixed plug flow digesters are technically simpler than fully mixed digesters. However, as thesubstrate is not mixed with a large amount of digested reactor content (like in the CSTR) it can be

difficult to maintain a well balanced process for each feed, especially when digesting easily

degradable substances. Additionally, in plug flow systems without vertical mixing the digester contentseparates by gravity, which means that the material is not only undergoing different digestion phases

in the horizontal direction, but also vertically.

3.5.3 Batch systems

In a batch process the reactor is filled completely with fresh substrate, inoculated with digested

material containing anaerobic biomass. No feeding or withdrawing of digested material takes place.

The gas production from a batch digester is exactly like shown in Figure 5: it first increases and then

decreases. This is why several batch digesters are operated next to each other, each in a different

phase of the process. In this way the gas production of the entire plant is maintained more or less

stable.

3.5.4 Accumulation systems

The name of this system indicates its mode of operation: material is accumulated in the digester. An

amount of previously digested material is used as an inoculum. These reactors resemble batch

digesters, but without filling them completely from the start. New material is added regularly, until the

digester is completely filled. When the last feed is digested the reactor is emptied and the

accumulation can start again. An example of a (mostly unplanned) accumulation digester is the

manure storage cellar on a farm.


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3.5.5 Dry digestion

Dry digestion takes place at dry matter concentrations of more than 20% (maximum 50%). Most full

scale dry digester systems are large batch systems that use percolate recycling. In those digesters a

fixed amount of organic waste is brought into the system and liquid is constantly recycled through the

mass. Without percolation the formed organic acids accumulate and zones of high acidity develop

rapidly, halting the digestion process. Especially with easily degradable waste such as kitchen waste

this is a problem. Mechanical mixing could be an alternative to percolation, material with high dry

matter content is difficult to mix well. In general this technology seems too complicated for small scale

application.

3.5.6 Practical considerations

When a digester type and basic configuration has been chosen the design process is not finished.

The chosen configuration should be dimensioned and designed in more detail. The needed digester

type and volume are mainly determined by the characteristics of the substrate (mainly hydrolysis rate,

maximum biodegradability), assuming that the reactor conditions (e.g. temperature) can be chosen

and set as desired. The technical choices made during the design phase are critical for the

subsequent use of the digester, and therefore the design criteria should be well thought over.

Important choices do not only have to be made for the substrate and the needed volume, but also for

very practical issues like storage of feed and digestate, facilities for mixing of different substrates,

ways of introducing the feed and removing the gas and digestate, mixing of the digester liquid, gasstorage, monitoring and control, etc.

3.6 Potential problems

3.6.1 Acidification

Acidification of the digester is one of the most common situations in which the anaerobic digestion

process steps are out of balance. This is a result of the relation between acidification and methane

formation. The methane producing organisms (methanogens) use acetic acid (formed in step 3, theacetogenesis, see Figure 2) to finally form methane. When there are not enough methanogens

present to consume the acetate, acids will accumulate in the digester. It is not only the acetic acid that

is not consumed, but also the acids formed in step 2 (the acidogenesis) that accumulate. These

accumulated acids cause a pH drop in the digester, which negatively affects the methanogens, and

can reach toxic levels. As a result, even less acid is transformed by methanogens. This is a vicious

circle, which can be started by a sudden feed of an excessive amount of easily acidifiable substrate to

the anaerobic digester. Correct process operation is the key to avoiding acidification problems.

3.6.2 Scum layer

Scum layers are formed by material floating on top of the liquid in the digester. When thick enough,

this floating layer can cause blockages of effluent pipes and biogas outlets. Fat is the most common

cause of scum layer formation, but also light materials such as straw can start to float. Different

floating materials can cling together, causing a dense layer. Adequate mixing can prevent the

formation of these layers, and mixing can also be used to destroy the scum layer when it forms.

However, mixing does not bring guaranteed success. As long as the floating materials are moist, they

can be incorporated again in the digester liquid, for example by adjusting the mixing equipment or by

introducing a form of mixing if it was not installed yet. If scum layers are left for too long, they can dry

out and form a crust that is difficult to remove.


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3.6.3 Foaming

Foaming is a common problem with many digesters. It occurs in many forms, and no general

prevention or remediation rules exist. When a foam layer is stable and reaches high enough, it can

block the gas outlets. Remedies that can be tried are mixing the top layer or spraying water on the

surface. Changing the feed composition (if possible) can also reduce foaming.

3.6.4 Sediment layer

Sand, grit and other inert materials can form a sediment layer on the bottom of the digester. This layercan cause blockages and wear on moving parts, but in any case it reduces the effective reactor

volume. Introducing this type of materials in the digester should be avoided. If a sediment layer is

formed it should be removed before damage occurs or before the effective volume is reduced too

much.


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4. Existing small scale digesters

This chapter presents an overview of existing small scale digesters as encountered in literature and

on the internet, as a basis for idea generation for the development of a robust digester that is easy to

build and operate, and will work in Western Europe. This list of digesters is not exhaustive, as the

number of variations in design, technology and configuration is endless. Because of this large

variation it is not useful to go into much depth for each digester, or to try to adapt them for use in the

UK. The digesters are categorised based on their main distinctive characteristic, and the country in

which they are found most frequently. The country name does not imply that the system is only used

in that particular country. Digesters used in (tropical) developing countries would need to be equipped

with heating system in order to obtain efficient biogas production for the situation in UK. However, the

different digester configurations used worldwide can provide useful ideas for the development of asuitable new digester concept for CCN.

In 2005 there were more than 25 million small scale biogas plants in operation worldwide, and the

installation rate from that time was over 1 million plants per year (AGAMA 2007). This indicates that

small scale anaerobic digestion for biogas production is in principle an interesting technique. In

Western Europe biogas production is done mainly in large digesters because of economy of scale,

and the direct link governments make between biogas and electricity production. Subsidies are givenfor produced electricity, and a certain scale is needed to make electricity generation feasible.

4.1 Fixed dome (China-type)

4.1.1 Basic principle

Digesters of the fixed dome type have been built since 1936 in China and it is the most common type

applied in developing countries. The digester is usually built of bricks, stone and/or poured concrete

(Marchaim 1992). Many variations to the fixed dome digester have been developed. Different

variations of the so-called Chinese type (Figure 10 and Figure 11) make use of a removable manhole

cover in the top of the dome. In India the two most common fixed dome digester models are the

Janata and the Deenbandhu design (Figure 12), both without a manhole in the top. These digestersare usually between 5 m3

and 10 m3.

Figure 10. Chinese type fixed dome digester with vertical walls (Fraenkel 1986).


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1. mixing tank with inlet pipe,

2. digester,

3. compensating and removal tank,

4. gas holder,

5. gas pipe,

6. weighted entry hatch with gas tight seal,

7. difference in level = gas pressure in cm water

column,

8. supernatant scum layer,

9. accumulation of thick sludge,

10. accumulation of grit and stones,

11. zero line filling height without gas pressure.

Figure 11. Chinese type fixed dome digester almost completely dome shaped (Nijaguna 2002).

Figure 12. Two types of Indian fixed dome digesters: Janata (left) and Deenbandhu (right) (Mital, 1997)

The gas tight chamber is formed by a hemispherical top sealed by layers of mortar. Feeding is donesemi-continuously (e.g. once a day) through the inlet pipe, displacing the same volume of effluent.

Biogas is stored under the dome, leading to quite high gas pressures (between 1 and 1.5 m of water),

which is why the top and bottom are hemispherical. Leakage of gas was a main problem in older

dome digesters and can still be an issue in newer ones. In 1992 there were about 5 million family-

sized fixed dome digesters operating in China, with volumes of 6, 8 and 10 m3

(Marchaim 1992). The

costs for a Deenbandhu plant as given by the Indian government are shown in Table 5.

Table 5. Costs for Deenbandhu digesters in India. Data probably from 2005, not specified. (MNRE 2005).

Plant capacity (m3) Cost per plant in India (Rs)*

North-eastern region Hilly areas Rest of the country

1 8800 7150 5500

2 14400 11700 9000

3 16800 13650 10500

4 21600 17550 13500

* Exchange rate mid 2005: 1 = 73 Rs / 1000 Rs = 13.8

The design and construction of these digesters is well-known and a lot of experience is available, also

freely accessible through the internet. Construction material is chosen on site to keep costs low. In

China the design of these digesters is standardised based on several design aspects (Marchaim

1992): gas pressure, average rate of gas production, gas storage, digester size, geometric forms,

loads and forces. At ambient pressure the water level is 95% of the total volume, and the gas


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pressure should be equal to, or below, 120 cm of water. The diameter to height ratio is usually 2:1.

Both cow and pig manure are common substrates, and for both materials an HRT of 35 - 40 days is

adopted, applying a solids concentration of less than 10%.

4.1.2 Polyethylene dome

Leaking of gas through the dome is the main problem with standard fixed dome biogas plants,especially in areas where skilled labour and good quality materials are scarc

Small Scale AD for CCN Phase1 Report

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