Opportunities for improvement of process technology for biomethanation processes

Green Process Synth 1 (2012): 49–59 © 2012 by Walter de Gruyter • Berlin • Boston. DOI 10.1515/greenps-2011-0025

Review

Opportunities for improvement of process technology for biomethanation processes

Meena Krishania *, Virendra Kumar, Virendra Kumar Vijay and Anushree Malik

Center for Rural Development and Technology , Indian Institute of Technology Delhi, Room No. 275, Block III, IITD, Haus Khas, New Delhi, Delhi 110016 , India ,e-mail: [email protected]

* Corresponding author

Abstract

Biomethanation process is a promising eco-friendly solution for the treatment of organic biomass, which leads to effi cient bioenergy production. In this review, it is pointed out that more research and development is necessary to fi nd the best operational conditions and dedicated reactors for treating the target substrates and to make them accessible to rural areas. The present review focuses on the various approaches which can be used to solve the constraints occurring during the gas production. A brief description of possible pretreatments for converting complex lignocellulosic waste to biogas, with the desired range of operational parameters for anaerobic diges-tion, is highlighted. In addition, suitable reactors for different substrates are reviewed.

Keywords: bioenergy; biomass; biomethanation; pretreatment; reactors.

1. Introduction

As fossil fuel resources are very limited and their demand is high, the gap can be met with the energy generation from renewable resources. One of the feasible renewable energy sources for India is from biomass. Biomass availability in the country is very high – 150 million MT/annum. Production of biofuels from biomass can slow down the climate change, which contributes to reducing the greenhouse emissions [1] . Among several renewable technologies, the “ biomethana-tion technology ” is a commercially proven technology and is widely used for treating biomass. India has built about 4 million family sized biogas plants in the past [2] .

Recently, biomethanation technology has become more attractive as a source of renewable energy, due to its low tech-nological cost and high process effi ciency. A variety of sub-strates such as waste water, animal waste, industrial waste, municipal solid waste, agricultural residues, energy crops [3]

and water based resources like algae [4] , are extensively used for this anaerobic technology. Methane production through biomethanation has been evaluated as one of the most ener-gy-effi cient and environmentally benign ways of producing vehicle biofuels. Thus, it can provide multiple benefi ts to the user.

In the biomethanation process (Figure 1 ), the organic waste is converted into methane and enriched manure by a large consortium of microorganisms in the absence of air, also known as anaerobic digestion [4] . The important processes in anaerobic digestion are hydrolysis, acidogenesis, acetogene-sis, and methanogenesis, where the hydrolysis step is an extra cellular process. Hydrolytic and acidogenic bacteria excrete an enzyme to catalyze the hydrolysis of complex organic materials into smaller units. The hydrolyzed substrates are then utilized by acidogenic bacteria. Products such as acetate, hydrogen and carbon dioxide can directly be used by metha-nogenic bacteria producing methane and carbon dioxide, while other organic products such as alcohol and volatile fatty acids are further oxidized by acetogenic bacteria in syntrophy with the methanogens [5] . Since the whole process depends on the growth of microorganisms, many parameters like pH, temperature, carbon/nitrogen (C/N) ratio, organic loading rate, reactor designing, inoculums and hydraulic retention time (HRT) are of infl uence [6] . To harness fully anaerobic digestion, these parameters should be optimized. Tuning of pre-treatment, additives and reactor designing with respect to the feedstock, can solve the major limitations, such as low gas production from agricultural residues, large hydraulic reten-tion time and low gas production in winters.

2. Biomass to bioenergy

From ancient time, biomass has always been a good source of energy for mankind and it contributes around 14 % of the world ’ s energy supply [7] . In India, 80 % of the total energy consumed in rural areas comes from biomass fuels such as fi re wood, crop residues, live stock dung [1] , municipal solid waste, algae, industrial waste and agricultural residues, which are easily available in abundance [4] . Traditionally, biomass has been utilized for cooking through direct combustion in chullahs producing dust which is harmful for health. Presently agriculture residue usage is limited to the production of pro-ducer gas, in which the burning process ruins the equally important fertilizer and compost value of biomass. There are several methods to convert biomass to energy [8] . The

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50 M. Krishania et al.: Improvement of process technology for biomethanation processes

use of methane as a biofuel is a feasible way to utilize ligno-cellulosic materials as the energy source (Figure 1 ). Among several technologies such as bioethanol, biodiesel, biohydro-gen, the anaerobic digestion technology has proved to be a viable and promising technology; it is also known as a good treatment method of organic wastes. Recent life cycle assess-ment studies have demonstrated that biogas derived methane (biomethane) is one of the most energy effi cient and envi-ronmentally sustainable vehicle fuels [9] . At the same time, nutrients contained in the remaining digestate can be used for crop production and play a remarkable role in promoting sus-tainable biomass production systems. Biogas is generated by renewable feedstocks.

Organic waste

Carbohydrate, proteins, fats

Hydrolysis

Sugar, amino acids, fatty acids

Acidogenesis

Aceticacid

Volatile fatty acids, alcohols and other compounds

Acetogenesis

Methanogenesis

Biogas

Acetic acid, H2, CO2

H2,CO2

Figure 1 Process of biomethanation.

Agricultural residue

Pretreatment

Biomethanationtechnology

Biogas

Heat

Electricity

Vehicular fuel

Digestate

Organic fertilizer

Figure 2 Applications of biomethane.

Upgraded biogas with > 90 % methane content can be used directly as vehicle fuel or it can be injected into the gas grid. Methods for upgrading biogas for natural gas or vehicle fuel quality include: absorption of the gases in liquid solvents [9] , adsorption on solids or pressure swing adsorption (PSA) and membrane separation and chemical conversion to other com-pounds [10] . These separation processes are energy-intensive, require signifi cant capital investment and support services. Moreover, these processes are only considered economically viable at a larger scale. Biogas has other various applications (Figure 2 ) like electricity generation, lighting, cooking fuel and simultaneously formation of organic manure [8] . Biogas maintains the aspects of health and hygiene for rural areas.

3. Feedstocks for biomethanation systems

Feedstocks can be proteins from waste, fats, or carbohydrate cellulose which, with the help of microbes, can be converted into biogas energy. Table 1 shows the feedstocks availability for biogas production. Categories considered for biogas pro-duction are mentioned below.

3.1. Animal and human waste resources

These include cattle, sheep, goat and pig wastes (dung, lit-ter, urine, and meat), fi shery wastes, poultry wastes, slaughter house waste and human waste, etc.

3.2. Lignocellulosic waste

Lignocellulosic waste includes energy crops, agricultural residues, deoiled seed cakes, weeds, spoilt fodder, leaves and urban solid waste, etc.


M. Krishania et al.: Improvement of process technology for biomethanation processes 51

3.3. Industrial waste

Industrial waste includes that from distilleries , dairy plants , pulp and paper industries , poultry industries , sugar industries , food processing industry, and kitchen waste.

3.4. Aqueous waste materials

Aqueous waste materials include sea weeds, marine algae and water hyacinth, etc. The volume of biomass from agricultural waste and crop residue in India is 278,710 kt/year [1] .

Agricultural residues are a good source for anaerobic diges-tion, due to the large availability of biomass. Agricultural resi-dues contain 15 – 20 % lignin and the C/N ratio varies from 130 to 150 [8] . Because of these complex characteristics (Table 2 ), agricultural residues need pre-treatment and additives to

enhance the biogas production [29] . Figure 4 shows the struc-ture before and after pretreatment.

4. Pretreatment for biomethanation systems

Agricultural residues or lignocellulosic biomass are the most abundant renewable resources to be converted to biofuels. An agricultural residue contains a high percentage of lignocel-luloses, which are hardly biodegradable by anaerobic bacteria (Figure 5 ) [22] . Chemical pre-treatments have proven to be promising methods to improve biogas production of ligno-cellulosic biomass [22] . Extensive research has been done to develop various effective pretreatment techniques on different varieties of lignocellulosic biomass feedstocks; none has been commercialized for lignocellulosic biomethane production

Hydrolysis

Lignocellulosic wasteCelluloses, hemicellulose, lignin, extractives, etc.

DelignificationIndustrialchemicals

Furfural, coal,char, biosorbents,ion exchangers

Pyrolysis

Microbialcultivation

Humification

SSF

Biofertilizers

Enzymes

Bioethanol

Carboxylicacids

VFAs

Methanation

Biogas

BiohydrogenAcidogenesis

Combustion,bioconversion

Biodiesel

Transesterification

Fermentation

Reducingsugars

Food, feed andmedicine

Figure 3 Biomass to biofuels [11].

Table 1 Feedstocks for biogas production.

Feedstocks Total waste (fresh)

Gas yield (m 3 /kg of dry matter) Pretreatment requirement References

Waste (kg/day/head)

Cattle 10 – 15 0.34 No [8, 12] Poultry 0.75 0.46 No [8, 13] Sheep 0.75 0.37 No [8, 14] Pig 1.3 0.39 Yes [8, 15] Night soil 0.75 0.38 No [8, 16] Kitchen 0.25 0.30 Yes [8, 17]

Residue yield (tonnes/ha)

Rice straw 1.2 0.61 Yes [8, 18] Wheat straw 3.5 0.41 Yes [8, 19] Water hyacinth 5 0.40 Yes [8, 20] Marine algae 3.3 0.40 No [8, 21]



Figure 4 Structure of lignocellulosic residues before and after pretreatment.

Table 2 Properties of common lignocellulosic materials.

Biomass Cellulose ( % )

Hemi-cellulose ( % )

Lignin ( % )

Volatile solid ( % )

Fixed carbon ( % )

Ash ( % )

Heating value ( % )

References

Wheat straw 39.2 26.1 15 80.6 11.7 7.7 18.9 [22, 23] Rice straw 44.3 33.5 20.4 72.7 11.8 15.5 14.5 [23, 24] Rice husk 34.4 29.3 19.2 59.5 11.8 17.1 12.3 [24, 25] Sugarcane bagasse

45.0 20.0 30.0 70.9 7.0 22.1 14.3 [23, 26]

Maize stover 37.5 30.0 8.4 75.2 19.3 6.9 16.5 [27, 28]

due high cost. Only a few of pretreatments are reported. Pre-treatment is aim at the improvement of anaerobic digestion effi ciency and the choice of suitable reactors makes the pro-cess even more effi cient [23, 24] . The different pretreatments for lignocellulosic materials, are briefl y explained below.

4.1. Alkali pre-treatment

Alkali pre-treatment is done with NaOH, Ca(OH) 2 (lime) or ammonia to remove lignin. This hydrolysis process is for breaking the lignin and cellulose linkages to make a part of the hemicelluloses and cellulose easily accessible to enzymes. NaOH is the best and capable of increasing biogas production by 16 % [30] . Furthermore, treatment of the sludge with dilute NaOH (e.g., 1.6 g/l) at room or low temperature (25 – 35 ° C) helps to improve the volatile solids (VS) removal by 40 – 90 % [31] . A similar treatment by 5 g/kg NaOH on municipal solid waste, has also improved the formation of biogas by 35 % [32] . Alkaline pre-treatment proved to be more effective on agricultural residues than on wood materials [33] .

4.2. Ammonia fi ber explosion (AFEX)

AFEX is one of the alkaline physico-chemical pre-treatment processes. In this treatment, substrates are exposed to liquid ammonia at a relatively high temperature (e.g., 90 – 100 ° C) for

a period of, e.g., 30 min, followed by an immediate reduction of pressure [34] . The main advantage of this treatment is that no inhibitory intermediates are formed during this process (e.g., furfural) and it is recyclable after pretreatment (protect the environment). The important parameters in the AFEX treat-ment are ammonia concentration, water quantity, temperature, pressure reduction and time [35] . For example, the optimum conditions in the pretreatment of switchgrass were reported to be about 100 ° C, ammonia loading of 1:1 kg of ammonia/kg of dry matter, and 5 min retention time [36] . There are also some disadvantages of using the AFEX process compared to some other processes. AFEX is a more suitable treatment for biomass that contains less lignin, and the AFEX pretreatment does not signifi cantly solubilize hemicellulose compared to other pre-treatment processes such as dilute-acid pretreatment [34] .

4.3. Steam treatment

In steam explosion, pre-treatment of wheat straw is fi rst treated with steam at 200 – 220 ° C and 15 – 22 bars. Then, the residue is delignifi ed by 2 % H 2 O 2 at 50 ° C for 5 h under pH 11.5 [37] . The steam explosion pre-treatment results in a signifi cant loss of hemicelluloses, and about 11 – 12 % lignin removal, while the alkaline peroxide post-treatment results in 81 – 88 % removal of the original lignin, which altogether removes 92 – 99 % of the original lignin from wheat straw [38] . Steam pretreatment also includes a risk on the production of compounds, like furfural, hydroxymethylfurfural (HMF), and soluble phenolic compounds. These compounds inhibit meth-ane production and reduce the digestibility.

The effect of thermal steam/liquid hot water (LHW) pre-treatment is improved by adding an external acid. Addition of an external acid lowers the optimal temperature, catalyzes the solubilization of the hemicellulose, and gives a better enzy-matic hydrolysable substrate. Compared to steam pretreat-ment the liquid hot water has the major advantage is that the solubilized hemicellulose and lignin products are present in lower concentrations due to higher water input in the treat-ment [34] .

4.4. Acid/thermochemical pre-treatment

Acid treatment of lignocellulosic materials at a high tempera-ture can effi ciently enhance the enzymatic hydrolysis. Sulfuric acid is the most applied acid, while other acids such as hydro-chloric acid and nitric acid can also be used. Dilute-acid pre-



treatment can be performed either at short retention times (e.g., 5 min) at high temperature (e.g., 180 ° C) or in a relatively long retention time (e.g., 30 – 90 min) at lower temperatures (e.g., 120 ° C) [30] . Dilute-acid hydrolysis is the most commonly applied method amongst the chemical pre-treatment methods. At elevated temperatures (e.g., 140 – 190 ° C) and low concen-trations of acid (e.g., 0.1 – 1 % sulfuric acid), the dilute acid treatment can achieve high reaction rates and signifi cantly improve cellulose hydrolysis. Almost 100 % hemicellulose removal is possible by dilute-acid pretreatment. Dilute acid pre-treatment can disrupt lignin and increases the susceptibil-ity of cellulose to enzymatic hydrolysis, but it not effective in dissolving lignin [37] . Thermo chemical pre-treatment of chicken manure with NaOH and H 2 SO 4 at 100 ° C was found to increase both methane yield and biodegradability [38] .

4.5. Radiation treatment

Lignocellulosic biomass can easily be treated by the use of high energy radiation methods for digestion ( γ -ray, ultra-sound, electron beam, pulsed electrical fi eld, UV, and micro-wave heating) [39] . High energy radiations methods are usually slow, energy-intensive, and prohibitively expensive. The high energy radiations change the lignocellulosic struc-ture by enhancement of a specifi c surface area, decrease of the degrees of polymerization and crystallinity of cellulose, hydrolysis of hemicellulose, and partial depolymerization of lignin [32] .

Ultrasound pretreatment has been started to be used for biogas production. In this method, infl uential factors are the enzyme, particle size, acid concentration, duration of ultra-sonication and power of ultrasound. Under optimal conditions [using enzyme, particle size < 0.18 mm, acid concentration (v/v) 3 % , ultrasound power 120 W and 180 s ultrasound time] the yield of sugars was 26.01 (g/l) which was 94.49 % of the expected [34] . Braguglia et al. reported that activated sludge was sonicated at an energy input of 5000 or 2500 kJ/kg dry solids, corresponding to a disintegration degree of approxi-mately 8 or 4 % , respectively. Sonication proved to be effec-tive both in increasing volatile solid (VS) destruction and cumulative biogas production. In addition to sonication, other methods such as cavitation, repeated freezing and defreezing, and heating at low temperatures of, e.g., 60 – 170 ° C for 5 – 30 min or high temperatures of 180 – 200 ° C for 10 s, can improve cell disruption and lysing [34] .

4.6. Milling

Milling can improve susceptibility to enzymatic hydrolysis, by reducing the size of the material, and degree of crystallinity of lignocelluloses. This helps to improve the enzymatic deg-radation of these materials towards biogas production [31] . Methane yield increases with decrease in particle size. Biogas production from rice straw with combination of grinding, heating, and ammonia treatment (2 % ) results in the highest biogas yield [12] . In another study, the grinding of municipal

OH

OH

OH

OH

Cellulose

Hemicellulose

Lignin

Biogaswith low yield and

productivity and high residue

Biogaswith high yield and

productivity and fewer residues

Without pretreatment

Microfibril

Pretreatment

Degradingenzymes

?

Macrofibril

Cellulose fiber

Lignocelluloses

Solid waste

Degradingenzymes

OH

OH

OHOHO

O O

O O

O O

O OCH2OHCH2OH

CH2OH CH2OH

Figure 5 Effect of pretreatment on accessibility of degrading enzymes.



solid waste from 2.2 to 1.1 mm had no effect on mesophilic digestion, but improved thermophilic digestion by 14 % [31] .

Other pretreatments, like concentrated acids, wet oxidation, solvents and metal complexes are effective, but too expensive compared to the value of glucose [40] . In oxidative pretreat-ment, often a lot of sugars get lost, because of non-selective oxidation. Also, soluble lignin compounds are formed, which can be inhibiting in the subsequent conversion step of the hemicellulose to methane. Carbon dioxide explosion based pretreatment is diffi cult to judge, as not much research has been done on this process. The effect of the pretreatments is, however, infl uenced by biomass composition and operating conditions. All of these pretreatments have advantages and disadvantages and future research is needed for optimization. Currently, research into pretreatment is facing several techno-economic challenges, including the environmental impact, delignifi cation, co-fermentation, pretreatment product digest-ibility, energy demand and processing costs. Future research should address these challenges to promote the commercial-ization of agricultural residue biomethanation.

5. Optimization of operational parameters for

biomethanation

5.1. Temperature

The optimized temperature range in thermophilic (50 – 60 ° C), mesophilic (32 – 35 ° C) and psychophilic (up to 20 ° C) is already known. A constant temperature is important for pre-venting negative effects on biogas production. The growth rate of microbes is higher at a thermophilic temperature; the pro-cess becomes faster and more effi cient. However, hydraulic retention time (HRT) depends on the temperature; the higher the temperature, the lower the HRT. The thermophilic tem-peratures also result in imbalance and high risk for ammonia inhibition. An increase in temperature also increases ammo-nia toxicity [4, 41] . Mesophilic temperature digesters have improved degradation rates when compared with a thermo-philic digester, but HRT is higher at a mesophilic temperature [42] . Methanogenesis is effective at a thermophilic temper-ature. The temperature needs to be optimized for single or multi stage reactors [43] .

5.2. pH

The biomethanation process takes place within a relatively narrow pH range, from 6.5 to 8.5 already known. It is neces-sary for the pH to be in the desired range because it directly effects the growth of microbes. The optimal pH of metha-nogenesis is around pH 7.0; the optimum pH of hydrolysis and acidogenesis has been reported to be between 5.5 and 6.5 [44] . This variation in pH makes the biomethanation pro-cess work effectively in a two-stage process; hydrolysis/aci-dogenesis and acetogenesis/methanogenesis separately. The pH value increased with ammonia accumulation during the degradation of protein, while the accumulation of volatile fatty acid (VFA), which is due to the degradation of organic

matter (1 g of volatile acids produced/g of volatile solids) [43] decreases the pH value. This is important to maintain the pH in the desired range for effi cient gas production.

5.3. Fatty acids

Fatty acids are a key intermediate in the biomethanation pro-cess, which is also capable of inhibiting methanogenesis in high concentrations. Biomethanation processes will alter the pH, particularly the production of fatty acids. In the process, fermentation of sugar is inhibited by total VFA concentra-tions above 4 g/l [45] . The most common VFAs present in anaerobic digesters are acidic acid and propionic acid; con-centrations > 3000 mg/l have previously been shown to cause digester failure [46] . A sudden increase in the organic load-ing rate is expected to cause an accumulation of high VFAs, since acetogens grow at a slower rate, subsequent to a drop in pH (high VFAs inhibit acidogensis). The formation of vola-tile fatty acids from fats/lipids and ammonia from proteins beyond a particular range, inhibits the methane production. Free ammonia and excess VFAs inhibit methanogenesis [47, 48] .

5.4. Solid concentration

The degradable part of feed material in a unit volume of slurry is defi ned as solid concentration. The total solids (TS) concentration of the waste infl uences the pH, temperature and effectiveness of the microorganisms in the decomposi-tion process. The solid concentration is optimized according to the reactor design. Normally 7 – 9 % solid concentration is best suited for fl oating dome reactors [6, 49] . The continuous stirred tank reactor (CSTR) was simulated over a range of % TS concentration of 4 – 10 [50] . Table 3 shows the % TS in different feedstocks for biomethanation. A high organic load-ing rate (OLR) reduces the HRT and capital cost generated by the size of digesters.

5.5. Hydraulic retention time (HRT)

Most anaerobic systems are designed to retain the waste for a fi xed number of days. The number of days the materials stay in the tank is called the hydraulic retention time or HRT [52] . The HRT is equal to the volume of the tank divided by the daily fl ow [HRT = volume (V)/fl ow (Q)]. In tropical countries like India, HRT varies from 30 to 50 days; it varies accord-ing to weather changes [53] . HRT is important since it estab-lishes the quantity of time available for bacterial growth and subsequent conversion of the organic material to gas. The HRT varies with the feedstocks, concentration of solids and temperature. An increase in temperature reduces the HRT of substrate into the digesters.

5.6. C:N ratio

The C/N (carbon to nitrogen) ratio in the feedstock is focused, because a high-level of nitrogen ( > 80 mg/l) as undissociated ammonia with a low C/N ratio can cause toxicity, and a low-



level of nitrogen with a high C/N ratio, can inhibit the rate of digestion. It is necessary to maintain a proper C/N ratio of substrate in the desired range. During the biomethanation pro-cess, microorganisms utilize carbon 25 – 30 times more than nitrogen [6] . The C/N ratio should also be optimized accord-ing to the type of reactor; the two-stage reactor has been reported as a reliable process with C/N ratios of < 20 [53] . Table 3 shows the C/N ratios of different organic wastes.

5.7. Mixing

Mixing is a physical operation which creates uniformities in fl uids and eliminates any concentration and temperature gra-dients. The main aim of stirring the digester contents is to provide an intimate contact between microorganisms and the substrate for enhancing the biomethanation process. Mixing does not always take place continuously, because excessive mixing can reduce biogas production. It is reported that slow mixing allow the digester to better absorb the disturbance of shock loading than high mixing of the reactor contents [54] . Excessive mixing can disrupt the structure of the granules (microbial biomass); reducing the rate of oxidation of fatty acids can lead to digester instability [55, 56] .

5.8. Additives and nutrients

For the growth and survival of microorganisms, nutrients are required for effi cient biogas production. For proper anaerobic digestion, a nutrient ratio of C:N:P:S = 600:15:5:1 is required [3] . Many trace elements are important for developing the biofi lm in reactors, like Fe, Zn, Cu, Ni and Co. Fe at 5 mg/l, Zn at 1 mg/l, Cu at 0.1 mg/l, Ni at 1.2 mg/l and Co at 4.8 mg/l improve the biomethanation process [10] . Addition of nickel can increase acetate utilization rate of methane form-ing bacteria, as the F430 enzyme in the bacteria contains nickel [57] . For digestion of agricultural residues, the addi-tion of micronutrients and macronutrients are absolutely nec-essary. Besides these elements, some substrates also enhance the biogas production and its composition. Addition of pectin to cattle dung slurry increases the biogas yield. It not only enhances biogas, but also imparts stability to the process, dur-ing a period of fl uctuating temperatures [58] .

Addition of 5 % commercial charcoal to cow dung slurry on a dry weight basis, raised the yield by 17 – 35 % [59] . Addition

of inert materials like vermiculite, bagasse, gulmohar leaves, wheat straw, ground nut shells and lugiminous plant leaves, increase the biogas yield, gas composition and extent of bio-degradation [41] .

5.9. Microbial biomass

At least 300 different species of bacteria are found in the feces of a single individual. Most of these bacteria are anaerobic and facultative anaerobic. E. coli is the common example. Bacteria may be divided into three groups: 1) aerobes, 2) facultative anaerobes, and 3) anaerobes, including the methane-forming bacteria. Aerobes are active and can degrade a substrate only in the presence of free oxygen. However, strict aerobes die in an anaerobic digester because of the unavailability of oxy-gen. During the degradation of waste within an anaerobic digester, facultative anaerobic bacteria like the Enterobacter spp ., produce a variety of acids and alcohols, carbon dioxide and hydrogen from carbohydrates, lipids and proteins [43] . Anaerobes are active in the absence of oxygen and some anaerobes are strong acid producers, such as, Streptococcus spp. [60] . In anaerobic digestion, strict anaerobes, methano-gens, are used to convert the acetate, alcohol, carbon dioxide and hydrogen into methane by methane forming bacteria like Methanobacterium, Methanococcus etc. For effi cient degrada-tion of waste in biomethanation, a specifi c group of microor-ganisms is necessary. Anaerobes survive and degrade substrate most effi ciently when the oxidation-reduction potential (ORP) of their environment is between 200 and 400 mV [43] .

6. Reactor design

Various digester confi gurations are employed using differ-ent approaches, such as one-stage or two-stage digesters, wet or dry/semi-dry digesters, batch or continuous digesters [61] , attached or non-attached biomass digesters, high-rate digesters and digesters with a combination of different approaches [62] . The fermentation works out normally with the solid content (6 – 10 % TS) known as wet fermentation and at a high concen-tration ( > 20 % ) known as dry fermentation. Some researchers have reported that the fermentation can proceed at TS concen-trations up to 32 % [16] . The gas production varies considerably with time, and several units must be operated simultaneously to

Table 3 The solid content and C/N ratio of some common organic materials [51] .

Materials Dry matter content ( % ) Water content ( % ) C/N ratio

Dry rice straw 83 17 70Dry wheat straw 82 18 90Green grass 24 76 37Human excrement 20 80 8Pig excrement 18 82 18Cattle excrement 17 83 24Poultry waste 47 53 10Water hyacinth 18 82 25Pongamia deoiled cake 92.5 7.5 8.7



maintain a constant gas supply [8] . Major operation problems during digestion of agricultural residues are listed below:

Untreated agricultural biomass feeding into a conventional • biogas reactor is diffi cult. Additional process of pretreatment required for agricultural • biomass. Poor decomposition due to fl oating and scum formation. • Volatile fatty acid (VFA) over-production, leading to di-• gester failure.

The choice of reactor type is determined according to waste characterization, particularly by solid contents. In industrial

countries such as in Europe, renewable energy production, including biogas production, is becoming more and more popu-lar. In the agricultural and industrial sectors, high-tech digesters of various kinds are implemented. High total solid substrates are mainly treated in CSTRs, while soluble organic wastes are treated using up fl ow anaerobic sludge blanket (UASB) reac-tors [24] . Figure 6 shows the different types of bioreactors used for biogas production from different wastes. CSTR and plug fl ow reactors are best for agricultural residues [22, 24] . Table 4 lists the suitable digesters for the different substrates.

Batch biogas reactors (Figure 6 A ) are mainly used for experimental setups. These digesters require one time feeding

Shaker

Digester Collector Receiver

InletA

C

E

D

F

B Gas outlet Cover seal with clayLoose cover

Gas

Slurry

PC

M

Reactants

Coolingwater exit

Partitionwall

Outletpit

Outlet

Liquid level

Inlet

Slurry

Central guide

Gas outletGasholder

Mixingpit

Inletpipe

Gas outlet

Liquid outletSettler/gas separator

Sludge blanket

Liquid

Liquid inlet

Coolingwater enter

Products

CSTR(Jacketed)

TC

Displacement tankOutlet pipe

Figure 6 Showing different types of common biogas reactors in India: (A) laboratory batch reactor; (B) fi xed dome reactor; (C) fl oating dome reactor; (D) continues stirrer tank reactor (CSTR); (E) plug fl ow; and (F) up fl ow anaerobic sludge blanket (UASB) [61, 62] .



and a particular HRT for the analysis. The design is suitable for batch fi lling of any fermentable materials. A major disad-vantage is that their gas-output is not steady. Mostly lab scale analysis is suitable for these reactors.

The fi xed-dome reactors (Figure 6 B) contain reactors with a fi xed gas holder, which is situated on top of the digester. Gas pressure increases with the volume of gas stored and the height difference between the slurry level in the digester and the slurry level in the compensation tank [72] . In comparison with the fl oating drum, the relative construction costs are low, due to the absence of moving parts and rusting steel parts. If well constructed, fi xed dome plants have a long life span [8] . Underground construction saves space and protects the digester from temperature changes. It is generally used for low TS % (6 – 10) [29] , e.g., animal waste and human waste.

Floating-drum reactors (Figure 6 C) consist of an under-ground digester and a moving gas-holder also known as Khadi and village industries commission (KVIC) [29] . The gasholder fl oats either directly on the fermentation slurry or in a water jacket of its own. The gas is collected in the gas drum, which rises according to the amount of gas stored. The gas pressure is constant, determined by the weight of the gas holder [72] . The construction is relatively easy; construction mistakes do not lead to major problems in operation and gas yield. They are mainly used for animal waste with 2 – 9 % TS [8] .

Continues stirring is available in this CSTR reactor (Figure 6 D) for perfect mixing, due to the high exposure of substrate and enzyme taking place. CSTR reactors are most frequently used for liquid substrates in a range of 2 – 8 % TS concentration, but they can handle more solid reactions up to 22 % TS [73] . Energy crops and crop residues can be digested either alone or in co-digestion with other materials, employing either wet or dry processes. In the agricultural sector, one possible solu-tion to processing crop biomass is co-digestion together with animal manures in a CSTR reactor [74, 75] . Animal manures typically have low solids content ( < 10 % TS). Agricultural residues have high solids content (90 % TS), and thus, the

anaerobic digestion technology applied in co-digestion pro-cessing mostly uses the CSTRs.

Previous experiences have shown that fl oating tendencies of most biomass feedstocks is a major problem; plug fl ow biogas reactors (Figure 6 E) using agricultural biomass feed-stocks, must be designed for horizontal movement of digest-ing biomass. Plug fl ow reactors, rectangular in plan, are ideal for this situation [76] . Most agricultural biomass feedstocks quickly fl oat, unless the digestor contents are continuously agitated [77] . Plug fl ow is only used for partially fl oating sol-ids. Plug fl ow digestors are liquid-based and may need larger length:width ratios to avoid mixing.

An up fl ow anaerobic sludge blanket (Figure 6 F) fuses with an anaerobic process, whilst forming a blan-ket of granular sludge which suspends in the tank [78] . Wastewater fl ows upwards through the blanket and is pro-cessed (degraded) by the anaerobic microorganisms. The upward fl ow, combined with the settling action of grav-ity, suspends the blanket with the aid of fl occulants. Such reactors are generally used for industrial waste water, and typically suited to dilute waste water stream (3 % TS with a particle size < 0.75) [73] .

Many other varieties of reactors have been developed for different types of feedstocks [6] .

Based on a good planning, installation and operation know-how, most of these plants are running economically on a regu-lar basis. In order to improve the performance of anaerobic digestion, research is continuously going on in various reac-tors. Some examples of innovation reactors are DRANCO, BIMA, COMPO GAS, IBR etc.

7. Conclusion

Until now, biogas production was mainly carried out with biomass with a low percentage of lignin. Inclusion of highly lignifi ed biomass for production of biogas is neces-sary to meet energy requirements for future generations.

Table 4 Suitable digesters for the different substrates.

Reactor types Waste Biogas yield HRT (days) Temperature ( ° C) TS ( % ) References

Batch Cattle dung 0.29 m 3 /kg VS 15 atm. T 3.5 [63] Agricultural waste 0.21 m 3 /kg TS 53 atm. T 5 [56] Plant biomass 0.68 m 3 /kg VS 180 15 – 35 5 [64] Food waste 0.3 1 m 3 /kg VS 20 35 3 [65]

Floating type Cattle dung 0.27 m 3 /kg VS 50 atm. T 5.28 [61] Agriculture waste – – – – – Food waste 0.5 m 3 /kg TS 4 atm. T 5 [66]

Plug fl ow Cattle dung 1.31 m 3 /m 3 /d 15 35 15.2 [67] Leafy waste 0.5 m 3 /m 3 /d 35 18 [68]

CSTR Cattle dung 0.5 m 3 /m 3 /d 35 37 9.2 [62] Agricultural waste 0.29 m 3 /kg COD 20 55 1.9 (COD) [24] Industrial water 0.66 – 1.47 m 3 /m 3 /d 1 30 N.A. [69]

UASB Agricultural waste 0.267 m 3 /kg COD 20 55 2.8 (COD) [24] Food waste 25 l/ g VS 16 35 4.5 [70] Industrial water 0.1 m 3 /kg COD 0.6 atm. T – [71]



Presently, agriculture residue usage is limited to the produc-tion of producer gas in which the burning process ruins the equally important fertilizer and compost value of biomass. Degradation of lignifi ed biomass is diffi cult, but it could be done through pre-treatment, use of additives and suitable reactor designing, which can also solve the major limitations like low gas production from agricultural residues in win-ters and large hydraulic retention time. Further, optimization of all operating parameters like pH, temperature, C/N ratio, HRT and inoculums, help to fully harness the biomethana-tion technology, maximizing the production of biomethane per unit of biomass

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Received September 21, 2011; accepted November 29, 2011

Ms. Meena Krishania is pur-suing her PhD at the Indian Institute of Technology Delhi. She received a BTech (Biotechnology Engineering) degree from Rajasthan University with Honors in 2007 and an MTech (Chemical Engineering) degree from MNIT Jiapur in 2009 with third rank. She has published around 10 papers in

different international journals, national and conferences. She has written a book for international publication. Her research interests include biofuels and renewable resources.


Opportunities for improvement of process technology for biomethanation processes

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