Biogas Generation through Anaerobic Digestion ...

Research Journal of Chemistry and Environment_________________________________________

Vol.18(5) May (2014) Res. J. Chem. Environ

Review Paper:

Biogas Generation through Anaerobic Digestion ProcessAn Overview Deepanraj B. 1, Sivasubramanian V.2 and Jayaraj S.1 1. Department of Mechanical Engineering, National Institute of Technology Calicut, Kerala, INDIA 2. Department of Chemical Engineering, National Institute of Technology Calicut, Kerala, INDIA *[email protected]

multiple benefits to the users and can protect the environment. This system produces biogas, a combination of methane and carbon dioxide with little amount of some other gases through anaerobic digestion of organic materials. In anaerobic digestion the organic matter is decomposed by the intensive reaction of a large range of microorganisms in the absence of oxygen 3. The process consists of a complex series of reactions that convert a wide array of polymeric substances such as carbohydrates, proteins and lipids having carbon atoms at various oxidation and/or reduction states, to one-carbon molecules in its most oxidized state (CO2) and its most reduced state (CH4).

Abstract Depleting petroleum resources, ever increasing petroleum price and the rapid addition of greenhouse gases into the atmosphere rekindle the researchers to develop new techniques to obtain clean and sustainable energy from the renewable sources. There are many such resources existing in reality and ready to serve the mankind on demand. Out of the many renewable options, wind, solar and biomass energies are considered as the major sources. This paper focuses on biogas technology which is a very attractive way to utilize biomass sources for fulfilling partial energy requirements. Biogas system can provide multiple benefits to the users and aid to protect the environment. Biogas systems produce biogas through anaerobic digestion of organic materials.

In 1630, Jan Baptista Van Helmont first tested the release of flammable gases from decaying organic matter4. In 1776, Count Alessandro Volta experimented and found that the amount of inflammable gas produced had a direct correlation with the amount of decaying organic matter4,5. In 1808, Sir Humphrey Davy investigated that methane was available in the gases produced during the anaerobic digestion of cattle manure and collected 0.3 liter of methane from cattle manure4. In 1875, Wouter Sluys, a Dutch farmer used methane first for the purpose of illumination. In 1895, England recovered biogas by applying the sewage treatment and the same was used in street lamps4,6. The first biogas plant in India was constructed during 1897 in Bombay7.

All biodegradable biomass materials are suitable for feeding into biomass digesters. Common feedstock includes agricultural wastes, crop residues, animal wastes, forest residues etc. This paper presented an overview of anaerobic digestion process including various stages and microorganisms involved in the biogas production, the parameters that affect biogas production, types of digesters and the byproducts. It also discusses the results of the experiments conducted using various feedstock.

In the 1930s Boswell made the most important scientific breakthroughs in the history of development of biogas technologies by combining different types of organic waste with manure as substrate. Also, he developed microbiology to identify anaerobic bacteria and the environment to promote effective methane production6, 8.

Keywords: Biogas, Anaerobic digestion, Bio-methanation, Bioenergy.

Introduction

During World War II, due to the shortage of crude oil, biogas was used as an alternate fuel to drive German trucks. After the war, biogas technology was not popular because of cheap crude oil availability4. After the crude oil crisis in 1970s, research on biogas technology development picked up speed and again started taking part in the supply of energy for our daily usage.

The availability of conventional fuel resources, increase in fuel cost and the increasing concern for environmental issues have challenged researchers to develop new techniques to obtain clean and sustainable energy through the utilization of renewable energy sources.1,2 Biogas technology introduces a very attractive way to utilize biomass sources for fulfilling partial energy requirements. Biogas system can provide

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stage and the last stage is called as methane formation stage. The chemical reactions and the bacteria involved in all the four stages are given in table 1 and the process flow chart is shown in fig. 1.

Stages of anaerobic digestion Anaerobic digestion process is a well understood chemical reaction which occurs in four different stages. They are hydrolysis, acidogenesis, acetogenesis and methanogenesis9, 10 . The second and third stages are called as acid formation

Table 1 Chemical reaction and bacteria involved in the anaerobic digestion 16, 17 Stage

Type of conversion

Bacteria involved

Stage-I Hydrolysis (C6H10O5)n + nH2O = n(C6H12O6)

Proteins to soluble peptides and amino acids

Clostridium, Proteus vulgaris, Vibrio, Bacillus, Peptococcus, Bacteriodes,

Carbohydrates to soluble sugars

Clostridium, Acetovibrio celluliticus, Staphylococcus, Bacteriodes

Lipids to fatty acids or alcohols

Clostridium, Micrococcus, Staphylococcus

Amino acids to fatty acids, acetate and NH3

Lactobacillus, Escherichia, Bacillus, Staphylococcus, Pseudomonas, Sarcina, Desulfovibrio, Selenomonas, Streptococcus,Veollonella, Desulfobacter, Desulforomonas.

Sugars to intermediary fermentation products

Clostridium, Eubacterium limosum, Streptococcus

Stage III Acetogenesis CH3CH2OH + H2O  CH3COOH + 2H2 2CH3CH2OH + 2CO2  CH4 + 2CH3COOH CH3CH2COOH + 2H2O  CH3COOH + 3H2 + CO2 CH3CH2CH2COOH + 2H2O  2CH3COOH + 2H2 CH3CHOHCOOH + H2O  CH3COOH + CO2 + 2H2

Higher fatty acids or alcohols to hydrogen and acetate

Clostridium, Syntrophomonas wolfeii

Volatile fatty acids and alcohols to acetate or hydrogen

Syntrophomonas wolfei, Syntrophomonas wolinii

Stage IV Methanogenesis CH3COOH  CH4 + CO2 CO2 + 4 H2  CH4 + 2H2O

Acetate to methane and carbondioxide

Methanosaeta, Methanosarcina

Hydrogen and carbondioxide to methane

Methanobacterium formicicum, Methanobrevibacterium, Methanoplanus, Methanospirilum

Stage-II Acidogenesis C6H12O6 + 2H2O  2CH3COOH + 4H2+CO2 C6H12O6 + 2H2  2CH3CH2COOH + 2H2O C6H12O6  CH3CH2CH2COOH + 2H2+ 2CO2 C6H12O6  2CH3CH2OH + 2CO2 C6H12O6  2CH3CHOHCOOH

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Fig. 1: Flow chart for stages of anaerobic digestion process 12 Hydrolysis: The first stage in the anaerobic digestion process is hydrolysis. In this stage, the complex organic polymers are converted into simple soluble molecules. During this stage, the lipids (fats) are converted into fatty acids, carbohydrates (polysaccharides) into simple sugars (monosaccharides) and proteins into amino acids11. Hydrolysis stage is carried out by different groups of facultative or obligate fermentative bacteria through excreting extracellular enzymes13. Lipases convert lipids to long-chain fatty acids, proteases converts proteins to amino acids and the polysaccharides such as cellulose, starch andpectin are hydrolyzed to monosaccharides by cellulases, amylases and pectinases 14.

other products are taken to third stage for further degradation by acetogens. Acetogenesis: Third stage of anaerobic digestion process is acetogenesis. In this stage, the volatile fatty acids having more than two carbon atoms (from acidogenesis stage) are converted into acetic acids, hydrogen and carbon dioxide with the help of acetogens 9. Methanogenesis:In the last stage, the methanogenic bacteria (methogens) produce methane by consuming acetic acid, hydrogen and some carbon dioxide. Around 66% of methane is formed from acetic acids by means of acetate decarboxylation and remaining 34% of methane is formed from carbon dioxide reduction 16.

Acidogenesis: In the second stage, the soluble compounds produced through hydrolysis are converted into volatile fatty acids (C1-C5), hydrogen, carbon dioxide, ethanol and some organic nitrogen and sulfur compounds4. The acids produced in this stage are acetic acid (CH3COOH), propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH) and valeric acid (CH3CH2CH2CH2COOH)15. The acetic acid formed in this stage is directly taken to last stage and the

Parameters affecting anaerobic digestion process There are many factors that affect the performance of anaerobic digestion. Some of the factors which are having major influence in anaerobic digestion are elaborated below:

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Research Journal of Chemistry and Environment_________________________________________ Temperature: Temperature is the most important parameter to be considered in anaerobic digestion. Different species of methogens function optimally in three different temperature ranges11 : 45–60°C thermophilic, 20–45°C mesophilic) and below 20°C psychrophilic. The rate of biogas production increase with an increase in temperature. In biogas digestion process, only mesophilic and thermophilic temperature ranges are considered important because anaerobic digestion reaction essentially stops below 10°C11. The bacteria available for digestion process are sensitive to temperature fluctuation, so, it is necessary to maintain a constant temperature. Thermophilic bacteria are more efficient in terms of retention time, loading rate and gas yield, but they need higher heat input and are also sensitive to temperature fluctuations and environmental variables than mesophilic7, 11. Influence of temperature on the rate of anaerobic digestion process is shown in fig. 218.


below the optimum range and can inhibit methanogens, because they are very sensitive to acid conditions9,11. Reduction in pH can be controlled by the addition of chemicals such as sodium carbonate, sodium bicarbonate, gaseous ammonia, ammonium hydroxide, lime, potassium and sodium hydroxide 9. Sambo et al21 investigated the effect of pH (4, 7 and 9) using cow dung as feed and reported that pH 7 gives maximum biogas yield followed by 9 and then 4. Sivakumar et al37 studied the biogas potential from spoiled milk and effect of pH investigated by them is shown in fig. 3.

Fig 3: Effect of pH anaerobic digestion of spoiled milk37

Retention period: The time period for which the organic material remains inside the digester for biogas generation is known as retention period. The retention period will vary depends on the type of feedstock and the temperature used 22. Solids retention time (SRT) and hydraulic retention time (HRT) are the two significant retention times in anaerobic digestion process. SRT refers the time that bacteria (solids) remain inside the digester. HRT is commonly used to denote substrate retention time. It is the time spent by the input slurry, inside the digester from the instant of its entry to its exit 4, 17.

Fig. 2: Influence of temperature on the rate of anaerobic digestion18 Solid to water content: Water and raw material should be added together to generate slurry with required consistency. Production of biogas is inefficient if the slurry is too dilute or too thick. The optimum solid concentration may vary from 725% depending on the type of raw material used 17. Sewage waste contains very low solid content and so optimum level can be achieved by adding solid matters like crop residues, weed plants etc. Budiyono et al19 experimented the effect of total solid contents (2.6, 4.6, 6.2, 7.4, 9.2, 12.3 and 18.4%. of TS) on biogas yield using cattle manure in a 400 ml batch digester and found that 7.4 and 9.2% of total solids achieved better performance on biogas yield than other total solid percentages.

Organic loading rate: Organic loading rate (OLR) is an important parameter which affects the biogas production in anaerobic digestion, particularly when the digestion takes place in continuous flow mode17. OLR is a measure of biological conversion capacity of the anaerobic digestion system 7. It can be expressed as the amount of raw material (kg of volatile solids) fed to the digester per unit volume per day 11, 4. Overloading easily affects the digestion process due to accumulation of acids. The optimum loading rate is in between 0.5 kg and 2 kg of total volatile solids per unit volume of the digester per day which can be chosen based on type of raw material, retention time and the process temperature 4.

pH level: The optimum pH level of the anaerobic digester is at 6.7 to7.5 20. The pH level will not be constant throughout the process. The volatile fatty acids production rate is much higher than the methane production rate, resulting in pH level

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Research Journal of Chemistry and Environment_________________________________________ C/N ratio: The relationship between the amount of carbon and nitrogen present in the raw materials is represented by the C/N ratio15. The carbon-nitrogen (C/N) ratio is one of the important factors in the production of biogas. The elements of carbon (in the carbohydrates) and nitrogen (in the form of proteins, ammonia nitrates) are the major food for anaerobic bacteria22. The consumption of carbon by bacteria is 30 times faster than the nitrogen consumption. Therefore, for optimum rate, the availability of carbon in the substrate should be 2030 times higher than nitrogen (i.e. C/N ratio between 20 and 30) 23-26.


before taken into the digester. Particle size is one of the important parameter which plays significant role in the biogas production. Smaller particle size leads to increase the substrate utilization because smaller particle size provides increased microbial activity16, 41. Sharma et al42 investigated the effect of particle size on agricultural and forest residues with five particle sizes (0.088, 0.40, 1.0, 6.0 and 30.0 mm) and reported that the maximum quantity of biogas was produced from 0.088 and 0.40 mm particle size. Ultrasonic pretreatment disintegrates the particles, creates favorable conditions for biodegradation and enhance biogas production40. Application of ultrasonic pretreatment increases daily biogas production and significantly reduces volatile solids during anaerobic digestion43. Braguglia et al44 investigated the waste activated sludge with ultrasonic pretreatment and reported that the biogas production of ultrasonic treated sludge was higher upto 30% than untreated sludge.

If the C/N ratio is high, then rapid consumption of nitrogen by methogens takes place and results in lower bio-gas production. Lower C/N ratio leads to ammonia accumulation and pH values exceeding 8.5 which is toxic to methogens27. To maintain the optimum C/N ratio in the digester, substrates of high C/N ratio can be co-digested with lower C/N ratio substrates 23. The effect of C/N ratio of various feeds on biogas production showed that C/N ratio of 26:1 gives maximum biogas yield compared to others. The typical C/N ratios for different organic materials available are shown in table 2.

Thermal pretreatment includes conventional thermal hydrolysis, microwave thermal treatment and steam explosion. Biogas production can be increased when the organic and inorganic compounds in the feedstock are partially solubilized during thermal pretreatment43, 45. During the thermal pretreatment, the organic and inorganic compounds in the feedstock are partially solubilized before hydrolysis which reduces digester volume and increases the biogas production43. Bien et al46 investigated the sewage sludge with thermal pretreatment (170-180°C) and found that the methane production increased by 25% compared to untreated sludge. Chemical pretreatment technique includes acid, alkaline, oxidation and ozonolysis pretreatment. Application of these chemical pretreatments results in higher solublization and biodegradation of cellulose, hemicelluloses and lignin, which are the main components of biomass 40. Moset et al47 evaluated the methane yield of pig slurry with acid pretreatment (H2SO4) and found that 10% acid treated pig slurry generates 20% higher methane yield than untreated slurry.

Mixing/agitation: Mixing or agitation is required in the digester to maintain homogeneity and process stability 29. Mixing helps to combine the fresh incoming material with microorganisms and prevents from thermal stratification and scum formation in the digester. Mixing maintains uniformity in substrate concentration, temperature and other environmental factors. Also, it prevents solid deposition at the bottom of the digester30. Mixing can be done either by using mechanical stirrers or by recirculation of the digester slurry using centrifugal pumps16. Some of the results for operating parameters from available literature are presented in table 3. Pretreatments In anaerobic digestion process, increase in biogas production and higher calorific value of the gas can be achieved by applying pretreatments to substrate26, 40. Pretreatment breaks down the complex structure of organic compounds into simpler molecules and makes them more susceptible to microbial degradation26. Pretreatment could be done by various techniques like mechanical, thermal, chemical, biological pretreatment etc40.

Biological pretreatment is carried out by using biological agent (microorganisms) with the substrates48. The microorganisms commonly used in the biological pretreatment are brown-rot fungi, white-rot fungi and soft-rot fungi, which degrade lignin and hemicelluloses in the substrate49. Brown-rot fungi mainly attack cellulose whereas white and soft rot-fungi attack both cellulose and lignin49. Some of the results for pretreatment from available literature are presented in table 4.

Mechanical pretreatments such as milling and ultrasonic pretreatments are used in anaerobic digestion process. Milling is used to reduce the particle size of the feedstock

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Table 2 Typical C/N ratios of different materials 4, 11 S.N. 1 2 3 4 5 6 7 8 9 10

Material Cow, buffalo, sheep, pig and horse manure Poultry manure Night soil Fish scarps Slaughter house waste Sawdust Grass clipping and hay Bagasse, Wheat and rice straw Corn stalks Kitchen vegetable scraps

N (%) 1.4-3.8

C/N ratio 15-40

6.3 6 6.5 7-14 0.1-0.25 2-4 0.3-0.5 0.8 3.3

5.2 6-10 5.1 2-4 200-511 10-20 120-150 60 16

Table 3 Effect of different operating parameters Parameter experimented Temperature

Feedstock

Parameter range

Result

Food waste

Temperature range from 30 to 55°C Three different temperature of 20, 35 and 45°C

Solid to water content Solid to water content

Sea weed (Laminaria Digitata) Water hyacinth Jatropha seed cake

Solid to water content

Water hyacinth

Highest gas production and COD removal rate was achieved at 50°C 40 Maximum biogas and methane yield produced at 35°C followed by 45 and 20°C 41 Maximum biogas yield obtained at 25g/l 42 Maximum biogas yield obtained at 20%TS followed by 15,25 and 10% TS 34 7% of total solids produced the best performance 44

pH

Cow dung

pH

Bioethanol waste Spoiled milk

Temperature

pH C/N ratio

Waste water and bagasse

C/N ratio

Beef cattle feces and water hyacinth

Six different solid concentrations from 5g/l to 30g/l Four different solid concentrations of 10, 15, 20 and 25% of total solids were investigated Substrates with five different solid concentrations 3, 5, 7, 9 and 11% of total solids were experimented Three different pH 4, 7, 9 were investigated Three different pH 6, 7, 8 were investigated Seven different pH (5, 5.5, 6, 6.5, 7, 7.5, 8) Seven different C/N ratios (6.62, 9.27, 13.19, 19.56, 24.53, 31.76 and 64.58) were investigated. Three different C/N ratios (20, 25 and 30)

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Better biogas yield was at pH 7, followed by 9 and 4 21 Better biogas yield was at pH 7, followed by 8 and 6 45 pH 7 produced better yield than other pH 46 C/N ratio of 25.53 gave better yield than others 47 Highest CH4 content was provided by C/N ratio of 30 39



Table 4 Effect of different pretreatment techniques in biogas production Pretreatment type

Feedstock used

Ultrasonic (31kHz for 64 sec)

Sewage sludge

Results obtained

Palm oil mill effluent

Volatile solid removal rate increases from 45.8% to 50.3% 50 16% increase in methane production than untreated effluent 51

Thermal (microwave, 175°C)

Waste activated sludge

24.25% increase in biogas production than untreated sludge 52

Chemical (Alkali treated)

Plant residues and cattle dung

31-42% higher digestion efficiency than untreated samples 53

Chemical (Acid treated)

Waste activated sludge

Increase of biogas production by 21% 54

Biological (white-rot fungi)

olive mill wastewaters

COD removal was increased from 34 to 65 % 55

Ultrasonic

(20 kHz)

digestion, cattle dung is used as one of the solid waste feedstock. For the year 2010, the estimated cattle and buffalo populations are 224 and 97 million respectively74. It has been calculated that from a livestock population of about 458 million, 354 million tons of dung is available per year4. China, Korea, Taiwan and Philippines mostly use pig manure as feedstock11. Mostly used animal wastes worldwide are cow dung, pig waste, poultry manure, horse dung, camel dung, elephant dung, fishery waste slaughter house wastes, etc. Out of these some of them like poultry wastes are rich in organic nitrogen and relatively lower carbon source75. This leads to the process slow and unsuccessful. This problem can be overcome by co-digestion of different feedstock having low and high nitrogen values76. Quantities of fresh dung available from different animals and their biogas yield are presented in table 7.

Feedstock All biodegradable biomass materials are well suitable for anaerobic digestion. Feedstock may be solid, slurries and both dilute and concentrated liquids56. Common feedstock includes agricultural wastes and crop residues, animal wastes, aquatic waste, forest residues and municipal solid wastes 4, 57. The detailed list of feedstock under each category is given in table 5. Agricultural wastes and crop residues: Many agricultural crops and processes yield residues that can be used as a feedstock for biogas generation. Among the world countries, India stands second in producing rice, wheat, cotton, tea, cashew nuts and some other crops58. In India, no crop has been grown specifically for anaerobic digestion to produce biogas. But, there is abundant biomass source available in the form of agricultural waste and crop residue which can be used as feedstock for anaerobic digestion59, 60. Crop Stubbles, straw, spoiled fodder, sugar cane trash and bagasse, weeds, tobacco waste, rice and coffee husks, fruit and vegetable processing wastes, oil cakes, etc are some of the types of agricultural wastes and crop residues used in anaerobic digestion 4, 22. Table 6 shows the biogas yield potential from different agricultural wastes and crop residues.

Aquatic plants: Aquatic plants such as water hyacinth, various species micro and macro algae, sea weeds etc. are the best sources for the biogas production. Because they have easily hydrolysable sugars and low lignin content, they do not compete with land resources used in arable food crop cultivation 79, 80. Water hyacinth and micro algae are mostly used as feed material because of its higher gas yield11. It is calculated that the efficiency of biomass production per hectare of microalgae is about 5–30 times that of crop plants81, 82. The relatively high lipid, protein and starch contents and the absence of lignin makes microalgae as an efficient feed material for methane production81. Biogas yield from different aquatic plants is given in Table 8.

Animal wastes: Animals produce a substantial amount of wastes, as animal breeding activity has been highly developed 72. The world’s cattle population is estimated as 1300 million in which India stands first rank with the population of 281.7 million73. In rural India for anaerobic

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moisture content of the substrate (Wet and dry) process temperature (mesophilic and thermophilic) etc. Some of these categories are explained as follows:

Classification of anaerobic digestion system Anaerobic digester systems can be classified into several types based on feeding mode (Batch and continuous system),

Table 5 Feedstock for anaerobic digestion 4, 22 Category Agricultural wastes and crop residues Animal wastes Aquatic plants Forest residues

Types of waste Crop Stubbles, straw, spoiled fodder, sugar cane trash and bagasse, weeds, tobacco waste, rice and coffee husks, fruit and vegetable processing wastes, oil cakes, etc. Cattle dung, goat and sheep manure, pig manure, elephant dune, fishery waste, slaughter house wastes. Algae, sea weeds, water hyacinth, etc. Dead trees, plants, twigs, barks, roots, leaves, etc.

Table 6 Biogas yield from Agricultural and crop residues Agricultural and crop residues Residues Sugar beat leaves Tops of sugar beets

Vegetable and fruit wastes

Methane yield (m3 per kg of volatile solids) 0.2161 0.32-0.34

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Cauliflower leaves

Methane yield (m3 per kg of volatile solids) 0.190 71

Radish shoots

0.304 71

Wastes

Maize (whole crop silage)

0.3961

Carrot (leaves)

0.241 71

Maize straw

0.3414,63

Carrot (petiole)

0.309 71

Sunflower (whole crop silage)

0.361

Cabbage leaves

0.309 71

Sunflower oil cake

0.22764

Onion outer peel

0.400 65,71

Corn cob mix

0.35-0.3665

Potato peel

0.267 71

Wheat grain

0.37-0.465

Lemon pressings

0.473 71

Wheat straw

0.314,66

Banana peel

0.24-0.32 65,71

Oat husk

0.24267

Mango peel

0.37-0.52 65,71,

Oat straw

0.3262

Pine apple peel

0.357 71

Rice husk

0.1314

Pomegranate peel

0.312 71

Rice straw

0.3-0.3868,69

Grape pressings

0.28 65,71

Coffee husk

0.2014

Coriander leaves

0.325 71

Grass cuttings

0.362

Coriander stem

0.309 71

Barley Stalks

0.2314,70

--

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Table 7 Fresh dung availability from different animals and their biogas yield 4,77,78

Animal

Fresh dung (kg/animal/day)

Volatile matter (%)

Cow Buffalo Camel Horse Elephant Pig Sheep/Goat Poultry

10 15 20 15 40 1.2 2.0 0.10

71.3 81-84 81.1 43.4 91 54.3 63.3

Biogas yield (liter per kg of volatile solids) 72 79 119 111 272 149 113-135 191

Methane composition in biogas (%) 50 50 53 55 44 56 54 56

Table 8 Biogas yield from different aquatic plants 83- 85

Species Water hyacinth Arthrospira platensis Chlorella kessleri Euglena gracilis Dunaliella salina Scenedesmus obliquus (Green algae) Scenedesmus obliquus (Green algae) Phaeodactylum tricornutum Phaeodactylum tricornutum

Mesophilic Mesophilic Mesophilic Mesophilic Mesophilic

Volatile Solids (% on dry basis) 74 -

Biogas yield (liter per kg of volatile solids) 398 481 335 485 505

Methane composition in biogas (%) 77 61 65 67 64

Mesophilic

71.8

181.81

74.3

Thermophilic

71.8

241.28

77.1

Mesophilic Thermophilic

82.7 82.7

421 400

75.1 78.6

Temperature condition

Wet and dry systems: The process can be termed as wet or dry based on the total solid concentration of the feed substrate. Dry digestion system processes the substrates with solid concentration between 20-40%. Wet system processes the substrates with solid concentration less than 15% 10, 16. Dry system processes thick substrates and slurry and hence needs more energy input to process and move. In wet systems, the substrates can be transported easily through standard pumps with less energy input. Also, in wet systems the circulation of material is easy and the contact between the bacteria and their food are more resulting in the increased the gas production rate 16.

Batch and continuous systems: In the batch process, the substrate is fed into the digester only once (with or without inocula) and sealed completely for the entire retention time. After the retention period, the digested substrate is emptied from the digester and the new substrate is filled to start the process again 86. In this type, the production of gas is noncontinuous 20. At the beginning and end of the process, the gas production is low and at the middle of the process, the gas production is high. So, in order to get constant gas supply from batch systems, many batch type digesters can be operated in parallel 20. In continuous process, the digester is fed continuously with the substrate and the equal amount of digested substrate is removed, resulting in constant and continuous production of biogas. This type of system uses mechanical agitator or biogas recirculation to mix the digester contents continuously 75.

Thermophilic and Mesophilic systems: The temperature of the mesophilic systems digester is maintained at less than 45°C whereas in thermophilic system, the temperature of the

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Research Journal of Chemistry and Environment_________________________________________ digester is maintained in between 45°C to 60°C16. Mesophilic systems are more tolerant to changes in environmental conditions than thermophilic systems and so they are more stable. In thermophilic system, the higher process temperature enhances the reaction faster and decreases the substrate retention time compared to mesophilic system 87. To achieve higher temperature in case of thermophilic systems, it requires more energy input.


Janata Model, Deenbandhu Model, CAMARTEC model. The cost of fixed dome plants is less compared to floating dome plants.

Single stage and multistage systems: In a single stage system, all the stages of the anaerobic digestion takes place in a single, sealed reactor 88. The major problem in the single stage system is during the acid formation stage, the acidogenic bacteria reduce the pH of the digester which affects the methane formation due to difference in the growth of acidogenic and methanogenic bacterial communities 10. In two stage or multi stage systems, separate digester vessels are employed for both acid and methane formation to bring maximum control over microorganisms during acid and methane formation10,88. In the first digester vessel, hydrolysis, acidogenesis and acetogenesis take place and the product of first vessel is taken to the second vessel for methane formation. The temperature of the second digester vessel can be maintained either mesophilic or thermophilic based on the need. For dry batch systems or wet continuous systems, single stage systems are preferred, whereas in twostage system, continuous and wet processes are preferred 88.

Fig. 4: Floating drum digester

Types of anaerobic digesters/reactors Floating drum type digester: A floating-drum type digester consists of a cylindrical or dome-shaped digester, a metallic floating drum or gas-holder, inlet tank, outlet tank, inlet pipe, outlet pipe and partition wall 89. The gas in the gas holder is kept at constant pressure by moving the gas holder up and down with the help of central guide pipe during accumulation and discharge of gas 4. Some of the floating drum type biogas plants are KVIC (Khadi and Village Industries Commission) Model, Pragathi Model, Ganesh Model etc. The schematic of floating drum KVIC model plant is shown in fig 4. These type of plants have many advantages like maintenance of biogas at constant pressure, integrated arrangement for scum breaking, volume of gas can be observed by sensing the drum position, etc.

Fig 5: Fixed dome digester

Products of anaerobic digestion Biogas: The principle product of anaerobic digestion is biogas which contains methane (55-65%) and carbon dioxide (30-40%) as major composition with traces of water vapour, hydrogen sulfide and hydrogen 65. The biogas can be directly used for cooking and in combined heat and power (CHP) units. The biogas can be upgraded by removing carbon dioxide and hydrogen sulfide using scrubbers and the upgraded biogas (methane) can be used in automobiles and fuel cells 65.

Fixed dome digester: Chinese developed the fixed dome type plants first 4. A fixed-dome plant is made up of a closed, dome-shaped digester with an immovable fixed gas-holder as shown in fig 5. The gas produced is stored in the space between the slurry and the roof of the plant90. Here, the pressure of the gas inside the plant is not constant. Some of the fixed dome type plants are Chinese fixed-dome plant,

Digestate: In anaerobic digestion process, digestate is the byproduct. The digestate can be fibrous, liquor, or a sludgebased combination of the both, which is an extra material that cannot be used by the microbes9. In multi-stage digestion systems, different forms of digestate come from different

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9. Khanal S. K., Anaerobic biotechnology for bioenergy production principles and applications, Wiley-Blackwell, Singapore (2008)

digester tanks. But in single-stage digestion systems, the two fractions will be combined and the separation can be done by further processing.

10. Gerardi M. H., The microbiology of anaerobic digesters, John Wiley and Sons Inc., USA (2003)

After checking the contaminants the digestate can be used as fertilizer and soil improving material. Essential plant nutrients like nitrogen, potassium and phosphorous are largely available in the digestate which makes it suitable for soil amendment7. Digestate in the form of liquor can be used again in digestion process or it can be used in irrigation.

11. Nijaguna B. T., Biogas Technology, New Age International (P) Limited Publishers, New Delhi (2012) 12. Girard M., Palacios J. H., Belzile M., Godbout S and Pelletier F., Biodegradation in Animal Manure Management, Biodegradation - Engineering and Technology, Dr. Rolando Chamy (Ed.), (2013)

Conclusion 13. Jingquan L., Optimization of anaerobic digestion of sewage sludge using thermophilic anaerobic pre-treatment, Ph. D. Thesis submitted to Technical University of Denmark (2002)

The literature covering the biogas technology and anaerobic digestion was reviewed and presented in this paper to understand the fundamentals and over view clearly. This review also covered and discussed the stages of anaerobic digestion, feedstock, types of digesters and products of anaerobic digestion. The parameters that affect the biogas production and the parameters considered for the design of anaerobic digester are presented in this paper.

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