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Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen.^[1] It is used for industrial or domestic purposes to manage waste and/or to release energy.

The digestion process begins with bacterial hydrolysis of the input materials to break down insoluble organic polymers, such as carbohydrates, and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.^[2] The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.^[3]

It is used as part of the process to treat biodegradable waste and sewage sludge.^[4] As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.^[5]

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide and traces of other ‘contaminant’ gases.^[1] This biogas can be used directly as cooking fuel, in combined heat and power gas engines^[6] or upgraded to natural gas-quality biomethane. The use of biogas as a fuel helps to replace fossil fuels. The nutrient-rich digestate also produced can be used as fertilizer.

The technical expertise required to maintain industrial-scale anaerobic digesters, coupled with high capital costs and low process efficiencies, has so far been a limiting factor in its deployment as a waste treatment technology.^{[citation needed]} Anaerobic digestion facilities have, however, been recognized by the United Nations Development Programme as one of the most useful decentralized sources of energy supply, as they are less capital-intensive than large power plants.^[7] With increased focus on climate change mitigation, the re-use of waste as a resource and new technological approached which has lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom (2011),^[8] Germany ^[9] and Denmark (2011).^[10]

Contents 1 History 2 Process 2.1 Process stages 2.2 Configuration 2.2.1 Batch or continuous 2.2.2 Temperature 2.2.3 Solids content 2.2.4 Complexity 2.3 Residence time 3 Feedstocks 3.1 Moisture content 3.2 Contamination 3.3 Substrate composition 4 Applications 4.1 Waste treatment 4.2 Power generation 4.3 Grid injection 4.4 Fertiliser and soil conditioner 5 Products 5.1 Biogas 5.2 Digestate 5.3 Wastewater 6 See also 7 References 8 External links

History

Scientific interest in the manufacturing of gas produced by the natural decomposition of organic matter was first reported in the 17th century by Robert Boyle and Stephen Hale, who noted that flammable gas was released by disturbing the sediment of streams and lakes.^[11] In 1808, Sir Humphry Davy determined that methane was present in the gases produced by cattle manure.^[12][13] The first anaerobic digester was built by a leper colony in Bombay, India, in 1859. In 1895, the technology was developed in Exeter, England, where a septic tank was used to generate gas for the sewer gas destructor lamp, a type of gas lighting. Also in England, in 1904, the first dual-purpose tank for both sedimentation and sludge treatment was installed in Hampton. In 1907, in Germany, a patent was issued for the Imhoff tank,^[14] an early form of digester.^{[citation needed]}

Through scientific research, anaerobic digestion gained academic recognition in the 1930s. This research led to the discovery of anaerobic bacteria, the microorganisms that facilitate the process. Further research was carried out to investigate the conditions under which methanogenic bacteria were able to grow and reproduce.^[15] This work was developed during World War II, during which in both Germany and France, there was an increase in the application of anaerobic digestion for the treatment of manure.

Process

Many microorganisms are involved in the process of anaerobic digestion, including acetic acid-forming bacteria (acetogens) and methane-forming archaea (methanogens). These organisms feed upon the initial feedstock, which undergoes a number of different processes, converting it to intermediate molecules, including sugars, hydrogen, and acetic acid, before finally being converted to biogas.^[16]

Different species of bacteria are able to survive at different temperature ranges. Ones living optimally at temperatures between 35 and 40 °C are called mesophiles or mesophilic bacteria. Some of the bacteria can survive at the hotter and more hostile conditions of 55 to 60 °C; these are called thermophiles or thermophilic bacteria.^[17] Methanogens come from the domain of archaea. This family includes species that can grow in the hostile conditions of hydrothermal vents, so are more resistant to heat, and can, therefore, operate at high temperatures, a property unique to thermophiles.^[18]

As with aerobic systems, the bacteria, the growing and reproducing microorganisms within anaerobic systems, require a source of elemental oxygen to survive,^[19] but in anaerobic systems, there is an absence of gaseous oxygen. Gaseous oxygen is prevented from entering the system through physical containment in sealed tanks. Anaerobes access oxygen from sources other than the surrounding air, which can be the organic material itself or may be supplied by inorganic oxides from within the input material. When the oxygen source in an anaerobic system is derived from the organic material itself, the ‘intermediate’ end products are primarily alcohols, aldehydes, and organic acids, plus carbon dioxide. In the presence of specialised methanogens, the intermediates are converted to the ‘final’ end products of methane, carbon dioxide, and trace levels of hydrogen sulfide.^[20][21] In an anaerobic system, the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.^[11]

Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to be fully effective. Therefore, common practice is to introduce anaerobic microorganisms from materials with existing populations, a process known as “seeding” the digesters, typically accomplished with the addition of sewage sludge or cattle slurry.^[22]

Process stages

There are four key biological and chemical stages of anaerobic digestion:^[13]

1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis

In most cases, biomass is made up of large organic polymers. For the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts, or monomers, such as sugars, are readily available to other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysis of these high-molecular-weight polymeric components is the necessary first step in anaerobic digestion.^[23] Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids.

Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules, such as volatile fatty acids (VFAs) with a chain length greater than that of acetate must first be catabolised into compounds that can be directly used by methanogens.^[24]

The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here, VFAs are created, along with ammonia, carbon dioxide, and hydrogen sulfide, as well as other byproducts.^[25] The process of acidogenesis is similar to the way milk sours.

The third stage of anaerobic digestion is acetogenesis. Here, simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.^[26]

The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. These components make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8.^[27] The remaining, indigestible material the microbes cannot use and any dead bacterial remains constitute the digestate.

A simplified generic chemical equation for the overall processes outlined above is as follows:

Configuration

Anaerobic digesters can be designed and engineered to operate using a number of different process configurations:

• Batch or continuous
• Temperature: Mesophilic or thermophilic
• Solids content: High solids or low solids
• Complexity: Single stage or multistage

Batch or continuous

Anaerobic digestion can be performed as a batch process or a continuous process.

In a batch system biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process.

In its simplest form batch processing needs inoculation with already processed material to start the anaerobic digestion. In a typical scenario, biogas production will be formed with a normal distribution pattern over time. Operator can use this fact to determine when they believe the process of digestion of the organic matter has completed. There can be severe odour issues if a batch reactor is opened and emptied before the process is well completed.

A more advanced type of batch approach has limited the odour issues by integrating anaerobic digestion with in-vessel composting. In this approach inoculation takes place through the use of recirculated degasified percolate. After anaerobic digestion has completed, the biomass is kept in the reactor which is then used for in-vessel composting before it is opened ^[28]

As the batch digestion is simple and requires less equipment and lower levels of design work, it is typically a cheaper form of digestion.^[29] Using more than one batch reactor at a plant can ensure constant production of biogas.

In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow; first in – first out). Here, the end products are constantly or periodically removed, resulting in constant production of biogas. A single or multiple digesters in sequence may be used. Examples of this form of anaerobic digestion include continuous stirred-tank reactors, upflow anaerobic sludge blankets, expanded granular sludge beds and internal circulation reactors.^[30][31]

Temperature

The two conventional operational temperature levels for anaerobic digesters are determined by the species of methanogens in the digesters:^[32]

• Mesophilic digestion takes place optimally around 30 to 38 °C, or at ambient temperatures between 20 and 45 °C, where mesophiles are the primary microorganism present.
• Thermophilic digestion takes place optimally around 49 to 57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present.

A limit case has been reached in Bolivia, with anaerobic digestion in temperature working conditions of less than 10 °C. The anaerobic process is very slow, taking more than three times the normal mesophilic time process.^[33] In experimental work at University of Alaska Fairbanks, a 1000 litre digester using psychrophiles harvested from “mud from a frozen lake in Alaska” has produced 200–300 litres of methane per day, about 20 to 30% of the output from digesters in warmer climates.^[34]

Mesophilic species outnumber thermophiles, and they are also more tolerant to changes in environmental conditions than thermophiles. Mesophilic systems are, therefore, considered to be more stable than thermophilic digestion systems.

Though thermophilic digestion systems are considered to be less stable and the energy input is higher, more energy is removed from the organic matter. The increased temperatures facilitate faster reaction rates and, hence, faster gas yields. Operation at higher temperatures facilitates greater sterilization of the end digestate. In countries where legislation, such as the Animal By-Products Regulations in the European Union, requires end products to meet certain levels of reduction in the amount of bacteria in the output material, this may be a benefit.^[35]

Certain processes shred the waste finely and use a thermal pretreatment stage (hygienisation) to significantly enhance the gas output of the following standard mesophilic stage. The hygienisation process is also applied to reduce the pathogenic micro-organisms in the feedstock. Hygienisation may be achieved by using a Landia BioChop hygienisation unit ^[36] or similar method of combined heat treatment and solids maceration.

A drawback of operating at thermophilic temperatures is that more heat energy input is required to achieve the correct operational temperatures, which may not be outweighed by the increase in the outputs of biogas from the systems. Therefore, it is important to consider an energy balance for these systems.

Solids content

In a typical scenario, three different operational parameters are associated with the solids content of the feedstock to the digesters:

• High solids (dry—stackable substrate)
• High solids (wet—pumpable substrate)
• Low solids (wet—pumpable substrate)

High solids (dry) digesters are designed to process materials with a solids content between 25 and 40%. Unlike wet digesters that process pumpable slurries, high solids (dry – stackable substrate) digesters are designed to process solid substrates without the addition of water. The primary styles of dry digesters are continuous vertical plug flow and batch tunnel horizontal digesters. Continuous vertical plug flow digesters are upright, cylindrical tanks where feedstock is continuously fed into the top of the digester, and flows downward by gravity during digestion. In batch tunnel digesters, the feedstock is deposited in tunnel-like chambers with a gas-tight door. Neither approach has mixing inside the digester. The amount of pretreatment, such as contaminant removal, depends both upon the nature of the waste streams being processed and the desired quality of the digestate. Size reduction (gringing) is beneficial in continuous vertical systems, as it accelerates digestion, while batch systems avoid grinding and instead require structure (e.g. yard waste) to reduce compaction of the stacked pile. Continuous vertical dry digesters have a smaller footprint due to the shorter effective retention time and vertical design.

Wet digesters can be designed to operate in either a high-solids content, with a total suspended solids (TSS) concentration greater than ~20%, or a low-solids concentration less than ~15%.^[37][38]

High solids (wet) digesters process a thick slurry that requires more energy input to move and process the feedstock. The thickness of the material may also lead to associated problems with abrasion. High solids digesters will typically have a lower land requirement due to the lower volumes associated with the moisture.^{[citation needed]} High solids digesters also require correction of conventional performance calculations (e.g. gas production, retention time, kinetics, etc.) originally based on very dilute sewage digestion concepts, since larger fractions of the feedstock mass are potentially convertible to biogas.^[39]

Low solids (wet) digesters can transport material through the system using standard pumps that require significantly lower energy input. Low solids digesters require a larger amount of land than high solids due to the increased volumes associated with the increased liquid-to-feedstock ratio of the digesters. There are benefits associated with operation in a liquid environment, as it enables more thorough circulation of materials and contact between the bacteria and their food. This enables the bacteria to more readily access the substances on which they are feeding, and increases the rate of gas production.^{[citation needed]}

Complexity

Digestion systems can be configured with different levels of complexity:^[37]

In a single-stage digestion system (one-stage), all of the biological reactions occur within a single, sealed reactor or holding tank. Using a single stage reduces construction costs, but results in less control of the reactions occurring within the system. Acidogenic bacteria, through the production of acids, reduce the pH of the tank. Methanogenic bacteria, as outlined earlier, operate in a strictly defined pH range.^[40] Therefore, the biological reactions of the different species in a single-stage reactor can be in direct competition with each other. Another one-stage reaction system is an anaerobic lagoon. These lagoons are pond-like, earthen basins used for the treatment and long-term storage of manures.^[41] Here the anaerobic reactions are contained within the natural anaerobic sludge contained in the pool.

In a two-stage digestion system (multistage), different digestion vessels are optimised to bring maximum control over the bacterial communities living within the digesters. Acidogenic bacteria produce organic acids and more quickly grow and reproduce than methanogenic bacteria. Methanogenic bacteria require stable pH and temperature to optimise their performance.^[42]

Under typical circumstances, hydrolysis, acetogenesis, and acidogenesis occur within the first reaction vessel. The organic material is then heated to the required operational temperature (either mesophilic or thermophilic) prior to being pumped into a methanogenic reactor. The initial hydrolysis or acidogenesis tanks prior to the methanogenic reactor can provide a buffer to the rate at which feedstock is added. Some European countries require a degree of elevated heat treatment to kill harmful bacteria in the input waste.^[43] In this instance, there may be a pasteurisation or sterilisation stage prior to digestion or between the two digestion tanks. Notably, it is not possible to completely isolate the different reaction phases, and often some biogas is produced in the hydrolysis or acidogenesis tanks.

Residence time

The residence time in a digester varies with the amount and type of feed material, the configuration of the digestion system, and whether it be one-stage or two-stage.

In the case of single-stage thermophilic digestion, residence times may be in the region of 14 days, which, compared to mesophilic digestion, is relatively fast. The plug-flow nature of some of these systems will mean the full degradation of the material may not have been realised in this timescale. In this event, digestate exiting the system will be darker in colour and will typically have more odour.^{[citation needed]}

In two-stage mesophilic digestion, residence time may vary between 15 and 40 days.^[44]

In the case of mesophilic UASB digestion, hydraulic residence times can be 1 hour to 1 day, and solid retention times can be up to 90 days. In this manner, the UASB system is able to separate solids and hydraulic retention times with the use of a sludge blanket.^[45]

Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents, enabling the bacteria and the food to be in contact. They also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks.^{[citation needed]}

Feedstocks

The most important initial issue when considering the application of anaerobic digestion systems is the feedstock to the process. Almost any organic material can be processed with anaerobic digestion;^[46][47] however, if biogas production is the aim, the level of putrescibility is the key factor in its successful application.^[48] The more putrescible (digestible) the material, the higher the gas yields possible from the system.

Feedstocks can include biodegradable waste materials, such as waste paper, grass clippings, leftover food, sewage, and animal waste.^[1] Woody wastes are the exception, because they are largely unaffected by digestion, as most anaerobes are unable to degrade lignin, Xylophalgeous anaerobes (lignin consumers) or using high temperature pretreatment, such as pyrolysis, can be used to break down the lignin. Anaerobic digesters can also be fed with specially grown energy crops, such as silage, for dedicated biogas production. In Germany and continental Europe, these facilities are referred to as “biogas” plants. A codigestion or cofermentation plant is typically an agricultural anaerobic digester that accepts two or more input materials for simultaneous digestion.^[49]

Anaerobes can break down material with varying degrees of success from readily, in the case of short-chain hydrocarbons such as sugars, to over longer periods of time, in the case of cellulose and hemicellulose.^[50] Anaerobic microorganisms are unable to break down long-chain woody molecules, such as lignin.^[51]

Anaerobic digesters were originally designed for operation using sewage sludge and manures. Sewage and manure are not, however, the material with the most potential for anaerobic digestion, as the biodegradable material has already had much of the energy content taken out by the animals that produced it. Therefore, many digesters operate with codigestion of two or more types of feedstock. For example, in a farm-based digester that uses dairy manure as the primary feedstock, the gas production may be significantly increased by adding a second feedstock, e.g., grass and corn (typical on-farm feedstock), or various organic byproducts, such as slaughterhouse waste, fats, oils and grease from restaurants, organic household waste, etc. (typical off-site feedstock).^[52]

Digestors processing dedicated energy crops can achieve high levels of degradation and biogas production.^[38][53][54] Slurry-only systems are generally cheaper, but generate far less energy than those using crops, such as maize and grass silage; by using a modest amount of crop material (30%), an anaerobic digestion plant can increase energy output tenfold for only three times the capital cost, relative to a slurry-only system.^[55]

Moisture content

A second consideration related to the feedstock is moisture content. Dryer, stackable substrates, such as food and yard waste, are suitable for digestion in tunnel-like chambers. Tunnel-style systems typically have near-zero wastewater discharge, as well, so this style of system has advantages where the discharge of digester liquids are a liability. The wetter the material, the more suitable it will be to handling with standard pumps instead of energy-intensive concrete pumps and physical means of movement. Also, the wetter the material, the more volume and area it takes up relative to the levels of gas produced. The moisture content of the target feedstock will also affect what type of system is applied to its treatment. To use a high-solids anaerobic digester for dilute feedstocks, bulking agents, such as compost, should be applied to increase the solids content of the input material.^[56] Another key consideration is the carbon:nitrogen ratio of the input material. This ratio is the balance of food a microbe requires to grow; the optimal C:N ratio is 20–30:1.^[57] Excess N can lead to ammonia inhibition of digestion.^[53]

Contamination

The level of contamination of the feedstock material is a key consideration. If the feedstock to the digesters has significant levels of physical contaminants, such as plastic, glass, or metals, then processing to remove the contaminants will be required for the material to be used.^[58] If it is not removed, then the digesters can be blocked and will not function efficiently. It is with this understanding that mechanical biological treatment plants are designed. The higher the level of pretreatment a feedstock requires, the more processing machinery will be required, and, hence, the project will have higher capital costs.^[59]

After sorting or screening to remove any physical contaminants from the feedstock, the material is often shredded, minced, and mechanically or hydraulically pulped to increase the surface area available to microbes in the digesters and, hence, increase the speed of digestion. The maceration of solids can be achieved by using a chopper pump to transfer the feedstock material into the airtight digester, where anaerobic treatment takes place.

Substrate composition

Substrate composition is a major factor in determining the methane yield and methane production rates from the digestion of biomass. Techniques to determine the compositional characteristics of the feedstock are available, while parameters such as solids, elemental, and organic analyses are important for digester design and operation.^[60]

Applications

Using anaerobic digestion technologies can help to reduce the emission of greenhouse gases in a number of key ways:

• Replacement of fossil fuels
• Reducing or eliminating the energy footprint of waste treatment plants
• Reducing methane emission from landfills
• Displacing industrially produced chemical fertilizers
• Reducing vehicle movements
• Reducing electrical grid transportation losses

Waste treatment

Anaerobic digestion is particularly suited to organic material, and is commonly used for effluent and sewage treatment.^[61] Anaerobic digestion, a simple process, can greatly reduce the amount of organic matter which might otherwise be destined to be dumped at sea,^[62] dumped in landfills, or burnt in incinerators.^[63]

Pressure from environmentally related legislation on solid waste disposal methods in developed countries has increased the application of anaerobic digestion as a process for reducing waste volumes and generating useful byproducts. It may either be used to process the source-separated fraction of municipal waste or alternatively combined with mechanical sorting systems, to process residual mixed municipal waste. These facilities are called mechanical biological treatment plants.^[64][65][66]

If the putrescible waste processed in anaerobic digesters were disposed of in a landfill, it would break down naturally and often anaerobically. In this case, the gas will eventually escape into the atmosphere. As methane is about 20 times more potent as a greenhouse gas than carbon dioxide, this has significant negative environmental effects.^[67]

In countries that collect household waste, the use of local anaerobic digestion facilities can help to reduce the amount of waste that requires transportation to centralized landfill sites or incineration facilities. This reduced burden on transportation reduces carbon emissions from the collection vehicles. If localized anaerobic digestion facilities are embedded within an electrical distribution network, they can help reduce the electrical losses associated with transporting electricity over a national grid.^[68]

Power generation

In developing countries, simple home and farm-based anaerobic digestion systems offer the potential for low-cost energy for cooking and lighting.^{[33][69][70][71]} Anaerobic digestion facilities have been recognized by the United Nations Development Programme as one of the most useful decentralized sources of energy supply.^[7] From 1975, China and India have both had large, government-backed schemes for adaptation of small biogas plants for use in the household for cooking and lighting.^[72] At present, projects for anaerobic digestion in the developing world can gain financial support through the United Nations Clean Development Mechanism if they are able to show they provide reduced carbon emissions.^[73]

Methane and power produced in anaerobic digestion facilities can be used to replace energy derived from fossil fuels, and hence reduce emissions of greenhouse gases,^[74] because the carbon in biodegradable material is part of a carbon cycle. The carbon released into the atmosphere from the combustion of biogas has been removed by plants for them to grow in the recent past, usually within the last decade, but more typically within the last growing season. If the plants are regrown, taking the carbon out of the atmosphere once more, the system will be carbon neutral.^[75][76] In contrast, carbon in fossil fuels has been sequestered in the earth for many millions of years, the combustion of which increases the overall levels of carbon dioxide in the atmosphere.

Biogas from sewage works is sometimes used to run a gas engine to produce electrical power, some or all of which can be used to run the sewage works.^[77] Some waste heat from the engine is then used to heat the digester. The waste heat is, in general, enough to heat the digester to the required temperatures. The power potential from sewage works is limited – in the UK, there are about 80 MW total of such generation, with the potential to increase to 150 MW, which is insignificant compared to the average power demand in the UK of about 35,000 MW. The scope for biogas generation from nonsewage waste biological matter – energy crops, food waste, abattoir waste, etc. – is much higher, estimated to be capable of about 3,000 MW.^{[citation needed]} Farm biogas plants using animal waste and energy crops are expected to contribute to reducing CO₂ emissions and strengthen the grid, while providing UK farmers with additional revenues.^[78]

Some countries offer incentives in the form of, for example, feed-in tariffs for feeding electricity onto the power grid to subsidize green energy production.^[1][79]

In Oakland, California at the East Bay Municipal Utility District’s main wastewater treatment plant (EBMUD), food waste is currently codigested with primary and secondary municipal wastewater solids and other high-strength wastes. Compared to municipal wastewater solids digestion alone, food waste codigestion has many benefits. Anaerobic digestion of food waste pulp from the EBMUD food waste process provides a higher normalized energy benefit, compared to municipal wastewater solids: 730 to 1,300 kWh per dry ton of food waste applied compared to 560 to 940 kWh per dry ton of municipal wastewater solids applied.^[80][81]

Grid injection

Biogas grid-injection is the injection of biogas into the natural gas grid.^[82] As an alternative, the electricity and the heat can be used for on-site generation,^[83] resulting in a reduction of losses in the transportation of energy. Typical energy losses in natural gas transmission systems range from 1–2%, whereas the current energy losses on a large electrical system range from 5–8%.^[84]

In October 2010, Didcot Sewage Works became the first in the UK to produce biomethane gas supplied to the national grid, for use in up to 200 homes in Oxfordshire.^[85]

Fertiliser and soil conditioner

The solid, fibrous component of the digested material can be used as a soil conditioner to increase the organic content of soils. Digester liquor can be used as a fertiliser to supply vital nutrients to soils instead of chemical fertilisers that require large amounts of energy to produce and transport. The use of manufactured fertilisers is, therefore, more carbon-intensive than the use of anaerobic digester liquor fertiliser. In countries such as Spain, where many soils are organically depleted, the markets for the digested solids can be equally as important as the biogas.^[86]

Products

The three principal products of anaerobic digestion are biogas, digestate, and water.^[37][87][88]

Biogas

Typical composition of biogas^[89]

Matter	%
Methane, CH4	50–75
Carbon dioxide, CO2	25–50
Nitrogen, N2	0–10
Hydrogen, H2	0–1
Hydrogen sulfide, H2S	0–3
Oxygen, O2	0–2

Biogas is the ultimate waste product of the bacteria feeding off the input biodegradable feedstock (the methanogenesis stage of anaerobic digestion is performed by archaea – a micro-organism on a distinctly different branch of the phylogenetic tree of life to bacteria), and is mostly methane and carbon dioxide,^[90][91] with a small amount hydrogen and trace hydrogen sulfide. (As-produced, biogas also contains water vapor, with the fractional water vapor volume a function of biogas temperature).^[39] Most of the biogas is produced during the middle of the digestion, after the bacterial population has grown, and tapers off as the putrescible material is exhausted.^[92] The gas is normally stored on top of the digester in an inflatable gas bubble or extracted and stored next to the facility in a gas holder.

The methane in biogas can be burned to produce both heat and electricity, usually with a reciprocating engine or microturbine^[93] often in a cogeneration arrangement where the electricity and waste heat generated are used to warm the digesters or to heat buildings. Excess electricity can be sold to suppliers or put into the local grid. Electricity produced by anaerobic digesters is considered to be renewable energy and may attract subsidies.^[94] Biogas does not contribute to increasing atmospheric carbon dioxide concentrations because the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle.

Biogas may require treatment or ‘scrubbing’ to refine it for use as a fuel.^[95] Hydrogen sulfide, a toxic product formed from sulfates in the feedstock, is released as a trace component of the biogas. National environmental enforcement agencies, such as the U.S. Environmental Protection Agency‎ or the English and Welsh Environment Agency, put strict limits on the levels of gases containing hydrogen sulfide, and, if the levels of hydrogen sulfide in the gas are high, gas scrubbing and cleaning equipment (such as amine gas treating) will be needed to process the biogas to within regionally accepted levels.^[96] Alternatively, the addition of ferrous chloride FeCl₂ to the digestion tanks inhibits hydrogen sulfide production.^[97]

Volatile siloxanes can also contaminate the biogas; such compounds are frequently found in household waste and wastewater. In digestion facilities accepting these materials as a component of the feedstock, low-molecular-weight siloxanes volatilise into biogas. When this gas is combusted in a gas engine, turbine, or boiler, siloxanes are converted into silicon dioxide (SiO₂), which deposits internally in the machine, increasing wear and tear.^[98][99] Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are available at the present time.^[100] In certain applications, in situ treatment can be used to increase the methane purity by reducing the offgas carbon dioxide content, purging the majority of it in a secondary reactor.^[101]

In countries such as Switzerland, Germany, and Sweden, the methane in the biogas may be compressed for it to be used as a vehicle transportation fuel or input directly into the gas mains.^[102] In countries where the driver for the use of anaerobic digestion are renewable electricity subsidies, this route of treatment is less likely, as energy is required in this processing stage and reduces the overall levels available to sell.^[103]

Digestate

Digestate is the solid remnants of the original input material to the digesters that the microbes cannot use. It also consists of the mineralised remains of the dead bacteria from within the digesters. Digestate can come in three forms: fibrous, liquor, or a sludge-based combination of the two fractions. In two-stage systems, different forms of digestate come from different digestion tanks. In single-stage digestion systems, the two fractions will be combined and, if desired, separated by further processing.^[104][105]

The second byproduct (acidogenic digestate) is a stable, organic material consisting largely of lignin and cellulose, but also of a variety of mineral components in a matrix of dead bacterial cells; some plastic may be present. The material resembles domestic compost and can be used as such or to make low-grade building products, such as fibreboard.^[106][107] The solid digestate can also be used as feedstock for ethanol production.^[108]

The third byproduct is a liquid (methanogenic digestate) rich in nutrients, which can be used as a fertiliser, depending on the quality of the material being digested.^[109] Levels of potentially toxic elements (PTEs) should be chemically assessed. This will depend upon the quality of the original feedstock. In the case of most clean and source-separated biodegradable waste streams, the levels of PTEs will be low. In the case of wastes originating from industry, the levels of PTEs may be higher and will need to be taken into consideration when determining a suitable end use for the material.

Digestate typically contains elements, such as lignin, that cannot be broken down by the anaerobic microorganisms. Also, the digestate may contain ammonia that is phytotoxic, and may hamper the growth of plants if it is used as a soil-improving material. For these two reasons, a maturation or composting stage may be employed after digestion. Lignin and other materials are available for degradation by aerobic microorganisms, such as fungi, helping reduce the overall volume of the material for transport. During this maturation, the ammonia will be oxidized into nitrates, improving the fertility of the material and making it more suitable as a soil improver. Large composting stages are typically used by dry anaerobic digestion technologies.^[110][111]

Wastewater

The final output from anaerobic digestion systems is water, which originates both from the moisture content of the original waste that was treated and water produced during the microbial reactions in the digestion systems. This water may be released from the dewatering of the digestate or may be implicitly separate from the digestate.

The wastewater exiting the anaerobic digestion facility will typically have elevated levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). These measures of the reactivity of the effluent indicate its an ability to pollute. Some of this material is termed ‘hard COD’, meaning it cannot be accessed by the anaerobic bacteria for conversion into biogas. If this effluent were put directly into watercourses, it would negatively affect them by causing eutrophication. As such, further treatment of the wastewater is often required. This treatment will typically be an oxidation stage wherein air is passed through the water in a sequencing batch reactors or reverse osmosis unit.^{[112][113][114]}

See also

• Anaerobic digester types
• Bioconversion of biomass to mixed alcohol fuels
• Biogas
• Carbon dioxide air capture
• Dissolved oxygen
• Environmental issues with energy
• Eutrophication
• Hypoxia (environmental)
• Mass balance
• Mesophilic digester
• Methane capture
• Methane clathrate
• Microbiology of decomposition
• Relative cost of electricity generated by different sources
• Sewage treatment
• Sludge bulking
• Thermophilic digester
• Upflow anaerobic sludge blanket digestion (UASB)
• Wastewater quality indicators

References

This article includes inline citations, but they are not properly formatted. Please improve this article by correcting them.

External links

• Official Website of the Anaerobic Digestion and Biogas Association, Anaerobic Digestion and Biogas Association (ADBA)
• Online AD Cost Calculator, nnfcc.co.uk
• UK’s Official Information Portal on Anaerobic Digestion and Biogas, biogas-info.co.uk
• Glossary of Anaerobic Digestion terms, bioplex.co.uk
• Anaerobic digestion forum, listserv.repp.org
• Anaerobic digestion website, anaerobic-digestion.com
• US Government Information Sheet: Methane from anaerobic digesters, web.archive.org
• Anaerobic biodigester design for small tropical producers, ruralcostarica.com
• Biogas Community on WikiSpaces, biogas.wikispaces.com
• Online Anaerobic Digester Output Estimator, bioplex.co.uk
• Biogas Forum, forum.zorg-biogas.com
• American Biogas Council
• Introduction to Biogas and Anaerobic Digestion, information from eXtension’s Livestock and Poultry Environmental Learning Center

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v t e

Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal feces, and kitchen waste can be converted into a gaseous fuel called biogas. Biogas originates from biogenic material and is a type of biofuel.

Biogas is produced by the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material, and crops.^[1] Biogas comprises primarily methane (CH₄) and carbon dioxide (CO₂) and may have small amounts of hydrogen sulphide (H₂S), moisture and siloxanes.

The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel. Biogas can be used as a fuel in any country for any heating purpose, such as cooking. It can also be used in anaerobic digesters where it is typically used in a gas engine to convert the energy in the gas into electricity and heat.^[2] Biogas can be compressed, much like natural gas, and used to power motor vehicles. In the UK, for example, biogas is estimated to have the potential to replace around 17% of vehicle fuel.^[3] Biogas is a renewable fuel, so it qualifies for renewable energy subsidies in some parts of the world. Biogas can also be cleaned and upgraded to natural gas standards when it becomes biomethane.

Contents 1 Production 2 Composition 3 Benefits 4 Applications 4.1 Biogas upgrading 4.2 Biogas gas-grid injection 4.3 Biogas in transport 5 Legislation 6 Development around the world 6.1 In the United States 6.2 In the United Kingdom 6.3 In Germany 6.4 In the Indian subcontinent 6.5 China 6.6 In developing nations 7 See also 8 References 9 Further reading 10 External links

Production

Biogas is practically produced as landfill gas (LFG) or digester gas. A biogas plant is the name often given to an anaerobic digester that treats farm wastes or energy crops. Biogas can be produced using anaerobic digesters. These plants can be fed with energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste. During the process,as an air-tight tank transforms biomass waste into methane producing renewable energy that can be used for heating, electricity, and many other operations that use any variation of an internal combustion engine, such as GE Jenbacher gas engines.^[4] There are two key processes: Mesophilic and Thermophilic digestion.^[5] In experimental work at University of Alaska Fairbanks, a 1000-litre digester using psychrophiles harvested from “mud from a frozen lake in Alaska” has produced 200–300 litres of methane per day, about 20–30 % of the output from digesters in warmer climates.^[6]

Landfill gas is produced by wet organic waste decomposing under anaerobic conditions in a landfill.^[7][8] The waste is covered and mechanically compressed by the weight of the material that is deposited from above. This material prevents oxygen exposure thus allowing anaerobic microbes to thrive. This gas builds up and is slowly released into the atmosphere if the landfill site has not been engineered to capture the gas. Landfill gas is hazardous for three key reasons. Landfill gas becomes explosive when it escapes from the landfill and mixes with oxygen. The lower explosive limit is 5% methane and the upper explosive limit is 15% methane.^[9] The methane contained within biogas is 20 times more potent as a greenhouse gas than is carbon dioxide. Therefore, uncontained landfill gas, which escapes into the atmosphere may significantly contribute to the effects of global warming. In addition, landfill gas impact in global warming, volatile organic compounds (VOCs) contained within landfill gas contribute to the formation of photochemical smog.

Composition

Typical composition of biogas^[10]

Compound	Chem	%
Methane	CH4	50–75
Carbon dioxide	CO	25–50
Nitrogen	N2	0–10
Hydrogen	H2	0–1
Hydrogen sulfide	H2S	0–3
Oxygen	O2	0–0

The composition of biogas varies depending upon the origin of the anaerobic digestion process. Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55–75% methane,^[11] which for reactors with free liquids can be increased to 80-90% methane using in-situ gas purification techniques^[12] As-produced, biogas also contains water vapor. The fractional volume of water vapor is a function of biogas temperature; correction of measured gas volume for both water vapor content and thermal expansion is easily done via a simple mathematic algorithm^[13] which yields the standardized volume of dry biogas.

In some cases, biogas contains siloxanes. These siloxanes are formed from the anaerobic decomposition of materials commonly found in soaps and detergents. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or various other elements in the combustion gas. Deposits are formed containing mostly silica (SiO₂) or silicates (Si_xO_y) and can also contain calcium, sulfur, zinc, phosphorus. Such white mineral deposits accumulate to a surface thickness of several millimeters and must be removed by chemical or mechanical means.

Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are currently available.^[14]

Benefits

When biogas is used, many advantages arise. In North America, utilization of biogas would generate enough electricity to meet up to three percent of the continent’s electricity expenditure. In addition, biogas could potentially help reduce global climate change. Normally, manure that is left to decompose releases two main gases that cause global climate change: nitrous dioxide and methane. Nitrous dioxide (NO₂) warms the atmosphere 310 times more than carbon dioxide and methane 21 times more than carbon dioxide. By converting cow manure into methane biogas via anaerobic digestion, the millions of cows in the United States would be able to produce one hundred billion kilowatt hours of electricity, enough to power millions of homes across the United States. In fact, one cow can produce enough manure in one day to generate three kilowatt hours of electricity; only 2.4 kilowatt hours of electricity are needed to power a single one hundred watt light bulb for one day.^[15] Furthermore, by converting cow manure into methane biogas instead of letting it decompose, we would be able to reduce global warming gases by ninety-nine million metric tons or four percent.^[16]

Applications

Biogas can be utilized for electricity production on sewage works,^[17] in a CHP gas engine, where the waste heat from the engine is conveniently used for heating the digester; cooking; space heating; water heating; and process heating. If compressed, it can replace compressed natural gas for use in vehicles, where it can fuel an internal combustion engine or fuel cells and is a much more effective displacer of carbon dioxide than the normal use in on-site CHP plants.^[3]

Methane within biogas can be concentrated via a biogas upgrader to the same standards as fossil natural gas, which itself has had to go through a cleaning process, and becomes biomethane. If the local gas network allows for this, the producer of the biogas may utilize the local gas distribution networks. Gas must be very clean to reach pipeline quality, and must be of the correct composition for the local distribution network to accept. Carbon dioxide, water, hydrogen sulfide, and particulates must be removed if present.

Biogas upgrading

Raw biogas produced from digestion is roughly 60% methane and 29% CO₂ with trace elements of H₂S, and is not high quality enough to be used as fuel gas for machinery. The corrosive nature of H₂S alone is enough to destroy the internals of a plant. The solution is the use of biogas upgrading or purification processes whereby contaminants in the raw biogas stream are absorbed or scrubbed, leaving more methane per unit volume of gas. There are four main methods of biogas upgrading, these include water washing, pressure swing absorption, selexol absorption, and amine gas treating.^[18] The most prevalent method is water washing where high pressure gas flows into a column where the carbon dioxide and other trace elements are scrubbed by cascading water running counter-flow to the gas. This arrangement could deliver 98% methane with manufacturers guaranteeing maximum 2% methane loss in the system. It takes roughly between 3-6% of the total energy output in gas to run a biogas upgrading system.

Biogas gas-grid injection

Gas-grid injection is the injection of biogas into the methane grid (natural gas grid). Injections includes biogas:^[19] until the breakthrough of micro combined heat and power two-thirds of all the energy produced by biogas power plants was lost (the heat), using the grid to transport the gas to customers, the electricity and the heat can be used for on-site generation^[20] resulting in a reduction of losses in the transportation of energy. Typical energy losses in natural gas transmission systems range from 1–2%. The current energy losses on a large electrical system range from 5–8%.^[21]

Biogas in transport

If concentrated and compressed, it can also be used in vehicle transportation. Compressed biogas is becoming widely used in Sweden, Switzerland, and Germany. A biogas-powered train has been in service in Sweden since 2005.^[22][23] Biogas also powers automobiles and in 1974, a British documentary film entitled Sweet as a Nut detailed the biogas production process from pig manure, and how the biogas fueled a custom-adapted combustion engine.^[24][25] In 2007, an estimated 12,000 vehicles were being fueled with upgraded biogas worldwide, mostly in Europe.^[26]

Legislation

The European Union presently has some of the strictest legislation regarding waste management and landfill sites called the Landfill Directive.^{[citation needed]} The United States legislates against landfill gas as it contains VOCs. The United States Clean Air Act and Title 40 of the Code of Federal Regulations (CFR) requires landfill owners to estimate the quantity of non-methane organic compounds (NMOCs) emitted. If the estimated NMOC emissions exceeds 50 tonnes per year, the landfill owner is required to collect the landfill gas and treat it to remove the entrained NMOCs. Treatment of the landfill gas is usually by combustion. Because of the remoteness of landfill sites, it is sometimes not economically feasible to produce electricity from the gas. However, countries such as the United Kingdom and Germany now have legislation in force that provides farmers with long-term revenue and energy security.^[27]

Development around the world

In the United States

With the many benefits of biogas, it is starting to become a popular source of energy and is starting to be utilized in the United States more. In 2003, the United States consumed 147 trillion BTU of energy from “landfill gas”, about 0.6% of the total U.S. natural gas consumption.^[26] Methane biogas derived from cow manure is also being tested in the U.S. According to a 2008 study, collected by the Science and Children magazine, methane biogas from cow manure would be sufficient to produce 100 billion kilowatt hours enough to power millions of homes across America. Furthermore, methane biogas has been tested to prove that it can reduce 99 million metric tons of greenhouse gas emissions or about 4% of the greenhouse gases produced by the United States.^[28]

In Vermont, for example, biogas generated on dairy farms around the state is included in the CVPS Cow Power program. The Cow Power program is offered by Central Vermont Public Service Corporation as a voluntary tariff. Customers can elect to pay a premium on their electric bill, and that premium is passed directly to the farms in the program. In Sheldon, Vermont, Green Mountain Dairy has provided renewable energy as part of the Cow Power program. It all started when the brothers who own the farm, Bill and Brian Rowell, wanted to address some of the manure management challenges faced by dairy farms, including manure odor, and nutrient availability for the crops they need to grow to feed the animals. They installed an anaerobic digester to process the cow and milking center waste from their nine hundred and fifty cows to produce renewable energy, a bedding to replace sawdust, and a plant friendly fertilizer. The energy and environmental attributes are sold. On average, the system run by the Rowell brothers produces enough electricity to power three hundred to three hundred fifty other homes. The generator capacity is about three hundred kiloWatts.^[29]

In Hereford, Texas, cow manure is being used to power an ethanol power plant. By switching to methane biogas, the ethanol power plant has saved one thousand barrels of oil a day. Overall, the power plant has reduced transportation costs and will be opening many more jobs for future power plants that will be relying on biogas.^[30]

In the United Kingdom

There are currently around 60 non-sewage biogas plants in the UK, most are on-farm, but some larger facilities exist off-farm, which are taking food and consumer wastes.^[31]

On 5 October 2010, biogas was injected into the UK gas grid for the first time. Sewage from over 30,000 Oxfordshire homes is sent to Didcot sewage treatment works, where it is treated in an anaerobic digestor to produce biogas, which is then cleaned to provide gas for approximately 200 homes.^[32]

In Germany

Germany is Europe’s biggest biogas producer^[33] as it is the market leader in biogas technology.^[34] In 2010 there were 5,905 biogas plants operating throughout the whole country, in which Lower Saxony, Bavaria and the eastern federal states are the main regions.^[35] Most of these plants are employed as power plants. Usually the biogas plants are directly connected with a CHP which produces electric power by burning the bio methane. The electrical power is then fed into the public power grid.^[36] In 2010 the total installed electrical capacity of these power plants was 2,291 MW.^[35] The electricity supply was approximately 12.8 TWh which is 12.6 per cent of the total generated renewable electricity.^[37] Biogas in Germany is primarily extracted by the co-fermentation of energy crops (In German science mostly the term ‘NawaRo’ is used that means ‘nachwachsende Rohstoffe’ = renewable resources) mixed with excrements or rather manure, the main crop utilized is corn. Furthermore bio waste and industrial and agricultural residues such as waste from the food industry are expended.^[38] Therefore biogas production in Germany differs highly from the methods of production in the UK with regards to the selection of the raw materials that biogas is extracted from, because mainly landfill gas is used in the UK’s biogas plants.^[33]

Biogas production in Germany has developed rapidly over the last 20 years. The main reason for this development is the legally created frameworks. Governmental support of renewable energies started at the beginning of the 1990s with the Law on Electricity Feed (StrEG). This law guaranteed the producers of energy from renewable sources the feed into the public power grid, thus the power companies were forced to take all produced energy from independent private producers of green energy.^[39] In 2002 the Law on Electricity Feed was replaced by the Renewable Energy Source Act (EEG). This law even guaranteed a fixed compensation for the produced electric power over 20 years. The amount of ca. 0.08 Euro gave particular farmers the opportunity to become an energy supplier and gaining a further source of income in the same place.^[38] The German agricultural biogas production was given a further push in 2004 by implementing the so-called NawaRo-Bonus. This is a special bonus payment given for the usage of renewable resources i.e. energy crops.^[40] In 2007 the German government stressed its intention to invest further effort and support in improving the renewable energy supply to provide an answer on growing climate challenges and increasing oil prices by the ‘Integrated Climate and Energy Programme’.

This continual trend of renewable energy promotion induces a number of challenges facing the management and organisation of renewable energy supply that has also several impacts on the biogas production.^[41] The first challenge to be noticed is the high area-consuming of the biogas electric power supply. In 2011 energy crops for biogas production consumed an area of circa 800,000 ha in Germany.^[42] This high demand of agricultural areas generates new competitions with the food industries that did not exist yet. Moreover new industries and markets were created in predominately rural regions entailing different new players with an economic, political and civil background. Their influence and acting has to be governed to gain all advantages this new source of energy is offering. Finally biogas will furthermore play an important role in the German renewable energy supply if good governance is focused.^[41]

In the Indian subcontinent

In India, Pakistan and Bangladesh biogas produced from the anaerobic digestion of manure in small-scale digestion facilities is called gobar gas; it is estimated that such facilities exist in over two million households in India and in thousands in Pakistan, particularly North Punjab, due to the thriving population of livestock. It has become a popular source of fuel in many parts of Nepal. The digester is an airtight circular pit made of concrete with a pipe connection. The manure is directed to the pit, usually directly from the cattle shed. The pit is then filled with a required quantity of wastewater. The gas pipe is connected to the kitchen fireplace through control valves. The combustion of this biogas has very little odour or smoke. Owing to simplicity in implementation and use of cheap raw materials in villages, it is one of the most environmentally sound energy sources for rural needs. One type of these system is the Sintex Digester. Some designs use vermiculture to further enhance the slurry produced by the biogas plant for use as compost.^[43] In order to create awareness and associate the people interested in biogas, an association “Indian Biogas Association” (www.biogas-India.com)^[44] is formed. The “Indian Biogas Association” aspires to be a unique blend of; nationwide operators, manufacturers and planners of biogas plants, and representatives from science and research. The association was founded in 2010 and is now ready to start mushrooming. The sole motto of the association is “propagating Biogas in a sustainable way”.

The Deenabandhu Model is a new biogas-production model popular in India. (Deenabandhu means “friend of the helpless.”) The unit usually has a capacity of 2 to 3 cubic metres. It is constructed using bricks or by a ferrocement mixture. In India, the brick model costs slightly more than the ferrocment model; however, India’s Ministry of New and Renewable Energy offers some subsidy per model constructed.

In Pakistan, the Rural Support Programmes Network is running the Pakistan Domestic Biogas Programme^[45] which has installed over 1500 biogas plants and has trained in excess of 200 masons on the technology and aims to develop the Biogas Sector in Pakistan.

Also PAK-Energy Solution^[45] has taken the most innovative and responsible initiatives in biogas technology. In this regard, the company is also awarded by 1st prize in “Young Entrepreneur Business Plan Challenge” jointly organized by Punjab Govt. & LCCI.^{[45][46][47][48]} They have designed and developed Uetians Hybrid Model, in which they have combined fixed dome and floating drums and Uetians Triplex Model. Both of these designs have been innovated first time in Pakistan. 10 years of experience of an expert team of young engineers & entrepreneurs has been put together in research & development along with senior PhD advisory board.

Moreover, Pakistan Dairy Development Company has also taken an initiative to develop this kind of alternative source of energy for Pakistani farmers. Biogas is now running diesel engines, gas generators, kitchen ovens, geysers, and other utilities in Pakistan. In Nepal, the government provides subsidies to build biogas plant.

China

The Chinese have been experimenting with the applications of biogas since 1958. Around 1970, China had installed 6,000,000 digesters in an effort to make agriculture more efficient. During the last years the technology has met high growth rates.

In developing nations

Domestic biogas plants convert livestock manure and night soil into biogas and slurry, the fermented manure. This technology is feasible for small holders with livestock producing 50 kg manure per day, an equivalent of about 6 pigs or 3 cows. This manure has to be collectable to mix it with water and feed it into the plant. Toilets can be connected. Another precondition is the temperature that affects the fermentation process. With an optimum at 36 C° the technology especially applies for those living in a (sub) tropical climate. This makes the technology for small holders in developing countries often suitable.

Depending on size and location, a typical brick made fixed dome biogas plant can be installed at the yard of a rural household with the investment between 300 to 500 US $ in Asian countries and up to 1400 US $ in the African context. A high quality biogas plant needs minimum maintenance costs and can produce gas for at least 15–20 years without major problems and re-investments. For the user, biogas provides clean cooking energy, reduces indoor air pollution, and reduces the time needed for traditional biomass collection, especially for women and children. The slurry is a clean organic fertilizer that potentially increases agricultural productivity.

Domestic biogas technology is a proven and established technology in many parts of the world, especially Asia.^[49] Several countries in this region have embarked on large-scale programmes on domestic biogas, such as China^[50][51] and India. The Netherlands Development Organisation, SNV,^[52] supports national programmes on domestic biogas that aim to establish commercial-viable domestic biogas sectors in which local companies market, install and service biogas plants for households. In Asia, SNV is working in Nepal,^[53] Vietnam,^[54] Bangladesh,^[55] Bhutan, Cambodia,^[55] Lao PDR,^[56] Pakistan^[57] and Indonesia,^[58] and in Africa; Rwanda,^[59] Senegal, Burkina Faso, Ethiopia,^[60] Tanzania,^[61] Uganda, Kenya, Benin and Cameroon.

See also

References

AEBIG; Asociación Española de Biogas

Further reading

• Updated Guidebook on Biogas Development. United Nations, New York, (1984) Energy Resources Development Series No. 27. p. 178, 30 cm.
• Book: Biogas from Waste and Renewable Resources. WILEY-VCH Verlag GmbH & Co. KGaA, (2008) Dieter Deublein and Angelika Steinhauser

External links

This article’s use of external links may not follow Wikipedia’s policies or guidelines. Please improve this article by removing excessive or inappropriate external links, and converting useful links where appropriate into footnote references. (May 2011)

• American Biogas Council
• Spanish Biogas Association / Asociación Española de Biogas
• Biogas Video Book
• Biogas Videos on YouTube
• An Introduction to Biogas, University of Adelaide
• Biogas from manure and waste products – Swedish case studies
• The largest danish plant Lemvig Biogas – renewable energy and a sound economy
• An overview of biogas purification technologies
• Biogas Bonanza for Third World Development
• Biogas China
• Small Scale Biogas Plants
• Biogas Wiki with a lot of useful information about basic principles and documentation from projects of various sizes
• Micro Biogas Production in Kenya

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