Anaerobic digestion

Anaerobic digestion and regenerative thermal oxidiser component of Lübeck mechanical biological treatment plant in Germany, 2007

Sustainable energy

Renewable energy

Anaerobic digestion · Biomass

Hydroelectricity · Geothermal

Solar · Tidal

Wave · Wind

Energy conservation

Cogeneration · Energy efficiency

Geothermal · Green building

Microgeneration · Passive Solar

Organic Rankine Cycle

Sustainable transport

Biofuel · Electric vehicles

Plug-in hybrids

Environment Portal

v · d · e

Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen, 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.^[1]

It is used as part of the process to treat biodegradable waste and sewage sludge.^[2] 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.^[3]

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane and carbon dioxide. This biogas can be used directly as cooking fuel, in combined heat and power gas engines^[4] 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, had limited the level of its industrial application as a waste treatment technology.^[5] 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.^[6]

History

Gas street lamp

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.^[7] In 1808, Sir Humphry Davy determined that methane was present in the gases produced by cattle manure.^[8][9] 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,^[10] 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.^[11] 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.

Applications

Anaerobic digestion is particularly suited to organic material, and is commonly used for effluent and sewage treatment.^[12] Anaerobic digestion, a simple process, can greatly reduce the amount of organic matter which might otherwise be destined to be dumped at sea,^[13] dumped in landfills, or burnt in incinerators.^[14]

Almost any organic material can be processed with anaerobic digestion,^[15][16] including biodegradable waste materials, such as waste paper, grass clippings, leftover food, sewage, and animal waste. 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 pyrolisis, 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.^[17]

In developing countries, simple home and farm-based anaerobic digestion systems offer the potential for low-cost energy for cooking and lighting.^{[18][19][20][21]} Anaerobic digestion facilities have been recognized by the United Nations Development Programme as one of the most useful decentralized sources of energy supply.^[6] 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.^[22] 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.^[23]

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.^[24][25][26]

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

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,^[27] 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.^[28][29] 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.

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.^[30]

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.^[31]

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.^[32]

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.^[33][34]

Power generation

See also: Electrical energy efficiency on United States farms

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.^[35] 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.^[36]

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.^[37]

Grid injection

Biogas grid-injection is the injection of biogas into the natural gas grid.^[38] As an alternative, the electricity and the heat can be used for on-site generation,^[39] 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%.^[40]

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.^[41]

The process

Main article: Anaerobic respiration

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.^[42]

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.^[43] 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.^[44]

As with aerobic systems, the bacteria, the growing and reproducing microorganisms within anaerobic systems, require a source of elemental oxygen to survive,^[45] 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.^[46][47] In an anaerobic system, the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.^[7]

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.^[48]

Stages

The key process stages of anaerobic digestion

There are four key biological and chemical stages of anaerobic digestion:^[9]

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.^[49] 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.^[50]

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.^[51] 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.^[52]

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.^[53] 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:

C₆H₁₂O₆ → 3CO₂ + 3CH₄

Configuration

Farm-based maize silage digester located near Neumünster in Germany, 2007 - the green, inflatable biogas holder is shown on top of the digester.

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

A batch system is the simplest form of digestion. Biomass is added to the reactor at the start of the process in a batch, and is sealed for the duration of the process. Batch reactors suffer from odour issues that can be a severe problem when they are emptied. 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. As the batch digestion is simple and requires less equipment and lower levels of design work, it is typically a cheaper form of digestion.^[54]

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.^[55][56]

Temperature

The two conventional operational temperature levels for anaerobic digesters are determined by the species of methanogens in the digesters:^[57]

• 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.^[21] 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.^[58]

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 aand 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.^[59]

Certain processes shred the waste finely and use a thermal pretreatment stage (hygienisation) to significantly enhance the gas output of the following the 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 ^[60] 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

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%.^[61][62]

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.^[63]

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]}

Number of stages

Two-stage, low solids, UASB digestion component of a mechanical biological treatment system near Tel Aviv; the process water is seen in balance tank and sequencing batch reactor, 2005.

Digestion systems can be configured with different levels of complexity:^[61]

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.^[64] 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.^[65] 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.^[66]

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.^[67] 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

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.^[68]

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.^[69]

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

Anaerobic lagoon and generators at the Cal Poly Dairy, United States 2003

The most important initial issue when considering the application of anaerobic digestion systems is the feedstock to the process. Digesters typically can accept any biodegradable material; however, if biogas production is the aim, the level of putrescibility is the key factor in its successful application.^[70] The more putrescible (digestible) the material, the higher the gas yields possible from the system.

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.^[71]

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.^[72] Anaerobic microorganisms are unable to break down long-chain woody molecules, such as lignin.^[73] 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).^[74].

Digestors processing dedicated energy crops can achieve high levels of degradation and biogas production. ^[62][75][76] 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 AD plant can increase energy output tenfold for only three times the capital cost, relative to a slurry-only system.^[77]

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.^[78] 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.^[79] Excess N can lead to ammonia inhibition of digestion.^[75]

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.^[80] 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.^[81]

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.

Products

The three principal products of anaerobic digestion are biogas, digestate, and water.^[61][82][83]

Biogas

Main article: Biogas

Typical composition of biogas^[84]

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 holder with lightning protection rods and backup gas flare

Biogas carrying pipes

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,^[85][86] 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).^[63] 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.^[87] 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^[88] 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.^[89] 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.^[90] 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.^[91] Alternatively, the addition of ferrous chloride FeCl₂ to the digestion tanks inhibits hydrogen sulfide production.^[92]

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.^[93][94] Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are available at the present time.^[95] 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.^[96]

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.^[97] 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.^[98]

Digestate

Main article: 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.^[99][100]

Acidogenic anaerobic digestate

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.^[101][102] The solid digestate can also be used as feedstock for ethanol production.^[103]

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.^[104] 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.^[105][106]

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.^{[107][108][109]}

See also

	Renewable energy portal
	Energy portal
	Environment portal
	Sustainable development portal

• 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

1. ^ Anaerobic digestion reference sheet, waste.nl. Retrieved 25.10.07.
2. ^ Hydrolysis, Sewage Sludge and Anaerobic Digestion Retrieved 22.02.2010.
3. ^ Agricultural Biogas, www.clarke-energy.com, accessed 08.11.11
4. ^ GE Jenbacher Biogas Engines, www.clarke-energy.com, accessed 15.04.11
5. ^ Anaerobic digestion reference sheet, waste.nl. Retrieved 26.10.07.
6. ^ ^{a b} Biogas Bonanza for Third World Development, i-sis.org.uk, retrieved 4.11.07, cites United Nations Development Programme (UNDP) 1997 Report, Energy After Rio: Prospects and Challenges
7. ^ ^{a b} Fergusen, T. & Mah, R. (2006) Methanogenic bacteria in Anaerobic digestion of biomass, p49
8. ^ Cruazon, B. (2007) History of anaerobic digestion, web.pdx.edu. Retrieved 17.08.07.
9. ^ ^{a b} Anaerobic digestion, waste.nl. Retrieved 19.08.07.
10. ^ Water.me.vccs.edu. Retrieved 22 February 2010.
11. ^ Humanik, F. et al. (2007) Anaerobic digestion of animal manure, epa.gov. Retrieved 17.08.07.
12. ^ Anaerobic Digestion, wasteresearch.co.uk. Retrieved 24.10.07.
13. ^ Sea Dumping of Sewage Sludge, encyclopedia.com. Retrieved 22.02.2010.
14. ^ Ocean Dumping Ban Act (1988), bookrags.com. Retrieved 22.02.2010.
15. ^ An introduction to anaerobic digestion, anaerobic-digestion.com. Retrieved 17.08.07.
16. ^ Alfagy.com, retrieved 16.08.09
17. ^ Lemmer, A. & Oeschsner, H. Co-fermentation of grass and forage maize, Energy, Landtechnik, 5/11, p 56, ltnet.lv-h.de
18. ^ Friends of the Earth (2004) Anaerobic digestion Briefing Paper, foe.co.uk. Retrieved 17.08.07.
19. ^ Cardiff University (2005) Anaerobic Digestion Page, wasteresearch.co.uk. Retrieved 17.08.07.
20. ^ Doelle, H. W. (2001) Biotechnology and Human Development in Developing Countries, ejbiotechnology.info. Retrieved 19.08.07.
21. ^ ^{a b} Transfer of low-cost plastic biodigester technology at household level in Bolivia, lrrd.org
22. ^ Biogas Bonanza for Third World Development, i-sis.org.uk. Retrieved 4.11.07.
23. ^ The Clean Development Mechanism in Nepal in The Tiempo Climate Newswatch, tiempocyberclimate.org
24. ^ Juniper (2005) MBT: A Guide for Decision Makers – Processes, Policies & Markets, juniper.co.uk, (Project funding supplied by Sita Environmental Trust). Retrieved 22.11.06.
25. ^ Svoboda, I (2003) Anaerobic digestion, storage, olygolysis, lime, heat and aerobic treatment of livestock manures, scotland.gov.uk. Retrieved 17.08.07.
26. ^ Haase Mechanical Biological Treatment and Wet Anaerobic Digestion, haase-energietechnik.de. Retrieved 23.10.07.
27. ^ Digestion systems website, anaerobic-digestion.com. Retrieved 19.08.07.
28. ^ Benefits of Anaerobic Digestion, afbini.gov.uk^{[dead link]}. Retrieved 22 February 2010.
29. ^ Questions about biomass energy, dti.gov.uk. Retrieved 17.08.07.
30. ^ Global warming methane could be far more potent than carbon dioxide newmediaexplorer.org. Retrieved 17.08.07.
31. ^ Introduction and Spanish organic waste situation, compostnetwork.info. Retrieved 19.08.07.
32. ^ Renewable Energy Framework, esru.strath.ac.uk. Retrieved 8.11.07.
33. ^ EPA.gov, Retrieved 4.01.09.
34. ^ EPA.gov, Retrieved 4.01.09.
35. ^ 38% HHV Caterpillar Bio-gas Engine Fitted to Sewage Works | Claverton Group, claverton-energy.com
36. ^ Alfagy.com, "Be Green – Make Gas"
37. ^ CHP Feed-In Tariffs & Green Energy Financial Support, www.alfagy.com
38. ^ Half Britain’s homes could be heated by renewable gas, nationalgrid.com
39. ^ Biogas flows through Germany's grid 'big time', renewableenergyworld.com
40. ^ Transmission loss, energyvortex.com
41. ^ Shah, Dhruti (5 October 2010). "Oxfordshire town sees human waste used to heat homes". BBC NEWS. http://www.bbc.co.uk/news/uk-11433162. Retrieved 5 October 2010. 
42. ^ National Non-Food Crops Centre. Evaluation of Opportunities for Converting Indigenous UK Wastes to Fuels and Energy (Report), NNFCC 09-012
43. ^ Discovering Anaerobic Digestion and Biogas, face-online.org.uk. Retrieved 8.11.07.
44. ^ Methanogens, microbewiki.kenyon.edu. Retrieved 24.10.07.
45. ^ Aerobic and anaerobic respiration, sp.uconn.edu. Retrieved 24.10.07.
46. ^ Adapted from Beychok, M. (1967) Aqueous Wastes from Petroleum and Petrochemical Plants, First edition, John Wiley & Sons, LCCN 67019834
47. ^ What is the anaerobic digestion process?, anaerobic-digestion.com. Retrieved 17.08.07.
48. ^ The biogas plant, unu.edu. Retrieved 5.11.07.
49. ^ Sleat, R. & Mah, R. (2006) Hydrolytic Bacteria in Anaerobic digestion of biomass, p15
50. ^ Boone, D. & Mah, R. (2006) Transitional bacteria in anaerobic digestion of biomass, p35
51. ^ What is anaerobic digestion, sop.inria.fr. Retrieved 24.10.07.
52. ^ Anaerobic digestion, biotank.co.uk. Retrieved 24.10.07.
53. ^ Martin, A.D. (2007) Understanding Anaerobic Digestion, Presentation to the Environmental Services Association, 16.10.07, esauk.org. Retrieved 22.10.07.
54. ^ Anaerobic digestion, energy.ca.gov. Retrieved 18.06.09.
55. ^ BIOPAQ IC, paques.nl. Retrieved 19.08.07.
56. ^ Biological processes with Biomar technology^{[dead link]} envirochemie.com. Retrieved 19.08.07.
57. ^ Song, Y.C., Kwon, S.J., Woo, J.H. (2004) Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge, Water Res. 2004 Apr;38(7):1653–62
58. ^ Gupta, Sujata (2010-11-06). "Biogas comes in from the cold". New Scientist (London: Sunita Harrington): pp. 14. http://www.newscientist.com/article/mg20827854.000-cold-climates-no-bar-to-biogas-production.html. Retrieved 2011-02-04. 
59. ^ Animal by-products introduction, ec.europa.eu. Retrieved 24.10.07.
60. ^ [http://www.landia.co.uk/process+units/biochop+hygienisation BioChop Hygienisation Unit
61. ^ ^{a b c} Feasibility study concerning anaerobic digestion in Northern Ireland, eunomia.co.uk^{[dead link]}, Retrieved 19.08.07.
62. ^ ^{a b} Jewell, W.; Cummings, R.; Richards, B. (1993). "Methane fermentation of energy crops: Maximum conversion kinetics and in situ biogas purification". Biomass and Bioenergy 5 (3–4): 261–278. doi:10.1016/0961-9534(93)90076-G.  edit
63. ^ ^{a b} Richards, B.; Cummings, R.; White, T.; Jewell, W. (1991). "Methods for kinetic analysis of methane fermentation in high solids biomass digesters". Biomass and Bioenergy 1 (2): 65–26. doi:10.1016/0961-9534(91)90028-B.  edit
64. ^ Biomethanation in advances in biochemical engineering and biotechnology, books.google.com. Retrieved 24.10.07.
65. ^ Anaerobic Lagoons for Storage/Treatment of Livestock Manure, missouri.edu. Retrieved 8.11.07.
66. ^ Abstract: Methanogenic population dynamics during start-up of anaerobic digesters treating municipal solid waste and biosolids, interscience.wiley.com. Retrieved 24.10.07.
67. ^ Animal By-Products Regulations^{[dead link]}, defra.gov.uk. Retrieved 24.10.07.
68. ^ HIMET—A Two-Stage Anaerobic Digestion Process for Converting Waste to Energy, gastechnology.org. Retrieved 19.08.07.
69. ^ Finstein, M. S. (2006) ArrowBio process integrates preprocessing and advanced anaerobic digestion to recover recyclables and generate electricity, oaktech-environmental.com. Retrieved 19.08.07.
70. ^ Anaerobic digestion feedstock classification, wisbiorefine.org. Retrieved 24.10.07.
71. ^ Jerger, D. & Tsao, G. (2006) Feed composition in Anaerobic digestion of biomass, p65
72. ^ Money for old rope?^{[dead link]}, waste-management-world.com. Retrieved 24.10.07.
73. ^ Book Review: Biology of anaerobic microorganisms, aslo.org. Retrieved 24.10.07.
74. ^ www.bcfarmbiogas.ca/Feedstockenergy
75. ^ ^{a b} Richards, B.; Cummings, R. J.; Jewell, W. J. (1991). "High rate low solids methane fermentation of sorghum, corn and cellulose". Biomass and Bioenergy 1 (5): 249–260. doi:10.1016/0961-9534(91)90036-C.  edit
76. ^ Richards, B. (1991). "High solids anaerobic methane fermentation of sorghum and cellulose". Biomass and Bioenergy 1: 47–53. doi:10.1016/0961-9534(91)90051-D.  edit
77. ^ National Non-Food Crops Centre. Farm-Scale Anaerobic Digestion Plant Efficiency, NNFCC 11-015
78. ^ Management of Urban Biodegradable Waste, books.google.com. Retrieved 24.10.07.
79. ^ Anaerobic co-digestion of sewage sludge and rice straw, bvsde.ops-oms.org. Retrieved 24.10.07.
80. ^ Anaerobic digestion of classified municipal solid wastes, seas.ucla.edu. Retrieved 24.10.07.
81. ^ National Non-Food Crops Centre. Economic Assessment of Anaerobic Digestion Technology & its Suitability to UK Farming & Waste Systems (Report, 2nd Edition), NNFCC 10-010
82. ^ Abstract from Operation of Municipal Wastewater Treatment Plants Manual of Practice-MOP 11 Fifth Edition, e-wef.org. Retrieved 19.08.07.
83. ^ Anaerobic digestion, energy.ca.gov^{[dead link]}
84. ^ Basic Information on Biogas, kolumbus.fi. Retrieved 2.11.07.
85. ^ guide to biogas^{[dead link]}, adelaide.edu.au. Retrieved 19.08.07.
86. ^ How Anaerobic Digestion (Methane Recovery) Works, eere.energy.gov. Retrieved 19.08.07.
87. ^ Anaerobic digestion briefing sheet, foe.co.uk. Retrieved 24.10.07.
88. ^ GE Energy – Jenbacher Gas Engines for Power Generation, power-technology.com. Retrieved 19.08.07.
89. ^ work3.pdf UK Biomass Strategy 2007, defra.gov.uk^{[dead link]}, Retrieved 19.08.07.
90. ^ What is anaerobic digestion?, afbini.gov.uk. Retrieved 24.10.07.
91. ^ Removal of hydrogen sulfide from anaerobic digester gas, U.S. Patent, patentstorm.us. Retrieved 17.08.07.
92. ^ Abstract from Online Measurement of Dissolved and Gaseous-Hydrogen Sulfide in Anaerobic Biogas Reactors, cheric.org. Retrieved 24.10.07.
93. ^ Wheles, E. & Pierece, E. (2004) Siloxanes in landfill and digester gas, scsengineers.com. Retrieved 17.08.07.
94. ^ Biogas Upgrading and Utilisation, EEA Bioenergy, iea-biogas.net. Retrieved 25.10.07.
95. ^ Tower, P.; Wetzel, J.; Lombard, X. (2006-03). "New Landfill Gas Treatment Technology Dramatically Lowers Energy Production Costs". Applied Filter Technology. http://appliedfiltertechnology.com/_literature_37854/New_Landfill_Gas_Treatment_Tech_Dramatically_Lowers_Energy_Production_Costs. Retrieved 2009-04-30. ^{[dead link]}, appliedfiltertechnology.com^{[dead link]}
96. ^ Richards, B.; Herndon, F. G.; Jewell, W. J.; Cummings, R. J.; White, T. E. (1994). "In situ methane enrichment in methanogenic energy crop digesters". Biomass and Bioenergy 6 (4): 275–274. doi:10.1016/0961-9534(94)90067-1.  edit
97. ^ Biogas as a road transport fuel^{[dead link]} nfuonline.com. Retrieved 24.10.07.
98. ^ Haase biogas energy centre^{[dead link]} haase-energietechnik.de. Retrieved 19.08.07.
99. ^ Fact sheet on anaerobic digestion, waste.nl. Retrieved 19.08.07.
100. ^ Biomass and biogas^{[dead link]} globalwarming101.com. Retrieved 19.08.07.
101. ^ Oaktech Consultation Response to UK Source Segregation Requirement, alexmarshall.me.uk. Retrieved 19.08.07.
102. ^ UK Strategy for centralised anaerobic digestion, ingentaconnect.com. Retrieved 24.10.07.
103. ^ Solid Digestate to Ethanol, onlinelibrary.wiley.com Retrieved 11.18.10
104. ^ Biomass and biogas^{[dead link]}, globalwarming101.com. Retrieved 24.10.07.
105. ^ Vitoria Plant Information, ows.be. Retrieved 24.10.07.
106. ^ Kompogas Homepage^{[dead link]}, kompogas.ch. Retrieved 24.10.07.
107. ^ Abstract: Modelling a sequencing batch reactor to treat the supernatant from anaerobic digestion of the organic fraction of municipal solid waste, ingentaconnect.com. Retrieved 24.10.07.
108. ^ Clarke Energy Reverse Osmosis Unit^{[dead link]}, clarke-energy.co.uk. Retrieved 24.10.07.
109. ^ BOD Effluent Treatment, virtualviz.com. Retrieved 24.10.07.

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
• Low cost biodigester, Vietnam^{[dead link]}, cipav.org.co
• Appropedia article on home biogas systems
• Biogas Community on WikiSpaces, biogas.wikispaces.com^{[dead link]}
• Online Anaerobic Digester Output Estimator, bioplex.co.uk
• Biogas Forum, forum.zorg-biogas.com
• American Biogas Council
• [1], Solid Digestate to Ethanol
• Introduction to Biogas and Anaerobic Digestion, information from eXtension's Livestock and Poultry Environmental Learning Center
• Harper Adams Energy Limited, Information on the Anaerobic Digester at Harper Adams University College