Anaerobic digestion 1

Anaerobic digestion

Anaerobic digestion and regenerative thermaloxidiser component of Lübeck mechanical

biological treatment plant in Germany, 2007

Anaerobic digestion is a series of processes in which microorganismsbreak down biodegradable material in the absence of oxygen, used forindustrial or domestic purposes to manage waste and/or to releaseenergy.

The digestion process begins with bacterial hydrolysis of the inputmaterials in order to break down insoluble organic polymers such ascarbohydrates and make them available for other bacteria. Acidogenicbacteria then convert the sugars and amino acids into carbon dioxide,hydrogen, ammonia, and organic acids. Acetogenic bacteria thenconvert 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 wastemanagement system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobicdigesters can also be fed with purpose grown energy crops such as maize.

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, comprising ofmethane and carbon dioxide. This biogas can be used directly as cooking fuel, in combined heat and power gasengines[3] or upgraded to natural gas quality biomethane. The utilisation of biogas as a fuel helps to replace fossilfuels. The nutrient-rich digestate that is also produced can be used as fertilizer.

The technical expertise required to maintain industrial scale anaerobic digesters coupled with high capital costs andlow process efficiencies had limited the level of its industrial application as a waste treatment technology.[4]

Anaerobic digestion facilities have, however, been recognized by the United Nations Development Programme asone of the most useful decentralized sources of energy supply, as they are less capital intensive than large powerplants.[5]

Anaerobic digestion 2

History

Gas street lamp

Scientific interest in the manufacturing of gas produced by the naturaldecomposition of organic matter, was first reported in the seventeenth centuryby Robert Boyle and Stephen Hale, who noted that flammable gas was releasedby disturbing the sediment of streams and lakes.[6] In 1808, Sir Humphry Davydetermined that methane was present in the gases produced by cattle manure.[7]

[8] The first anaerobic digester was built by a leper colony in Bombay, India in1859. In 1895 the technology was developed in Exeter, England, where a septictank was used to generate gas for the sewer gas destructor lamp, a type of gaslighting. Also in England, in 1904, the first dual purpose tank for bothsedimentation and sludge treatment was installed in Hampton. In 1907, inGermany, a patent was issued for the Imhoff tank,[9] an early form of digester.

Through scientific research, anaerobic digestion gained academic recognition inthe 1930s. This research led to the discovery of anaerobic bacteria, themicroorganisms that facilitate the process. Further research was carried out toinvestigate the conditions under which methanogenic bacteria were able to growand reproduce.[10] This work was developed during World War II, during whichin both Germany and France there was an increase in the application ofanaerobic digestion for the treatment of manure.

Applications

Anaerobic digestion is particularly suited to organic material and is commonlyused for effluent and sewage treatment.[11] Anaerobic digestion is a simpleprocess that can greatly reduce the amount of organic matter, which mightotherwise be destined to be dumped at sea,[12] landfilled or burnt in an incinerator.[13]

Almost any organic material can be processed with anaerobic digestion.[14] [15] This includes biodegradable wastematerials such as waste paper, grass clippings, leftover food, sewage, and animal waste. The exception to this iswoody wastes that are largely unaffected by digestion, as most anaerobes are unable to degrade lignin. The exceptionbeing xylophalgeous anaerobes (lignin consumers), as used in the process for organic breakdown of cellulosicmaterial by a cellulosic ethanol start-up company in the U.S.[16] Anaerobic digesters can also be fed with speciallygrown energy crops such as silage for dedicated biogas production. In Germany and continental Europe, thesefacilities are referred to as biogas plants. A co-digestion or co-fermentation plant is typically an agriculturalanaerobic 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 cheap,low-cost energy for cooking and lighting.[18] [19] [20] [21] Anaerobic digestion facilities have been recognized by theUnited Nations Development Programme as one of the most useful decentralized sources of energy supply.[5] From1975, China (See Bioenergy in China) and India have both had large government-backed schemes for adaptation ofsmall biogas plants for use in the household for cooking and lighting.[22] At the present time, projects for anaerobicdigestion in the developing world can gain financial support through the United Nations Clean DevelopmentMechanism if they are able to show that 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 by-products. Anaerobic digestion 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

Anaerobic digestion 3

are called mechanical biological treatment plants.[24] [25] [26]

Utilising anaerobic digestion technologies can help to reduce the emission of greenhouse gasses in a number of keyways:• 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 lossesMethane and power produced in anaerobic digestion facilities can be utilized to replace energy derived from fossilfuels, and hence reduce emissions of greenhouse gases.[27] This is due to the fact that the carbon in biodegradablematerial is part of a carbon cycle. The carbon released into the atmosphere from the combustion of biogas has beenremoved by plants in order for them to grow in the recent past. This can have occurred within the last decade, butmore typically within the last growing season. If the plants are re-grown, taking the carbon out of the atmosphereonce more, the system will be carbon neutral.[28] [29] This contrasts to carbon in fossil fuels that has been sequesteredin the earth for many millions of years, the combustion of which increases the overall levels of carbon dioxide in theatmosphere.If the putrescible waste processed in anaerobic digesters were disposed of in a landfill, it would break down naturallyand often anaerobically. In this case, the gas will eventually escape into the atmosphere. As methane is about twentytimes more potent as a greenhouse gas than carbon dioxide, this has significant negative environmental effects.[30]

Digester liquor can be used as a fertiliser supplying vital nutrients to soils. The solid, fibrous component of thedigested material can be used as a soil conditioner to increase the organic content of soils. The liquor can be usedinstead of chemical fertilisers that require large amounts of energy to produce and transport. The use ofmanufactured fertilisers is, therefore, more carbon-intensive than the use of anaerobic digester liquor fertiliser. Incountries, such as Spain where there are many organically depleted soils, the markets for the digested solids can beequally as important as the biogas.[31]

In countries that collect household waste, the utilization of local anaerobic digestion facilities can help to reduce theamount of waste that requires transportation to centralized landfill sites or incineration facilities. This reducedburden on transportation reduces carbon emissions from the collection vehicles. If localized anaerobic digestionfacilities are embedded within an electrical distribution network, they can help reduce the electrical losses that areassociated with transporting electricity over a national grid.[32]

In Oakland, California at the East Bay Municipal Utility District’s (EBMUD) Main Wastewater TreatmentPlant(MWWTP), food waste is currently co-digested with primary and secondary municipal wastewater solids andother high-strength wastes. Compared to municipal wastewater solids digestion, food waste digestion has manybenefits. Anaerobic digestion of food waste pulp from the EBMUD food waste process provides a higher normalizedenergy benefit, compared to municipal wastewater solids:• 730 to 1,300 kWh per dry ton of food waste applied.• 560 to 940 kWh per dry ton of municipal wastewater solids applied.[33] [34]

Power generationBiogas 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. It turns out that 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 potential to increase to 150 MW, which is insignificant compared to the average power demand in the UK of about 35,000 MW. The

Anaerobic digestion 4

scope for biogas generation from non-sewage waste biological matter – energy crops, food waste, abattoir waste, etc.- is much higher, estimated to be capable of about 3,000 MW. Farm biogas plants using animal waste and energycrops are expected to contribute to reducing CO2 emissions and strengthen the grid while providing UK farmers withadditional revenues.[36]

Some countries offer incentives in the form of, for example, Feed-in Tariffs for feeding electricity onto the powergrid in order to subsidize green energy production.[37]

Grid injectionBiogas grid-injection is the injection of biogas into the natural gas grid.[38] As an alternative, the electricity and theheat can be used for on-site generation,[39] resulting in a reduction of losses in the transportation of energy. Typicalenergy losses in natural gas transmission systems range from 1–2%, whereas the current energy losses on a largeelectrical system range from 5–8%.[40]

In October 2010, Didcot Sewage Works became the first in the UK to produce biomethane gas supplied to thenational grid, for use in up to 200 homes in Oxfordshire.[41]

The processThere are many microorganisms that are involved in the process of anaerobic digestion including acetic acid-formingbacteria (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 attemperatures between 35–40 °C are called mesophiles or mesophilic bacteria. Some of the bacteria can survive at thehotter and more hostile conditions of 55–60 °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 conditionsof hydrothermal vents. These species are more resistant to heat and can, therefore, operate at high temperatures, aproperty that is unique to thermophiles.[44]

As with aerobic systems, the bacteria in anaerobic systems the growing and reproducing microorganisms withinthem require a source of elemental oxygen to survive.[45] In an anaerobic system, there is an absence of gaseousoxygen. Gaseous oxygen is prevented from entering the system through physical containment in sealed tanks.Anaerobes access oxygen from sources other than the surrounding air. The oxygen source for these microorganismscan be the organic material itself or may be supplied by inorganic oxides from within the input material. When theoxygen source in an anaerobic system is derived from the organic material itself, the 'intermediate' end-products areprimarily alcohols, aldehydes, and organic acids plus carbon dioxide. In the presence of specialised methanogens, theintermediates are converted to the 'final' end-products of methane, carbon dioxide, and trace levels of hydrogensulfide.[46] [47] In an anaerobic system the majority of the chemical energy contained within the starting material isreleased by methanogenic bacteria as methane.[6]

Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to befully effective. Therefore, it is common practice to introduce anaerobic microorganisms from materials with existingpopulations, a process known as "seeding" the digesters, and typically takes place with the addition of sewage sludgeor cattle slurry.[48]

Anaerobic digestion 5

Stages

The key process stages of anaerobic digestion

There are four key biological andchemical stages of anaerobicdigestion:[8]

1. Hydrolysis2. Acidogenesis3. Acetogenesis4. MethanogenesisIn most cases, biomass is made up of large organic polymers. In order for the bacteria in anaerobic digesters toaccess the energy potential of the material, these chains must first be broken down into their smaller constituentparts. These constituent parts or monomers such as sugars are readily available by other bacteria. The process ofbreaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysisof these high-molecular-weight polymeric components is the necessary first step in anaerobic digestion.[49] Throughhydrolysis 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 asvolatile fatty acids (VFAs) with a chain length that is greater than that of acetate must first be catabolised intocompounds that can be directly utilised by methanogens.[50]

The biological process of acidogenesis is where there is further breakdown of the remaining components byacidogenic (fermentative) bacteria. Here, VFAs are created along with ammonia, carbon dioxide, and hydrogensulfide, as well as other by-products.[51] The process of acidogenesis is similar to the way that milk sours.The third stage of anaerobic digestion is acetogenesis. Here, simple molecules created through the acidogenesisphase 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 utilise theintermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. It is thesecomponents that make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to bothhigh and low pHs and occurs between pH 6.5 and pH 8.[53] The remaining, non-digestible material that the microbescannot feed upon, along with any dead bacterial remains, constitutes the digestate.A simplified generic chemical equation for the overall processes outlined above is as follows:

C6H12O6 → 3CO2 + 3CH4

Anaerobic digestion 6

Configuration

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

Anaerobic digesters can be designed andengineered to operate using a number ofdifferent 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 ofdigestion. Biomass is added to the reactor atthe start of the process in a batch and issealed for the duration of the process. Batchreactors suffer from odour issues that can bea severe problem when they are emptied. Ina typical scenario, biogas production will beformed with a normal distribution pattern over time. Operator can use this fact to determine when they believe theprocess of digestion of the organic matter has completed. As the batch digestion is simple and requires lessequipment 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 instages to the reactor (continuous plug flow; first in – first out). Here, the end-products are constantly or periodicallyremoved, 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 (CSTRs), Upflow anaerobicsludge blanket (UASB), Expanded granular sludge bed (EGSB) and Internal circulation reactors (IC).[55] [56]

TemperatureThere are two conventional operational temperature levels for anaerobic digesters, which are determined by thespecies of methanogens in the digesters:[57]

• Mesophilic, which takes place optimally around 30-38 °C or at ambient temperatures between 20-45 °C wheremesophiles are the primary microorganism present

• Thermophilic, which takes place optimally around 49-57 °C at elevated temperatures up to 70 °C wherethermophiles are the primary microorganisms present

A limit case has been reached in Bolivia, with anaerobic digestion in temperature working conditions less than10 °C. The anaerobic process is very slow, taking more than three times the normal mesophilic time process.[21] Inexperimental work at University of Alaska Fairbanks, a 1000 litre digester using psychrophiles harvested from "mudfrom a frozen lake in Alaska" has produced 200–300 litres of methane per day, about 20–30 % of the output fromdigesters in warmer climates.[58]

There are a greater number of species of mesophiles than thermophiles. These bacteria are also more tolerant tochanges in environmental conditions than thermophiles. Mesophilic systems are, therefore, considered to be morestable than thermophilic digestion systems.As mentioned above, thermophilic digestion systems are considered to be less stable, the energy input is higher, and more energy is removed from the organic matter. However, 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

Anaerobic digestion 7

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 short high-temperature and -pressure pre-treatment (pasteurization/ hygienisation) stage that significantly enhances the gas output of the following standard mesophilic stage. Thehygienisation process is also applied in order to reduce the pathogenic micro-organisms in the feedstock.Hygienisation/pasteurization may be achieved by using a Landia BioChop hygienisation unit [60] or similar methodof combined heat treatment and solids maceration.A drawback of operating at thermophilic temperatures is that more heat energy input is required to achieve thecorrect operational temperatures. This increase in energy may not be outweighed by the increase in the outputs ofbiogas from the systems. Therefore, it is important to consider an energy balance for these systems.

SolidsIn a typical scenario, there are three different operational parameters associated with the solids content of thefeedstock 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 high-solids content between ~25-40%. Unlikewet digesters that process pumpable slurries, high solids (dry – stackable substrate) digesters are designed to processsolid substrates without the addition of water. There are three primary styles of dry digesters: continuous verticalplug flow and batch tunnel horizontal digesters. Continuous vertical plug flow are upright, cylindrical tanks wherefeedstock is continuously fed to the top of the digester and flows downward by gravity during digestion. In batchtunnel digesters, the feedstock is deposited in tunnel-like chambers with a gas-tight door. Neither approach hasmixing inside the digester. The amount of pretreatment such as contaminant removal depends both upon the natureof the waste streams being processed and the desired quality of the digestate. Grinding for size reduction is beneficialin continuous vertical systems as it accelerates digestion, while batch systems avoid grinding and instead requirestructure (e.g. yard waste) to reduce compaction of the stacked pile. Continuous vertical dry digesters have a smallerfootprint 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 typicallyhave a lower land requirement due to the lower volumes associated with the moisture.High solids digesters requirecorrection of conventional performance calculations (e.g. gas production, retention time, kinetics, etc.) originallybased on very dilute sewage digestion concepts, since larger fractions of the feedstock mass are potentiallyconvertible to biogas.[63]

Low-solids (wet) digesters can transport material through the system using standard pumps that require significantlylower energy input. Low-solids digesters require a larger amount of land than high-solids due to the increasevolumes associated with the increased liquid-to-feedstock ratio of the digesters. There are benefits associated withoperation in a liquid environment, as it enables more thorough circulation of materials and contact between thebacteria and their food. This enables the bacteria to more readily access the substances they are feeding off andincreases the speed of gas yields.

Anaerobic digestion 8

Number of stages

Two-stage, low-solids, UASB digestion component of a mechanical biologicaltreatment system near Tel Aviv, process water is seen in balance tank and

sequencing batch reactor, 2005

Digestion systems can be configured withdifferent levels of complexity:[61]

• One-stage or single-stage• Two-stage or multistageA single-stage digestion system is one inwhich all of the biological reactions occurwithin a single sealed reactor or holdingtank. Utilising a single stage reducesconstruction costs, however facilitates lesscontrol of the reactions occurring within thesystem. Acidogenic bacteria, through theproduction of acids, reduce the pH of thetank. Methanogenic bacteria, as outlinedearlier, operate in a strictly defined pHrange.[64] Therefore, the biological reactionsof the different species in a single-stagereactor can be in direct competition witheach other. Another one-stage reaction system is an anaerobic lagoon. These lagoons are pond-like earthen basinsused for the treatment and long-term storage of manures.[65] Here the anaerobic reactions are contained within thenatural anaerobic sludge contained in the pool.

In a two-stage or multi-stage digestion system, different digestion vessels are optimised to bring maximum controlover the bacterial communities living within the digesters. Acidogenic bacteria produce organic acids and morequickly grow and reproduce than methanogenic bacteria. Methanogenic bacteria require stable pH and temperaturein order to optimise their performance.[66]

Under typical circumstances, hydrolysis, acetogenesis, and acidogenesis occur within the first reaction vessel. Theorganic material is then heated to the required operational temperature (either mesophilic or thermophilic) prior tobeing pumped into a methanogenic reactor. The initial hydrolysis or acidogenesis tanks prior to the methanogenicreactor can provide a buffer to the rate at which feedstock is added. Some European countries require a degree ofelevated heat treatment in order to kill harmful bacteria in the input waste.[67] In this instance, there may be apasteurisation or sterilisation stage prior to digestion or between the two digestion tanks. It should be noted that it isnot possible to completely isolate the different reaction phases, and often there is some biogas that is produced in thehydrolysis or acidogenesis tanks.

ResidenceThe residence time in a digester varies with the amount and type of feed material, the configuration of the digestionsystem, 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, comparedto mesophilic digestion, is relatively fast. The plug-flow nature of some of these systems will mean that the fulldegradation of the material may not have been realised in this timescale. In this event, digestate exiting the systemwill be darker in colour and will typically have more odour.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 retentiontimes can be up to 90 days. In this manner, the UASB system is able to separate solid an hydraulic retention timeswith the utilisation of a sludge blanket.[69]

Anaerobic digestion 9

Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mixthe contents enabling the bacteria and the food to be in contact. They also allow excess material to be continuouslyextracted to maintain a reasonably constant volume within the digestion tanks.

Feedstocks

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

The most important initial issue whenconsidering the application of anaerobicdigestion systems is the feedstock to theprocess. Digesters typically can accept anybiodegradable material; however, if biogasproduction is the aim, the level ofputrescibility is the key factor in itssuccessful application.[70] The moreputrescible (digestible) the material thehigher the gas yields possible from thesystem.

Substrate composition is a major factor indetermining the methane yield and methaneproduction rates from the digestion ofbiomass. Techniques to determine thecompositional characteristics of thefeedstock are available, while parameters such as solids, elemental, and organic analyses are important for digesterdesign and operation.[71]

Anaerobes can break down material with varying degrees of success from readily, in the case of short-chainhydrocarbons such as sugars, to over longer periods of time, in the case of cellulose and hemicellulose.[72] Anaerobicmicroorganisms are unable to break down long-chain woody molecules such as lignin.[73] Anaerobic digesters wereoriginally designed for operation using sewage sludge and manures. Sewage and manure are not, however, thematerial with the most potential for anaerobic digestion, as the biodegradable material has already had much of theenergy content taken out by the animal that produced it. Therefore, many digesters operate with co-digestion of twoor 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-sitefeedstock), or various organic byproducts, such as slaughterhouse waste, fats oils and grease from restaurants,organic household waste, etc. (typical off-site feedstock)..Digestors processing dedicated energy crops can achieve high levels of degradation and biogas production. [62] [74]

[75] Slurry-only systems are generally cheaper but generate far less energy than those using crops such as maize andgrass silage; by using a modest amount of crop material (30 per cent), an AD plant can increase energy outputtenfold for only three times the capital cost, relative to a slurry-only system.[76]

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 that are produced. The moisture content of the target feedstock will also affect what type of system is applied to its treatment. In order to use a high-solids anaerobic digester for dilute feedstocks, bulking agents such as compost should be applied to increase the solid content of the input material.[77] Another key

Anaerobic digestion 10

consideration is the carbon:nitrogen ratio of the input material. This ratio is the balance of food a microbe requires inorder to grow. The optimal C:N ratio for the 'food' a microbe is 20–30:1.[78] Excess N can lead to ammoniainhibition of digestion.[74]

The level of contamination of the feedstock material is a key consideration. If the feedstock to the digesters hassignificant levels of physical contaminants such as plastic, glass, or metals, then pre-processing will be required inorder for the material to be used.[79] If it is not removed then the digesters can be blocked and will not functionefficiently. It is with this understanding that mechanical biological treatment plants are designed. The higher thelevel of pre-treatment a feedstock requires the more processing machinery will be required, and, hence, the projectwill have higher capital costs.[80]

After sorting or screening to remove any physical contaminants, such as metals, and plastics from the feedstock, thematerial is often shredded, minced, and mechanically or hydraulically pulped to increase the surface area available tomicrobes in the digesters and, hence, increase the speed of digestion. The maceration of solids can be achieved byusing a chopper pump to transfer the feedstock material into the airtight digester, where anaerobic treatment takesplace.

ProductsThere are three principal products of anaerobic digestion: biogas, digestate, and water.[61] [81] [82]

Biogas

Typical composition of biogas[83]

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 back-up gas flare

Biogas is the ultimate waste product of thebacteria feeding off the input biodegradablefeedstock (the methanogenesis stage ofanaerobic digestion is performed by archaea- a micro-organism on a distinctly differentbranch of the phylogenetic tree of life tobacteria), and is mostly methane and carbondioxide,[84] [85] with a small amounthydrogen and trace hydrogen sulfide.(As-produced, biogas also contains watervapor, with the fractional water vaporvolume a function of biogastemperature).[63] Most of the biogas isproduced during the middle of the digestion,

Anaerobic digestion 11

Biogas carrying pipes

after the bacterial population has grown, and tapers off as theputrescible material is exhausted.[86] The gas is normally stored ontop of the digester in an inflatable gas bubble or extracted andstored next to the facility in a gas holder.

The methane in biogas can be burned to produce both heat andelectricity, usually with a reciprocating engine or microturbine[87]

often in a cogeneration arrangement where the electricity andwaste heat generated are used to warm the digesters or to heatbuildings. Excess electricity can be sold to suppliers or put into thelocal grid. Electricity produced by anaerobic digesters isconsidered to be renewable energy and may attract subsidies.[88]

Biogas does not contribute to increasing atmospheric carbondioxide concentrations because the gas is not released directly intothe atmosphere and the carbon dioxide comes from an organicsource with a short carbon cycle.

Biogas may require treatment or 'scrubbing' to refine it for use as afuel.[89] Hydrogen sulfide is a toxic product formed from sulfatesin the feedstock and is released as a trace component of the biogas.National environmental enforcement agencies such as the U.S. Environmental Protection Agency‎ or the English andWelsh Environment Agency put strict limits on the levels of gasses containing hydrogen sulfide, and, if the levels ofhydrogen sulfide in the gas are high, gas scrubbing and cleaning equipment (such as amine gas treating) will beneeded to process the biogas to within regionally accepted levels.[90] An alternative method to this is by the additionof ferrous chloride FeCl2 to the digestion tanks in order to inhibit hydrogen sulfide production.[91]

Volatile siloxanes can also contaminate the biogas; such compounds are frequently found in household waste andwastewater. In digestion facilities accepting these materials as a component of the feedstock, low-molecular-weightsiloxanes volatilise into biogas. When this gas is combusted in a gas engine, turbine, or boiler, siloxanes areconverted into silicon dioxide (SiO2), which deposits internally in the machine, increasing wear and tear.[92] [93]

Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are available at thepresent time.[94] In certain applications, in situ treatment can be used to increase the methane purity by reducing theoffgas carbon dioxide content, purging the majority of it in a secondary reactor.[95]

In countries such as Switzerland, Germany, and Sweden, the methane in the biogas may be concentrated in order forit to be used as a vehicle transportation fuel or input directly into the gas mains.[96] In countries where the driver forthe utilisation of anaerobic digestion are renewable electricity subsidies, this route of treatment is less likely, asenergy is required in this processing stage and reduces the overall levels available to sell.[97]

DigestateDigestate is the solid remnants of the original input material to the digesters that the microbes cannot use. It alsoconsists 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, the different forms ofdigestate come from different digestion tanks. In single-stage digestion systems, the two fractions will be combinedand, if desired, separated by further processing.[98] [99]

Anaerobic digestion 12

Acidogenic anaerobic digestate

The second by-product (acidogenicdigestate) is a stable organic materialconsisting largely of lignin and cellulose,but also of a variety of mineral componentsin a matrix of dead bacterial cells; someplastic may be present. The materialresembles domestic compost and can beused as compost or to make low-gradebuilding products such as fibreboard.[100]

[101] The solid digestate can also be utilizedas feedstock for ethanol production.[102]

The third by-product is a liquid(methanogenic digestate) that is rich innutrients and can be used as a fertiliserdependent on the quality of the material being digested.[103] Levels of potentially toxic elements (PTEs) should bechemically assessed. This will be dependent upon the quality of the original feedstock. In the case of most clean andsource-separated biodegradable waste streams, the levels of PTEs will be low. In the case of wastes originating fromindustry, the levels of PTEs may be higher and will need to be taken into consideration when determining a suitableend 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 will hamper the growth of plants if it is used as asoil-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 theoverall volume of the material for transport. During this maturation, the ammonia will be broken down into nitrates,improving the fertility of the material and making it more suitable as a soil improver. Large composting stages aretypically used by dry anaerobic digestion technologies.[104] [105]

WastewaterThe final output from anaerobic digestion systems is water. This water originates both from the moisture content ofthe original waste that was treated but also includes water produced during the microbial reactions in the digestionsystems. This water may be released from the dewatering of the digestate or may be implicitly separate from thedigestate.The wastewater exiting the anaerobic digestion facility will typically have elevated levels of biochemical oxygendemand (BOD) and chemical oxygen demand (COD). These are measures of the reactivity of the effluent and showan ability to pollute. Some of this material is termed 'hard COD', meaning that it cannot be accessed by the anaerobicbacteria for conversion into biogas. If this effluent were put directly into watercourses, it would negatively affectthem by causing eutrophication. As such, further treatment of the wastewater is often required. This treatment willtypically be an oxidation stage wherein air is passed through the water in a sequencing batch reactors or reverseosmosis unit.[106] [107] [108]

Anaerobic digestion 13

References[1] Anaerobic digestion reference sheet (http:/ / www. waste. nl/ content/ download/ 472/ 3779/ file/ WB89-InfoSheet(Anaerobic Digestion).

pdf), waste.nl. Retrieved 25.10.07.[2] Hydrolysis, Sewage Sludge and Anaerobic Digestion (http:/ / www. anaerobic-digestion. com/ html/ wastewater_sludge_hydrolysis_w. php).

Retrieved 22.02.2010.[3] GE Jenbacher Biogas Engines (http:/ / www. clarke-energy. co. uk/ gas_engines. html), www.clarke-energy.com, accessed 15.04.11[4] Anaerobic digestion reference sheet (http:/ / www. waste. nl/ content/ download/ 472/ 3779/ file/ WB89-InfoSheet(Anaerobic Digestion).

pdf), waste.nl. Retrieved 26.10.07.[5] Biogas Bonanza for Third World Development (http:/ / www. i-sis. org. uk/ BiogasBonanza. php), i-sis.org.uk, retrieved 4.11.07, cites United

Nations Development Programme (UNDP) 1997 Report, Energy After Rio: Prospects and Challenges[6] Fergusen, T. & Mah, R. (2006) Methanogenic bacteria in Anaerobic digestion of biomass, p49[7] Cruazon, B. (2007) History of anaerobic digestion (http:/ / web. pdx. edu/ ~cruzan/ The Magic of Methane/ History of AD. htm),

web.pdx.edu. Retrieved 17.08.07.[8] Anaerobic digestion (http:/ / www. waste. nl/ content/ download/ 472/ 3779/ file/ WB89-InfoSheet(Anaerobic Digestion). pdf), waste.nl.

Retrieved 19.08.07.[9] Water.me.vccs.edu (http:/ / water. me. vccs. edu/ courses/ ENV149/ Imhoff. htm). Retrieved 22 February 2010.[10] Humanik, F. et al. (2007) Anaerobic digestion of animal manure (http:/ / www. epa. gov/ agstar/ pdf/ conference04/ humenik. pdf), epa.gov.

Retrieved 17.08.07.[11] Anaerobic Digestion (http:/ / www. wasteresearch. co. uk/ ade/ efw/ anaerobic. htm), wasteresearch.co.uk. Retrieved 24.10.07.[12] Sea Dumping of Sewage Sludge (http:/ / www. encyclopedia. com/ doc/ 1G1-9246063. html), encyclopedia.com. Retrieved 22.02.2010.[13] Ocean Dumping Ban Act (1988) (http:/ / www. bookrags. com/ research/ ocean-dumping-ban-act-1988-enve-02/ ), bookrags.com. Retrieved

22.02.2010.[14] An introduction to anaerobic digestion (http:/ / www. anaerobic-digestion. com/ html/ introduction-to-anaerobic-dige. html),

anaerobic-digestion.com. Retrieved 17.08.07.[15] Alfagy.com, retrieved 16.08.09 (http:/ / www. alfagy. com/ index. php?option=com_content& view=article& id=54& Itemid=31,)[16] The Effect of Lignin on Biodegradability (http:/ / forfuelfreedom. com/ pdfs/ Industry_Report. pdf), forfuelfreedom.com. Retrieved

08.08.08.[17] Lemmer, A. & Oeschsner, H. Co-fermentation of grass and forage maize (http:/ / ltnet. lv-h. de/ en/ volltext/ Lt20016/ LT20016E_412_413.

pdf), Energy, Landtechnik, 5/11, p 56, ltnet.lv-h.de[18] Friends of the Earth (2004) Anaerobic digestion Briefing Paper (http:/ / www. foe. co. uk/ resource/ briefings/ anaerobic_digestion. pdf),

foe.co.uk. Retrieved 17.08.07.[19] Cardiff University (2005) Anaerobic Digestion Page (http:/ / www. wasteresearch. co. uk/ ade/ efw/ anaerobic. htm), wasteresearch.co.uk.

Retrieved 17.08.07.[20] Doelle, H. W. (2001) Biotechnology and Human Development in Developing Countries (http:/ / www. ejbiotechnology. info/ content/ vol4/

issue3/ issues/ 02/ ), ejbiotechnology.info. Retrieved 19.08.07.[21] Transfer of low-cost plastic biodigester technology at household level in Bolivia (http:/ / www. lrrd. org/ lrrd19/ 12/ mart19192. htm),

lrrd.org[22] Biogas Bonanza for Third World Development (http:/ / www. i-sis. org. uk/ BiogasBonanza. php), i-sis.org.uk. Retrieved 4.11.07.[23] The Clean Development Mechanism in Nepal in The Tiempo Climate Newswatch (http:/ / www. tiempocyberclimate. org/ newswatch/

feature060212. htm), tiempocyberclimate.org[24] Juniper (2005) MBT: A Guide for Decision Makers – Processes, Policies & Markets (http:/ / www. juniper. co. uk/ Publications/ downloads.

html), 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 (http:/ / www. scotland.

gov. uk/ Resource/ Doc/ 1057/ 0002224. pdf), scotland.gov.uk. Retrieved 17.08.07.[26] Haase Mechanical Biological Treatment and Wet Anaerobic Digestion (http:/ / www. haase-energietechnik. de/ en/ Products_and_Services/

), haase-energietechnik.de. Retrieved 23.10.07.[27] Digestion systems website (http:/ / www. anaerobic-digestion. com/ ), anaerobic-digestion.com. Retrieved 19.08.07.[28] Benefits of Anaerobic Digestion (http:/ / www. afbini. gov. uk/ index/ services/ specialist-advice/ renewable-energy/

re-anaerobic-digestion-intro/ re-anaerobic-digestion-benefits. htm), afbini.gov.uk. Retrieved 22 February 2010.[29] Questions about biomass energy (http:/ / www. dti. gov. uk/ energy/ sources/ renewables/ news-events/ press-materials/ background/

biomass/ page24314. html), dti.gov.uk. Retrieved 17.08.07.[30] Global warming methane could be far more potent than carbon dioxide (http:/ / www. newmediaexplorer. org/ sepp/ 2005/ 02/ 01/

global_warming_methane_could_be_far_worse_than_carbon_dioxide. htm) newmediaexplorer.org. Retrieved 17.08.07.[31] Introduction and Spanish organic waste situation (http:/ / www. compostnetwork. info/ index. php?id=44), compostnetwork.info. Retrieved

19.08.07.[32] Renewable Energy Framework (http:/ / www. esru. strath. ac. uk/ EandE/ Web_sites/ 04-05/ demand_side_management/ framework5. htm),

esru.strath.ac.uk. Retrieved 8.11.07.[33] EPA.gov (http:/ / www. epa. gov/ region09/ waste/ organics/ ad/ EBMUDFinalReport. pdf), Retrieved 4.01.09.

Anaerobic digestion 14

[34] EPA.gov (http:/ / www. epa. gov/ region09/ waste/ organics/ ad/ index. html), Retrieved 4.01.09.[35] 38% HHV Caterpillar Bio-gas Engine Fitted to Sewage Works | Claverton Group (http:/ / www. claverton-energy. com/

38-hhv-caterpillar-bio-gas-engine-fitted-to-long-reach-sewage-works. html), claverton-energy.com[36] Alfagy.com (http:/ / www. alfagy. com/ index. php?option=com_content& view=article& id=54& Itemid=31), "Be Green – Make Gas"[37] CHP Feed-In Tariffs & Green Energy Financial Support (http:/ / www. alfagy. com/ index. php?option=com_content& view=article&

id=90& Itemid=77), www.alfagy.com[38] Half Britain’s homes could be heated by renewable gas (http:/ / www. nationalgrid. com/ corporate/ About+ Us/ climate/ press/ 020209.

htm), nationalgrid.com[39] Biogas flows through Germany's grid 'big time' (http:/ / www. renewableenergyworld. com/ rea/ / news/ article/ 2008/ 07/

biogas-flows-through-germanys-grid-big-time-53075), renewableenergyworld.com[40] Transmission loss (http:/ / www. energyvortex. com/ energydictionary/ energy_loss__transmission_loss. html), energyvortex.com[41] Shah, Dhruti (5 October 2010). "Oxfordshire town sees human waste used to heat homes" (http:/ / www. bbc. co. uk/ news/ uk-11433162).

BBC NEWS. . 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 (http:/ / www. nnfcc. co. uk/ tools/evaluation-of-opportunities-for-converting-indigenous-uk-wastes-to-fuels-and-energy-report-nnfcc-09-012)

[43] Discovering Anaerobic Digestion and Biogas (http:/ / www. face-online. org. uk/ resources/ factsheets/ discovering/ anerobic digestion andbiogas. pdf), face-online.org.uk. Retrieved 8.11.07.

[44] Methanogens (http:/ / microbewiki. kenyon. edu/ index. php/ Methanogens), microbewiki.kenyon.edu. Retrieved 24.10.07.[45] Aerobic and anaerobic respiration (http:/ / www. sp. uconn. edu/ ~terry/ Common/ respiration. swf), 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? (http:/ / www. anaerobic-digestion. com/ html/ the-anaerobic-digestion-proces. php),

anaerobic-digestion.com. Retrieved 17.08.07.[48] The biogas plant (http:/ / www. unu. edu/ unupress/ unupbooks/ 80434e/ 80434E0k. htm), 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 (http:/ / www-sop. inria. fr/ relation_ext/ slides/ water_treatment. pdf), sop.inria.fr. Retrieved 24.10.07.[52] Anaerobic digestion (http:/ / www. biotank. co. uk/ anaerobic_digestion. htm), biotank.co.uk. Retrieved 24.10.07.[53] Martin, A.D. (2007) Understanding Anaerobic Digestion (http:/ / www. esauk. org/ events/ Alastair_D_Martin. pdf), Presentation to the

Environmental Services Association, 16.10.07, esauk.org. Retrieved 22.10.07.[54] Anaerobic digestion (http:/ / www. energy. ca. gov/ research/ renewable/ biomass/ anaerobic_digestion/ index. html), energy.ca.gov.

Retrieved 18.06.09.[55] BIOPAQ IC (http:/ / www. paques. nl/ ?pid=42& parentid=41), paques.nl. Retrieved 19.08.07.[56] Biological processes with Biomar technology (http:/ / www. envirochemie. com/ Biological-Plants. 56. 0. html?& L=1) 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" (http:/ / www. newscientist. com/ article/ mg20827854.

000-cold-climates-no-bar-to-biogas-production. html). New Scientist (London: Sunita Harrington): pp. 14. . Retrieved 2011-02-04.[59] Animal by-products introduction (http:/ / ec. europa. eu/ food/ food/ biosafety/ animalbyproducts/ index_en. htm), ec.europa.eu. Retrieved

24.10.07.[60] untitled (http:/ / www. landiainc. com/ public/ File/ Brochurer/ Industri GB/ BioChop AK15A. C15 GB. pdf), landiainc.com[61] Feasibility study concerning anaerobic digestion in Northern Ireland (http:/ / eunomia. co. uk/ NI AD final report. pdf), eunomia.co.uk ,

Retrieved 19.08.07.[62] Jewell, W.; Cummings, R.; Richards, B. (1993). "Methane fermentation of energy crops: Maximum conversion kinetics and in situ biogas

purification". Biomass and Bioenergy 5: 261–278. doi:10.1016/0961-9534(93)90076-G.[63] 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: 65–26. doi:10.1016/0961-9534(91)90028-B.[64] Biomethanation in advances in biochemical engineering and biotechnology (http:/ / books. google. com/ books?id=-ZdRJfMKORwC&

pg=PA5& lpg=PA5& dq=acidogens+ reduce+ ph& source=web& ots=EJkTnAMkfq& sig=qlyQVlqbt_N_1m1c20NduxAEK90#PPP1,M1),books.google.com. Retrieved 24.10.07.

[65] Anaerobic Lagoons for Storage/Treatment of Livestock Manure (http:/ / extension. missouri. edu/ explore/ envqual/ eq0387. htm),missouri.edu. Retrieved 8.11.07.

[66] Abstract: Methanogenic population dynamics during start-up of anaerobic digesters treating municipal solid waste and biosolids (http:/ /www3. interscience. wiley. com/ cgi-bin/ abstract/ 71002223/ ABSTRACT), interscience.wiley.com. Retrieved 24.10.07.

[67] Animal By-Products Regulations (http:/ / www. defra. gov. uk/ ANIMALH/ by-prods/ ), defra.gov.uk. Retrieved 24.10.07.[68] HIMET—A Two-Stage Anaerobic Digestion Process for Converting Waste to Energy (http:/ / www. gastechnology. org/ webroot/ app/ xn/

xd. aspx?it=enweb& xd=4reportspubs\4_8focus\himetfocus. xml), gastechnology.org. Retrieved 19.08.07.

Anaerobic digestion 15

[69] Finstein, M. S. (2006) ArrowBio process integrates preprocessing and advanced anaerobic digestion to recover recyclables and generateelectricity (http:/ / www. oaktech-environmental. com/ documents/ ProceedingsSWANA2006Nashville. pdf), oaktech-environmental.com.Retrieved 19.08.07.

[70] Anaerobic digestion feedstock classification (http:/ / www. wisbiorefine. org/ proc/ anaerobic. pdf), 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? (http:/ / www. waste-management-world. com/ display_article/ 304404/ 123/ ARCHI/ none/ none/

Money-from-old-rope?/ ), waste-management-world.com. Retrieved 24.10.07.[73] Book Review: Biology of anaerobic microorganisms (http:/ / wwww. aslo. org/ lo/ toc/ vol_34/ issue_3/ 0647. pdf), aslo.org. Retrieved

24.10.07.[74]

This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand (http:/ / en. wikipedia. org/wiki/ Template:cite_doi/ 10. 1016. 2f0961-9534. 2891. 2990036-c. 0a?preload=Template:Cite_doi/ preload& editintro=Template:Cite_doi/editintro& action=edit)

[75] 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.

[76] National Non-Food Crops Centre. Farm-Scale Anaerobic Digestion Plant Efficiency, NNFCC 11-015 (http:/ / www. nnfcc. co. uk/ tools/farm-scale-anaerobic-digestion-plant-efficiency-nnfcc-11-015)

[77] Management of Urban Biodegradable Waste (http:/ / books. google. com/ books?doi=XfhO-iP3em4C& pg=PA193& lpg=PA193&dq=high+ solids+ anaerobic+ digestion+ bulking& source=web& ots=fEDEUaGKK4&sig=B9CbvsX37xfMDLF4ILVhMKp2D-U#PPP1,M1), books.google.com. Retrieved 24.10.07.

[78] Anaerobic co-digestion of sewage sludge and rice straw (http:/ / www. bvsde. ops-oms. org/ bvsaar/ cdlodos/ pdf/ anaerobiccodigestion495.pdf), bvsde.ops-oms.org. Retrieved 24.10.07.

[79] Anaerobic digestion of classified municipal solid wastes (http:/ / www. seas. ucla. edu/ stenstro/ r/ r10), seas.ucla.edu. Retrieved 24.10.07.[80] National Non-Food Crops Centre. Economic Assessment of Anaerobic Digestion Technology & its Suitability to UK Farming & Waste

Systems (Report, 2nd Edition), NNFCC 10-010 (http:/ / www. nnfcc. co. uk/ tools/economic-assessment-of-anaerobic-digestion-technology-and-its-suitability-to-uk-farming-and-waste-systems-report-2nd-edition-nnfcc-10-010)

[81] Abstract from Operation of Municipal Wastewater Treatment Plants Manual of Practice-MOP 11 Fifth Edition (http:/ / www. e-wef. org/timssnet/ products/ tnt_products. cfm?primary_id=MOP1131& Action=LONG& subsystem=ORD), e-wef.org. Retrieved 19.08.07.

[82] Anaerobic digestion (http:/ / www. energy. ca. gov/ development/ biomass/ anaerobic. html), energy.ca.gov[83] Basic Information on Biogas (http:/ / www. kolumbus. fi/ suomen. biokaasukeskus/ en/ enperus. html), kolumbus.fi. Retrieved 2.11.07.[84] guide to biogas (http:/ / www. adelaide. edu. au/ biogas/ Beginners), adelaide.edu.au. Retrieved 19.08.07.[85] How Anaerobic Digestion (Methane Recovery) Works (http:/ / www. eere. energy. gov/ consumer/ your_workplace/ farms_ranches/ index.

cfm/ mytopic=30003), eere.energy.gov. Retrieved 19.08.07.[86] Anaerobic digestion briefing sheet (http:/ / www. foe. co. uk/ resource/ briefings/ anaerobic_digestion. pdf), foe.co.uk. Retrieved 24.10.07.[87] GE Energy – Jenbacher Gas Engines for Power Generation (http:/ / www. power-technology. com/ contractors/ cogeneration/ jenbacher/ ),

power-technology.com. Retrieved 19.08.07.[88] work3.pdf UK Biomass Strategy 2007 (http:/ / www. defra. gov. uk/ environment/ climatechange/ uk/ energy/ renewablefuel/ pdf/

ukbio0507-), defra.gov.uk , Retrieved 19.08.07.[89] What is anaerobic digestion? (http:/ / www. afbini. gov. uk/ index/ services/ specialist-advice/ renewable-energy/

re-anaerobic-digestion-intro/ re-anaerobic-digestion-what-is. htm), afbini.gov.uk. Retrieved 24.10.07.[90] Removal of hydrogen sulfide from anaerobic digester gas (http:/ / www. patentstorm. us/ patents/ 5976373-description. html), U.S. Patent,

patentstorm.us. Retrieved 17.08.07.[91] Abstract from Online Measurement of Dissolved and Gaseous-Hydrogen Sulfide in Anaerobic Biogas Reactors (http:/ / www. cheric. org/

research/ tech/ periodicals/ vol_view. php?seq=23639), cheric.org. Retrieved 24.10.07.[92] Wheles, E. & Pierece, E. (2004) Siloxanes in landfill and digester gas (http:/ / www. scsengineers. com/ Papers/

Pierce_2004Siloxanes_Update_Paper. pdf), scsengineers.com. Retrieved 17.08.07.[93] Biogas Upgrading and Utilisation, EEA Bioenergy (http:/ / www. iea-biogas. net/ Dokumente/ Biogas upgrading. pdf), iea-biogas.net.

Retrieved 25.10.07.[94] Tower, P.; Wetzel, J.; Lombard, X. (2006-03). "New Landfill Gas Treatment Technology Dramatically Lowers Energy Production Costs"

(http:/ / www. appliedfiltertechnology. com/ Userfiles/ Docs/ AFT_SWANA_2006_Paper_Rev1. pdf). Applied Filter Technology. . Retrieved2009-04-30. , appliedfiltertechnology.com

[95] Richards, B. (1994). "In situ methane enrichment in methanogenic energy crop digesters". Biomass and Bioenergy 6: 275–274.doi:10.1016/0961-9534(94)90067-1.

[96] Biogas as a road transport fuel (http:/ / www. nfuonline. com/ x9498. xml) nfuonline.com. Retrieved 24.10.07.[97] Haase biogas energy centre (http:/ / www. haase-energietechnik. de/ en/ Products_and_Services/ Waste_Treatment/ Biogas_Engineering/

FE-488-e_Biogas_CHP. pdf) haase-energietechnik.de. Retrieved 19.08.07.[98] Fact sheet on anaerobic digestion (http:/ / www. waste. nl/ page/ 248), waste.nl. Retrieved 19.08.07.[99] Biomass and biogas (http:/ / www. globalwarming101. com/ content/ view/ 595/ 88888958/ ) globalwarming101.com. Retrieved 19.08.07.

Anaerobic digestion 16

[100] Oaktech Consultation Response to UK Source Segregation Requirement (http:/ / www. alexmarshall. me. uk/ index_files/ documents/ResponsetoConsultationonthesourcesegregationrequirementinParagraph7AofSchedule3totheWasteMan. pdf), alexmarshall.me.uk. Retrieved19.08.07.

[101] UK Strategy for centralised anaerobic digestion (http:/ / www. ingentaconnect. com/ content/ els/ 09608524/ 1995/ 00000052/ 00000003/art00039), ingentaconnect.com. Retrieved 24.10.07.

[102] Solid Digestate to Ethanol (http:/ / onlinelibrary. wiley. com/ doi/ 10. 1002/ bit. 22627/ abstract), onlinelibrary.wiley.com Retrieved11.18.10

[103] Biomass and biogas (http:/ / www. globalwarming101. com/ content/ view/ 595/ 88888958/ ), globalwarming101.com. Retrieved 24.10.07.[104] Vitoria Plant Information (http:/ / www. ows. be/ pub/ Vitoria_InfoEnviro. mei07. pdf), ows.be. Retrieved 24.10.07.[105] Kompogas Homepage (http:/ / www. kompogas. ch/ en/ ), kompogas.ch. Retrieved 24.10.07.[106] Abstract: Modelling a sequencing batch reactor to treat the supernatant from anaerobic digestion of the organic fraction of municipal solid

waste (http:/ / www. ingentaconnect. com/ content/ jws/ jctb/ 2007/ 00000082/ 00000002/ art00006?crawler=true), ingentaconnect.com.Retrieved 24.10.07.

[107] Clarke Energy Reverse Osmosis Unit (http:/ / www. clarke-energy. co. uk/ clarke_waste/ water_treatment. htm), clarke-energy.co.uk.Retrieved 24.10.07.

[108] BOD Effluent Treatment (http:/ / web. archive. org/ web/ 20080524072038/ http:/ / www. virtualviz. com/ wastewater. htm),virtualviz.com. Retrieved 24.10.07.

External links• Official Website of the Anaerobic Digestion and Biogas Association (http:/ / www. adbiogas. co. uk/ ), Anaerobic

Digestion and Biogas Association (ADBA)• Online AD Cost Calculator (http:/ / www. nnfcc. co. uk/ tools/

economic-assessment-of-anaerobic-digestion-technology-and-its-suitability-to-uk-farming-and-waste-systems-ad-cost-model-tool-nnfcc-10-010),nnfcc.co.uk

• UK's Official Information Portal on Anaerobic Digestion and Biogas (http:/ / www. biogas-info. co. uk/ ),biogas-info.co.uk

• Glossary of Anaerobic Digestion terms (http:/ / www. bioplex. co. uk/ glossary. shtml), bioplex.co.uk• Anaerobic digestion forum (http:/ / listserv. repp. org/ pipermail/ digestion_listserv. repp. org/ ), listserv.repp.org• Anaerobic digestion website (http:/ / www. anaerobic-digestion. com/ ), anaerobic-digestion.com• US Government Information Sheet: Methane from anaerobic digesters (http:/ / web. archive. org/ web/

20041124201613/ www. eere. energy. gov/ consumerinfo/ factsheets/ ab5. html?print), web.archive.org• Anaerobic biodigester design for small tropical producers (http:/ / www. ruralcostarica. com/ biodigester. html),

ruralcostarica.com• Low cost biodigester, Vietnam (http:/ / www. cipav. org. co/ lrrd/ lrrd9/ 2/ an92. htm), cipav.org.co• Appropedia article on home biogas systems• Biogas Community on WikiSpaces (http:/ / www. biogas. wikispaces. com/ ), biogas.wikispaces.com• Online Anaerobic Digester Output Estimator (http:/ / www. bioplex. co. uk/ estimator. shtml), bioplex.co.uk• Biogas Forum (http:/ / forum. zorg-biogas. com/ ), forum.zorg-biogas.com• American Biogas Council (http:/ / www. americanbiogascouncil. org/ )• (http:/ / onlinelibrary. wiley. com/ doi/ 10. 1002/ bit. 22627/ abstract), Solid Digestate to Ethanol• Introduction to Biogas and Anaerobic Digestion (http:/ / www. extension. org/ pages/

Introduction_to_Biogas_and_Anaerobic_Digestion), information from eXtension's Livestock and PoultryEnvironmental Learning Center

• Harper Adams Energy Limited (http:/ / www. haenergy. co. uk), Information on the Anaerobic Digester at HarperAdams University College

Article Sources and Contributors 17

Article Sources and ContributorsAnaerobic digestion  Source: http://en.wikipedia.org/w/index.php?oldid=435609590  Contributors: A. B., AbhisheksinghWIKI, Aconescs, Ag2003, Agre22, Alan Liefting, Amberroom, AndyDingley, Anna Lincoln, Ascidian, AstroHurricane001, Atasin11, Auntof6, Awis, AxelBoldt, Behun, Bogelund, Brandon, Brastein, Brighterorange, Brinerustle, Brusegadi, CStack3, Chowbok,Chriswaterguy, Cimbalom, Crusaderstar, Cult hero, Curtbeckmann, Cyclonenim, DARTH SIDIOUS 2, Dancter, Darkwind, David Woodward, Doodle77, Dorkapeter, Drphilharmonic, Ed lewis,Edward, Effenberger, Element16, EmmettLBrown, Energee5, Engineman, Epbr123, Euchiasmus, Eugene Kelly, Everyking, Fahadsadah, Farmideas, GHarrison14, GTZ-44-ecosan, GaiusCornelius, Gpanzani, Granitethighs, Gregalton, Gregkaye, Ground Zero, Gullinbrusti, Hai398, Harperweb, Hbent, Hmains, Hu12, Hugo-cs, Iknowtrash, Incrediblehunk, Interiot, Iridescent,Iulian28ti, J.delanoy, Jaimemh, Jamesmorrison, Jcorgan, Jeff Dahl, Jezhotwells, Jmhollen, John of Reading, Johnfos, Jorfer, Joseph Solis in Australia, Juliancolton, Jxm211, Jyril, Katana0182,Kjkolb, KlickingKarl, Ksd5, Kweeket, L Kensington, Lawrencekhoo, Lcmoreno, Ldbio130, Ling.Nut, Ljohnson13, Lkinkade, Lotje, Malleus Fatuorum, Mark.murphy, Marxspiro,Matthewireland, Maurreen, Mbeychok, Mcchino64, Michael Devore, Mion, Mirgy, Mmeijeri, Moreno.laura, Mp3banker, Mr3641, NCurse, NormaMcDonald, OhanaUnited, Onetoremember,Parutakupiu, Paul P, Pinethicket, Quale, Quasirandom, Qwertyman, RAM, Resqbrett, Rich Farmbrough, RichardF, Rjwilmsi, Robert.carpenter, Roger Davies, Ronz, Salvador95, Satori Son,Shenme, Simesa, SimonP, Skier Dude, Slawomir D-K, Spanjers, Stillnotelf, Tabletop, Tim10000, Trafford09, User A1, Vanished user 47736712, Velella, Vinayakbhogan, Vortexrealm,Wavelength, Welsh, Wikibiogas, Withlyn, Woohookitty, Yeeak61, Yellowdesk, Yousaf465, Кузнецов, 226 anonymous edits

Image Sources, Licenses and ContributorsImage:Haase Lubeck MBT.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Haase_Lubeck_MBT.JPG  License: Creative Commons Attribution 3.0  Contributors: Alex Marshall(Clarke Energy)Image:SGDL0001.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:SGDL0001.JPG  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Originaluploader was G J Coyne at en.wikipediaImage:Stages of anaerobic digestion.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Stages_of_anaerobic_digestion.JPG  License: Creative Commons Attribution 3.0 Contributors: Original uploader was Vortexrealm at en.wikipediaImage:Haase anaerobic digester.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Haase_anaerobic_digester.JPG  License: Creative Commons Attribution 3.0  Contributors:Original uploader was Vortexrealm at en.wikipediaImage:Anaerobic digesters overhead view.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Anaerobic_digesters_overhead_view.jpg  License: Creative CommonsAttribution-Sharealike 2.5  Contributors: Original uploader was Vortexrealm at en.wikipediaImage:Anaerobic Lagoon at Cal Poly.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Anaerobic_Lagoon_at_Cal_Poly.jpg  License: Creative Commons Attribution-Sharealike 2.5 Contributors: Kjkolb, Zephynelsson VonImage:Biogasholder and flare.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Biogasholder_and_flare.JPG  License: Creative Commons Attribution 3.0  Contributors: Originaluploader was Vortexrealm at en.wikipediaImage:Biogas pipes.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Biogas_pipes.JPG  License: Creative Commons Attribution 3.0  Contributors: Original uploader wasVortexrealm at en.wikipediaImage:Anaerobic digestate.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Anaerobic_digestate.JPG  License: Creative Commons Attribution-Sharealike 2.5  Contributors:Original uploader was Vortexrealm at en.wikipedia

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