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Anaerobic digestion 1

Anaerobic digestion

Anaerobic digestion and regenerative thermaloxidiser component of Lbeck 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]

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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

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Anaerobic digestion 3

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