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Anaerobic Digestion
Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen.[1] The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.
Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin sediments, where it is usually referred to as "anaerobic activity".[2][3] This is the source of marsh gas methane as discovered by Alessandro Volta in 1776.[4][5]
The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. In acetogenesis, bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide amongst other compounds. Finally, methanogens convert these products to methane and carbon dioxide.[6] The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.[7]
Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge. 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.[8]
Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide, and traces of other 'contaminant' gases.[1] This biogas can be used directly as fuel, in combined heat and power gas engines[9] or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.
With the re-use of waste as a resource and new technological approaches that have lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom (2011),[10] Germany,[citation needed] Denmark (2011),[11] and the United States.[12]
The Metropolitan Borough of Walsall is a metropolitan borough in the West Midlands, England. It is named after its largest settlement, Walsall, but covers a larger area which also includes Aldridge, Bloxwich, Brownhills, Darlaston, Pelsall and Willenhall. It also serves as the post town for nearby Cannock Chase District and Lichfield District respectively.
The borough had an estimated population of 254,500 in 2007.[2]
The current boundaries were set as part of the provisions of the Local Government Act 1972, with a change to the north of the borough in 1994.[3] It is bounded on the west by the City of Wolverhampton, the south by the Metropolitan Borough of Sandwell, to the south east by the City of Birmingham, and by the Staffordshire districts of Lichfield, Cannock Chase and South Staffordshire to the east, north and northwest respectively. Most of the borough is highly industrialised and densely populated, but areas around the north and east of the borough are open space.
In 1986 the borough became an effective unitary authority when the West Midlands County Council was abolished. However it remains part of the West Midlands for ceremonial purposes, and for functions such as policing, fire and public transport.
