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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 City of Leeds is a city and metropolitan borough in West Yorkshire, England. The metropolitan borough includes the administrative centre of Leeds and the towns of Farsley, Garforth, Guiseley, Horsforth, Morley, Otley, Pudsey, Rothwell, Wetherby and Yeadon.[4] It has a population of 793,139 (mid-2019 est.), making it technically the second largest city in England by population behind Birmingham, since London is not a single local government entity. It is governed by Leeds City Council.
The current city boundaries were set on 1 April 1974 by the provisions of the Local Government Act 1972, as part a reform of local government in England. The city is a merger of eleven former local government districts; the unitary City and County Borough of Leeds combined with the municipal boroughs of Morley and Pudsey, the urban districts of Aireborough, Garforth, Horsforth, Otley and Rothwell, and parts of the rural districts of Tadcaster, Wharfedale and Wetherby from the West Riding of Yorkshire.
For its first 12 years the city had a two-tier system of local government; Leeds City Council shared power with West Yorkshire County Council. Since the Local Government Act 1985 Leeds City Council has effectively been a unitary authority, serving as the sole executive, deliberative and legislative body responsible for local policy, setting council tax, and allocating budget in the city, and is a member of the Leeds City Region Partnership. The City of Leeds is divided into 31 civil parishes and a single unparished area.
