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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]
Plymouth (/ˈplɪməθ/ ⓘ PLI-məth) is a port city and unitary authority in Devon, South West England. It is located on Devon's south coast between the rivers Plym and Tamar, about 36 miles (58 km) southwest of Exeter and 193 miles (311 km) southwest of London. It is the most populous city south of London.
Plymouth's history extends back to the Bronze Age, evolving from a trading post at Mount Batten into the thriving market town of Sutton, which was formally re-named as Plymouth in 1439 when it was made a borough. The settlement has played a significant role in English history, notably in 1588 when an English fleet based here defeated the Spanish Armada, and in 1620 as the departure point for the Pilgrim Fathers to the New World. During the English Civil War, the town was held by the Parliamentarians and was besieged between 1642 and 1646. In 1690 a dockyard was established on the River Tamar for the Royal Navy and Plymouth grew as a commercial shipping port throughout the Industrial Revolution.
After absorbing nearby settlements in 1914, the borough was awarded city status in 1928. During World War II, Plymouth suffered extensive damage in the Plymouth Blitz, leading to post-war rebuilding that significantly shaped its modern appearance. A further expansion of its boundaries in 1967 contributed to its current status as the 30th-most populous built-up area in the UK and the second-largest city in the South West after Bristol, with a population in 2021 of 264,727.
Plymouth's economy, historically rooted in shipbuilding and seafaring, has transitioned towards a service-based economy since the 1990s. It maintains strong maritime connections, hosting HMNB Devonport, the largest operational naval base in Western Europe, and offering ferry links to Brittany and Spain. The city is also home to the University of Plymouth, reflecting its educational and cultural significance. Today, the city is governed locally by Plymouth City Council and is represented nationally by two Members of Parliament.
