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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]
Kent is a county in South East England and one of the home counties. It borders Greater London to the north-west, Surrey to the west and East Sussex to the south-west, and Essex to the north across the estuary of the River Thames; it faces the French department of Pas-de-Calais across the Strait of Dover. The county town is Maidstone. It is the fifth most populous county in England, the most populous non-Metropolitan county and the most populous of the home counties.
Kent was one of the first British territories to be settled by Germanic tribes, most notably the Jutes, following the withdrawal of the Romans.[3] Canterbury Cathedral in Kent, the oldest cathedral in England, has been the seat of the Archbishops of Canterbury since the conversion of England to Christianity that began in the 6th century with Saint Augustine. Rochester Cathedral in Medway is England's second-oldest cathedral. Located between London and the Strait of Dover, which separates England from mainland Europe, Kent has been the setting for both conflict and diplomacy, including the Battle of Britain in World War II and the Leeds Castle peace talks of 1978 and 2004.
England relied on the county's ports to provide warships through much of its history; the Cinque Ports in the 10th[4]–14th centuries and Chatham Dockyard in the 16th–20th centuries were of particular importance. France can be seen clearly in fine weather from Folkestone and the White Cliffs of Dover. Hills in the form of the North Downs and the Greensand Ridge span the length of the county and in the Vale of Holmesdale in between and to the south are most of the county's 26 castles.
Kent's economy is greatly diversified: agriculture, haulage, logistics and tourism are major industries. Because of its relative abundance of fruit-growing and hop gardens, Kent is known as "The Garden of England".[5] In northwest Kent, industries include extraction of aggregate building materials, printing and scientific research. Coal mining has also played its part in Kent's industrial heritage. Large parts of Kent are within the London commuter belt and its strong transport connections to the capital and the nearby continent make Kent a high-income county. Twenty-eight per cent of the county forms part of two Areas of Outstanding Natural Beauty: the North Downs and The High Weald.
