Anaerobic digestion is a process in which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste and/or to produce fuels. It is a widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide and traces of other gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality bio-methane. The nutrient-rich digestate also produced can be used as fertilizer.