A mixture of methane and carbon dioxide resulting from the anaerobic decomposition of organic waste materials, such as surplus food, manures, and sewage. The decomposition is carried out by methanogenic bacteria (see methanogen); these obligate anaerobes produce methane, the main component of biogas, which can be collected and used as an energy source for domestic processes, such as heating, cooking, and lighting; in some cases it can be injected into a pre-existing gas grid to supplement natural gas. The production of biogas is carried out in special digesters; these were traditionally used in China and India and have now been developed on a large scale in many parts of the world as a means of generating renewable energy from organic waste. As well as providing a source of fuel, these systems also enable sewage, which contains pathogenic bacteria, to be digested, thereby removing the danger to humans that could otherwise result from untreated domestic and agricultural waste.