Anaerobic digestion (AD) is a naturally occurring process in which organic matter is broken down by microorganisms without the presence of oxygen to form methane gas. In the context of renewable energy, AD relates to the containment of the process in order that the methane, or bio-gas, be used directly to power Combined Heat and Power engines (see below).
AD reduces emissions of greenhouse gases as it replaces the burning of carbon in fossil fuels with the burning of carbon in biodegradable material grown in the recent past, usually the last growing season.When the plants are re-grown they absorb the CO2 released previously, thus closing the cycle.

Uses of AD

AD has long been used as part of the sewage treatment process in order to minimise the amount of effluent dumped at sea – the methane is generally used to power the processing plant and heat the digester. More recently, AD has been used as part of integrated waste management systems, helping to reduce the emission of methane produced in landfill sites into the atmosphere. Generally speaking, the methane is simply burnt off rather than used for energy production due to lack of any nearby demand;as a greenhouse gas, methane is 20 times more potent than CO2
In developing countries, simple home and farm-based anaerobic digestion systems have been used for decades as a way of generating low-cost energy for cooking and lighting.
However, it is only relatively recently that AD has appeared on UK farms where it is generally used to treat waste, provide heating and lighting for farm buildings, and produce nutrient-rich digestate (residue) used as fertilizer. Farm AD plants are expected to contribute to reducing CO2 emissions and strengthen the grid, while providing UK farmers with additional revenues.
In October 2010, Didcot Sewage Works became the first in the UK to produce biomethane gas supplied to the national gas grid. Ecotricity are the only UK energy provider who currently offers a ‘Green Gas’ tariff (Frack Free!), the profits of which go into ‘Green Gas’ production development. 
As yet there is no small-scale (or micro) AD unit available to UK households. However, it is only likely to be a matter of time, since micro AD units would mean that organic waste was taken care of locally, thereby negating transportation emissions, and  the gas could be used either in a micro-CHP unit (see below) or to cook with, and the digestate as a good nutrient-rich soil improver.
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