Draft PPS18: Renewable Energy
Annex 1: Introduction Anaerobic Digestion
C2. Anaerobic digestion (AD) is a method of waste treatment that produces a gas with high methane content from organic materials such as agricultural, household and industrial residues and sewage sludge (feedstocks). The methane can be used to produce heat, electricity, or a combination of the two. The process has the benefit of using waste substances that are otherwise difficult to dispose of in an environmentally acceptable manner. Energy from AD is also effectively carbon neutral in that the carbon it releases is approximately equal to the carbon absorbed from the atmosphere by the plants which constitute the origin of the organic waste. It can therefore reduce overall quantities of carbon dioxide released in the atmosphere when it is used to replace energy from fossil fuels. When used for heating, the process is simple, with the minimum pre-treatment of the gas required, and the use of simple, well-proven technology.
C3. Methane is a significant contributor to global warming (around 21 times more potent than carbon dioxide over a period of 100 years). AD with energy recovery offers an effective means of trapping this gas and converting it to carbon dioxide, which is less potent as a greenhouse gas, while producing a renewable source of energy. By-products of AD may be put to beneficial uses such as compost and liquid fertiliser. Such products can help reduce the demand for synthetic fertilisers and other soil conditioners that may be manufactured using less sustainable methods.
C4. The AD process has been used widely in the UK agricultural sector in the form of small on-farm digesters producing biogas to heat farmhouses and other farm buildings. An AD project is most likely to be part of an integrated farm waste management system in which the feedstocks and products all play a part. However there is potential for larger scale centralised anaerobic digesters (CADs) using feedstocks imported from a number of sources.