Draft PPS18: Renewable Energy
Annex 1 Energy from Waste (Biological Processes) Technology: Anaerobic Digestion
C9. AD is the bacterial fermentation of organic waste in warm, oxygen-free conditions. This process converts complex organic molecules into an inflammable gas comprising methane and carbon dioxide, leaving liquid and solid residues. The gas is usually referred to as biogas. During this process, up to 60% of the digestible solids are converted into biogas. This gas can be used to fuel a generator, to supply heating systems, or to serve a range of industrial applications.
C10. The digestion process takes place in a sealed airless container (the digester) and needs to be warmed and mixed thoroughly to create the ideal conditions for the bacteria to convert the organic matter into biogas. There are two types of AD process:
- Mesophilic digestion. The digester is heated to 30-35°C and the feedstock remains in the digester typically for 15-30 days. Mesophilic digestion tends to be more robust and tolerant than the thermophilic process (see below), but gas production is less, larger digestion tanks are required and sanitisation, if required, is a separate process stage.
- Thermophilic digestion. The digester is heated to 55°C and the residence time is typically for 12-14 days. Thermophilic digestion systems offer higher methane production, faster throughput, and better pathogen ‘kill’, but require more expensive technology, greater energy input and a higher degree of operating and monitoring.
C11. A typical AD plant will comprise waste pre-treatment equipment, a digester tank, buildings to house ancillary equipment such as a generator, a biogas storage tank, a flare stack and associated pipework. If anaerobic digestion is to be carried out on municipal solid waste, pre-treatment facilities will be required to separate organic from inorganic waste. Plants that use sewage sludge or farm slurry will require post-digestion equipment to treat the resulting liquors.