Anaerobic digestion is a well-proven renewable energy technology. At the same time, it can reduce greenhouse gas emissions by capturing methane from the decomposition of organic materials, such as livestock manures and slurries, sewage sludge and food wastes. Anaerobic digestion involves harnessing the natural process whereby organic matter is broken down by bacteria in the absence of oxygen. The materials ferment in a closed vessel and produce a biogas which is a mixture of about 60% methane and 40% carbon dioxide, with other trace gases, such as hydrogen sulphide. This can be used as a renewable energy source, both for heat and power, and as a transport fuel.

The treated liquid (or digestate) can be used as a fertiliser. In certain circumstances, it may prove attractive to separate fibre from the digestate and sell this as a soil conditioner and low grade fertiliser. Anaerobic digestion can be carried out in small scale systems, for example located on the farm and operated by farmers, or to serve businesses (or clusters of businesses) with large food waste arisings. Alternatively it can be carried out in large centralised systems, for example to treat municipal food waste being diverted from landfill by local authorities or manures and slurries from several farms.

Anaerobic digestion can lead to significant reductions in methane emissions from manures and slurries. Agriculture accounts for 7% of all UK greenhouse gas emissions of which about a third is due to methane emissions (a greenhouse gas with a global warming potential 21 times that of carbon dioxide over a 100-year time horizon). About 86% of this methane comes from enteric fermentation in the digestive system of animals, and 14% from manures and slurries. Anaerobic digestion offers the opportunity to capture methane from manures and slurries and so can lead to reductions in emissions to the atmosphere.

All biodegradable wastes have a significant potential to produce greenhouse gases when they decompose in landfills, which account for 3% of UK greenhouse gas emissions. For rapidly degrading wastes, such as food or kitchen wastes, anaerobic digestion offers climate change and energy benefits over some alternative food waste treatment methods, such as IVC (In Vessel Composting). Other environmental benefits, when anaerobic digestion is operated well and the digestate applied correctly to land, include improved air and water quality. The odour from treated animal slurry is significantly lower and less offensive than that of raw slurry applications to land. By destroying pathogens, anaerobic digestion reduces the risk of pollution of water from faecal indicator organisms (FIOs).

For further details on Anaerobic Digestion please refer to the Scoping Study (.pdf) and the Technical Feasibility Study (.pdf) and appendices (.pdf)

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