Anaerobic Digestion
Cattle stocks best suit Anearobic Digestion
Anaerobic digestion is widely used as a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich digestate can be used as fertiliser.
Almost any organic material can be processed with anaerobic digestion, including waste paper and cardboard (which is of too low a grade to recycle, e.g. because of food contamination), grass clippings, leftover food, industrial effluents, sewage and animal waste. Anaerobic digestion produces a biogas made up of around 60 percent methane and 40 percent carbon dioxide (CO2).
This can be burnt to generate heat or electricity or can be used as a vehicle fuel. If used to generate electricity the biogas needs to be scrubbed. It can then power the anaerobic digestion process or be added to the national grid and heat for homes.
As well as biogas, anaerobic digestion produces a solid and liquid residue called digestate that can be used as a soil conditioner to fertilise land. The amount of biogas and the quality of digestates obtained will vary according to the feedstock used. More gas will be produced if the feedstock is putrescible, which means it is more liable to decompose.
Sewage and manure yield less biogas, as the animal that produced it has already taken out some of the energy content. In the UK, anaerobic digestion has until recently been limited to small on-farm digesters, but is widely used across Europe.
Denmark has a number of farm co-operative anaerobic digestion plants which produce electricity and district heating for local villages, biogas plants have been built in Sweden to produce vehicle fuel for fleets of town buses and Germany and Austria have several thousand on-farm digesters treating mixtures of manure, energy crops and restaurant waste, with the biogas used to produce electricity.
Anaerobic digestion provides an important opportunity to generate 100 percent renewable energy from biodegradable waste. Research clearly indicates the most sustainable way to treat our food waste is to have separate weekly collections for treatment by anaerobic digestion. Strong backing in the new Waste Strategy should mean that we start to fulfil this potential, with the widespread introduction of food waste collections and the construction of more anaerobic digestion plants across the UK.
Anaerobic Digestion Fact Sheet from Farming Futures (PDF - 1.5MB)
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