Anaerobic digestion is the biological process of reclaiming energy from organic waste
Organic matter is derived from once living materials and is capable of decay. A variety of organic matter can be used as a feedstock for anaerobic digestion, for example, manure, energy crops, food waste or sewage sludge.
The process of anaerobic digestion involves mixing and heating the feedstock, to a temperature of 400C, within a large digester tank. Over a period of about 83days all the energy, in the form of biogas, is extracted from the feedstock, leaving a digestate residue.
The biogas is compiled of approximately 60% methane and 40% carbon dioxide. This can then be combusted in a combined heat and power plant to provide heat and electricity or a boiler to provide heat. The biogas can also be treated and upgraded to bio-methane which can then be injected into the national gas grid. The digestate is a nitrogen rich fertiliser which can be used in place of petro-chemical fertilisers.
By displacing petro-chemical fertilisers the dependence on artificial fertisliers is reduced. Production of petro-chemical fertilisers is carbon intensive with 2 tonnes of CO2 generated for each tone of nitrogen produced. Anaerobic digestion harnesses the methane naturally derived from the decay of organic matter. Methane is 21 times more potent as a greenhouse gas than carbon dioxide. Therefore utilising this resource for the production of energy, not only displaces fossil-fuel derived energy, but prevents the natural leakage of methane into the atmosphere.
According to WRAP, households waste one third of the food they purchase and produce 7 million tonnes of food waste each year. A further 10 million tonnes of food waste are generated by commercial catering and food processing. By utilising waste as a local feedstock resource, not only is it diverted from landfill, where there is the risk of potential water contamination, but there are the additional benefits of reduction of transportation emissions and costs. The production and use of energy locally does not incur the transmission losses derived from centralised energy generation. Anaerobic digestion can provide a valued additional revenue stream for the waste industry or farms.
Anaerobic Digestion Biogas Association:
ABDA - a practical guide to AD
Greenfinch AD presentation
FOE 2007 briefing:
Biogen website with case studies: