If, however, this waste is captured and placed in a sealed, airless container known as a biodigester, then microbes within the solids begin to turn the slurry into CO2 and methane.
Research has shown that digesting just 6.6 tonnes of cow manure can produce enough gas to generate 186 kWh a day of electricity and 138 kWh a day of heat energy. This is sufficient to provide steady electricity for 20 homes, or heat for six. If small, farm based biodigesters were to make full use of the UK’s yield of animal waste and supply excess power to their neighbours, some 1.6 TWh of electricity – nearly 1 per cent of UK domestic electricity requirements – could be generated from what is essentially waste.
Food waste has similar potential. The Government’s Waste and Resources Action Programme calculates that biodigesting the 5.5 millions tonnes of food waste sent to landfill each year could generate between 477 and 761 GWh of electricity every year, meeting the needs of 164,000 households, and saving up to 3.6 million tonnes of CO2, partly by avoiding climate damaging methane emissions from the rotting produce.
The gas given off by the UK’s landfill and sewage processing sites is already widely used as a renewable source of electricity, but is under-exploited. Research by the European Environment Agency in 2006 showed that the UK could produce 85 TWh of heat energy from biodegradable waste and sewage – equivalent to one eighth of our current demand.
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This article first appeared in the Ecologist November 2007