Anaerobic digestion (AD) is the natural process of breaking down plant or animal matter by microorganisms in the absence of air.
AD plants take biodegradable materials – such as food processing waste or animal slurry – and turn them into a methane-rich gas and a fertiliser residue.
The process takes place in large, air-tight tanks which have a mixing or ‘stirring’ mechanism inside to encourage the naturally occurring microbes to break down the waste. The gas produced is captured and used for electricity generation. The residual nitrogen-rich sludge, called digestate, is then heated to over 70 ̊C to kill the microbes and turn it into fertiliser.
The methane gas is used as fuel to power an electric generator (or can be cleaned of CO2 and fed into the gas grid). The waste heat is then captured by a purpose-designed water jacket, and the steam/heated water is in turn used to dry the digestate ready for use in the agricultural and horticultural industries.
It’s a tried and tested technology – there are more than 7,300 plants in Europe – that has been used for many years, for example by the sewage treatment industry, to treat and extract value from human waste.
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