Anaerobic digestion (AD) is an alternative way of treating organic waste from composting. Unlike composting, the waste is broken down by bacteria that work in the absence of oxygen (hence the term “anaerobic”).
AD plants typically need organic waste that has more moisture than waste materials used for composting (e.g. sewage sludge on sewage farms, animal slurries and food waste on farms). Garden waste for example is generally not suitable due to the high proportions of wood. Grass cuttings can be treated but only benefit from AD in conditions where the grass hasn’t already begun to decompose.
The organic food waste is shredded and if necessary water added. It is then pumped into airtight vessels where the bacteria break down the organic waste in a process called methanogenesis, producing a biogas. This gas is a mixture of 65% methane and 35% carbon dioxide which can be burnt in a generating engine to produce renewable electricity. A final bi product of the process is a nitrogen rich liquid fertiliser (different from compost which is a solid) but both can be used as a fertiliser or soil improver by farmers.
A small experimental AD plant is operated at AmeyCespa’s Waterbeach Waste Management Park by Summerleaze. The plant treats small amounts of animal slurries, food and liquid waste. The gas produced is burnt in a special combustion area built beside the unit that currently burns the Landfill Gas to produce renewable electricity.
For more information about the AD plant at Waterbeach please see the Summerleaze website or contact the Waterbeach Site Manager Edgar Blanco, e-mail email@example.com