Anaerobic Digestion is the process of breaking down biodegradable products in the absence of oxygen that produces a methane and carbon dioxide rich biogas that can be use to produce energy. The process is relatively simple and uses a large amount of organic matter that would otherwise be sent to landfill. The carbon produced during the biodegrading process is part of the natural carbon cycle, whereby new and growing plants absorb the carbon released by the dead degrading materials. Thus, as long as the plants etc that are being used for the anaerobic digestion plant are replaced, then the process is carbon neutral.
Almost any type of material that is biodegradable can be used, such as paper, grass cuttings, food and animal waste. These systems are particularly suited for farms, where waste management has become an increasing issue. Indeed, the inclusion and recent tariff increase under the FiT scheme, had exactly this in mind.
The products from Anaerobic Digestion are a biogas in the form of methane, which can be used as fuel and a fertilizer which is very good for the soil.
The waste is delivered to the site and is sorted mechanically to ensure that all non-biodegradable materials are removed. The organic waste is then shredded and mixed with water. This slurry is pumped into a sealed vessel which has no oxygen in it (to help the process). The slurry in the vessel is then heated and stirred where it stays for up to 3 weeks (the digestion or fermentation stage).
During this time a gas comprising about 60% methane and 40% carbon dioxide is produced. This is often used as the source of heat energy to warn the slurry in the vessel. There is also enough methane left over to power an electricity generation system. Also produced is a fertilizer from the slurry (the water is often compressed out), which can be used for farming.
The process is normally continuous with filling and removal of the vessel happening at the same time.
Anaerobic digestion installations qualify under the UK Feed In Tariff scheme. The tariff rates have recently been increased to encourage a faster uptake of installations and the new rates that will apply from August 1st 2011 are:
14p/kWh for installed capacity up to 250kW
13p/kWh fir installed capacity between 250kW and 500kW
These compare with the previous tariff rates of 12.1p/kWh
You also benefit from the Export tariff of 3.1p/kWh for any electricity exported. There is also a landfill tax that you can avoid, which amounts to:
2011 £48 per tonne
2012 £56 per tonne
2013 £64 per tonne
2014 £72 per tonne