Anaerobic digestion 

Anaerobic digestion (AD) involves the break down of biodegradable material in the absence of oxygen by micro-organisms called methanogens. AD is already widely used to treat sewage and wastewater in the UK and can also be used to treat other organic wastes, including domestic and commercial food waste, manures and biofuel crops.

There are two main types of anaerobic digestion, defined by the temperatures at which the process is operated. The commonest form of AD, called mesophilic, is operated at a temperature of 35 to 40°C. Thermophilic processes reach temperatures of up to 60°C. Both systems operate in a liquid phase where the feedstock is processed to create a slurry which degrades in the absence of oxygen to produce an energy rich biogas. There are also systems which operate “dry” but these are not yet in common use and have yet to be proven in the treatment of food wastes in the UK. Other systems operate on a batch process. Where food waste controlled under the Animal By-product Regulations is treated in a mesophylic process, a pasteurisation stage is employed to ensure that the end product is free from harmful bacteria.

The process of anaerobic digestion provides a source of renewable energy, since the waste is broken down to produce biogas (a mixture of methane and carbon dioxide), which is suitable for energy production. The biogas can be used to generate electricity and heat to power on-site equipment and the excess electricity can be exported to the National Grid. We are also exploring other possible uses for the biogas including clean-up of the gas and injection to the gas grid, and using it as a vehicle fuel.

A further by-product of the process is biofertiliser (the digestate from the process), which is rich in nutrients such as nitrogen, phosphorus and other elements required for healthy plant growth and fertile soil. Biofertiiser from source segregated waste can be applied directly to agricultural land where it meets the PAS standard. Where waste is from mixed sources, the biofertiliser must be applied in accordance with an environmental permit for the receiving land.

Wanlip AD Plant is
part of the Leicester
City Council 25 year
PFI contract

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Poplars AD Plant will
treat waste from
households and
businesses incluing

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