Waste disposal contributes to climate change, for example through the release of methane from landfill or the burning of fossil fuel derived plastics. Sending untreated waste to landfill and incineration are the worst options for climate change – according to research by the Community Recycling Network.

Following the implementation in the UK of the EU Landfill Directive, much of the biodegradable waste stream which would previously have been sent to landfill has to be dealt with by other means. With strong public opposition from local communities to incineration, local authorities are increasingly looking for other means to tackle the UK’s growing waste problem. One such technique is the process of anaerobic digestion (AD).

Anaerobic Digestion is a process whereby organic waste is broken down in a controlled, oxygen free environment by bacteria naturally occurring in the waste material.

This biological process releases by-products, which can be easily captured:
• The biogas produced can be combusted when the AD plant is used with a combined heat and power plant (CHP), providing electricity (which can power the AD process or be added to the national grid) and heat for homes.
• Fibre and liquid digestates can provide soil improvers and fertilisers. This may become increasingly important as the UK Government looks to improve soil conditions.

Anaerobic processes can occur naturally or in a controlled environment such as a biogas plant. In controlled environments, organic materials such as sewage sludge and other relatively wet organic materials, along with various types of bacteria, are put in an airtight container called a digester where the process occurs. Depending on the waste feedstock and the system design, biogas is typically 55 to 75 percent pure methane.

Anaerobic digesters have been around for a long time, and they are commonly used for sewage treatment and for managing animal waste. Almost any organic material can be processed in this manner. This also includes materials such as waste paper, grass clippings and leftover food.

As a result, materials that are currently going to landfill can be utilised; natural methane emissions are reduced and conventional generation with its associated carbon emissions is displaced. AD takes under 35 days to stabilise waste, where as in landfill, biological waste may emit methane and leachate for over 35 years. The AD plant can be regarded as part of an integrated waste management plan and can also assist farmers in complying with legislation on the safe handling of waste: the process stabilises slurries, making them easier to handle and can reduce the odour by up to 80%.

In the UK we expect a number of Anaerobic Digestion Plants to be built over the next 10 years, as the Waste Disposal Authorities throughout the nation enter contracts to build new waste management facilities.

Targets for the North West have been established for Anaerobic digestion including the following:

Anaerobic digestion plants fuelled by farm biogas 2MW, 4 schemes by 2010

Anaerobic digestion plants fuelled by sewage gas 10.4MW, 12 schemes by 2010