Anaerobic digestion

Throughout Britain, some councils collect segregated organic rubbish from people's homes. This includes garden waste and vegetable peelings.  The organic waste is delivered to a special processing plant called an ‘Anaerobic Digestor’.  Anaerobic means "in the absence of oxygen".

The waste is placed into a large enclosed metal chamber. It is sealed, and in the absence of oxygen the waste rapidly decomposes through the action of bacteria.

There are no anaerobic digestion plants in operation in Nottinghamshire at this time, but it is a technology that could be employed in the future to treat the organic fraction of the waste.

The anaerobic digestion process produces three useful products:

• Firstly, biogas, which can be used as a fuel to produce electricity.
• Secondly, a liquid which can be used as agricultural fertiliser.
• Thirdly, a solid which can be used as a soil conditioner.

What is it?

Like composting, anaerobic digestion reduces the bulk of organic waste by converting it into a relatively stable solid residue (digestate) similar to compost. Unlike composting, however, anaerobic digestion requires an oxygen-free environment (i.e. anaerobic conditions) for the specialised bacteria to function. The process produces a flammable gas consisting mainly of methane and carbon dioxide (biogas).

Anaerobic digestion is particularly suited to wet, organic waste. As such it has been used in the water industry for sewage treatment since the end of the last century. Most large sewage treatment works use the process, recovering the biogas to meet on-site heat and power requirements.

The digestate usually requires stabilisation by composting before a saleable product can be produced. The biogas can be used to produce electricity, although up to one third of it may be needed to heat the digester itself, since the process requires warm conditions.