Composting is a biological process in which micro-organisms convert degradable organic matter into carbon dioxide and water vapour, using oxygen in the air, and leaving a bulk-reduced, stabilised residue known as compost.

Composting of household waste:

• may reduce the environmental impact of having to incinerate or landfill the waste
• provides compost that can improve soil structure, increase fertility and improve fertiliser efficiency
• may reduce the need for materials such as peat and fertilisers.

Composting is ideally applied to ‘green’ garden-type wastes. It is more problematic when used for kitchen waste because of difficulties with odour, insects and contamination, although some equipment suppliers claim to have overcome these problems. Successful composting (ie to produce a saleable product) can only be achieved if separated organic matter, as opposed to whole waste, is used as the feedstock.

Composting is regarded as recycling and has been included in the Project Integra recycling rate target of 40% by 2005. 

Householder or home composting has traditionally been part of the gardening culture of Britain. The amount of waste being composted at home is difficult to determine, as figures are only available for districts where authorities have introduced schemes to provide households with special compost bins.  Even then there is little evidence that the quantity of waste needing disposal has been reduced since most ‘home composters’ are already doing it. Many households cannot compost, either because they have no garden or no space.

Interest is increasing in using worms in home composting, particularly for kitchen wastes.

Worm composting (sometimes called 'vermiculture’) uses worms and micro-organisms to convert organic wastes into nutrient rich compost or humus. This is the same process which occurs naturally in areas of decaying vegetation. Wormeries are beginning to be introduced for home composting but experience with them is still fairly limited. Two types of worms are normally associated with the processing of organic waste: redworm or tiger worm.

Hampshire local authorities have promoted various offers for home composters at subsidised or reduced prices.

The number of centralised local authority commissioned organic waste composting facilities in the UK has been increasing rapidly since 1990, stimulated by the recycling credits scheme. In 1995 over 30 schemes were operating, taking waste from parks and gardens or source-separated garden waste taken by householders to household waste recycling centres HWRCs;  In Hampshire over 40,000 tonnes of green waste are processed through centralised facilities each year.

The majority of central composting schemes use the turned window method, a process in which piles of shredded and mixed organic waste approximately 3 metres high, 4 metres wide, and any length, are constructed. The windrows are turned regularly to ensure an even mixture, to provide aeration and to control temperature and moisture. Composting in such a way can produce odour problems and attract flies, if not properly managed, and there is also some concern about the release of airborne bacteria and fungal spores. In-vessel compost systems are available which reduce these problems but are more costly. 

The quality of incoming material for composting needs to be carefully controlled, otherwise contamination can be spread with the resultant compost. Also there is some concern over the extent to which green waste composting facilities attract new material into the system.

Waste-derived compost can be used as a soil conditioner and improver for residential gardening, parks and agriculture and for land reclamation, but concerns over its quality have limited its penetration of the major potential outlets in commercial horticulture and agriculture. The green waste processed centrally in Hampshire is turned into Pro-Grow soil conditioner and sold through the HWRCs.  It is a very high quality Soil Association accredited organic soil conditioner and is used for landscaping projects nationally.