In-vessel composting

Composting involves the use of naturally occurring aerobic processes which break down organic matter to produce an organic material suitable for use as a soil conditioner and a source of nutrients in agriculture and horticulture.

The main difference between composting systems largely relate to scale and whether the process occurs in the open or is enclosed (in-vessel). Most small scale domestic composting systems operate for only short periods at high temperatures and then become cold quite quickly, with the overall process often taking many months to complete. Larger scale systems, where the material is mechanically turned to form a windrow, are more efficient but are only suitable for green waste. They are however used extensively throughout the UK for the treatment of garden waste.

In-vessel systems have the advantage of operating at optimal conditions of temperature and moisture content, and because they are enclosed can accept for treatment a wider range of organic materials including food waste. An additional advantage is that air from the composting process, which can be odorous, can be captured and treated.

Waste materials are received into an enclosed reception hall and are preprocessed normally by shredding to achieve size reduction, and mixing to achieve the correct proportion of materials. It is important to ensure that green and food waste, which contains high concentrations of nitrogen, is mixed with sufficient amounts of brown, woody carbon rich material to ensure efficient treatment.

Composting is carried out in pre-formed tunnels or vessels which are loaded mechanically with the mixed feedstock. Water is applied to ensure the correct moisture content is maintained and air is drawn through the material to maintain a good supply of oxygen which accelerates the composting process.

Treatment occurs at temperatures of up to circa 60°C and for up to 21 days duration. The material may be removed part way through the process, remixed and returned to a tunnel for the composting process to be completed. The mixing of the material ensures that any cold areas in the compost and which therefore do not compost normally are broken up and remixed. After the process is completed the composting vessels are emptied and the material allowed to mature in the open for a period of between 4 to 6 weeks prior to use.

In-vessel composting plants

Biffa operate two 50,000 tonne per annum facilities, at Etwall, Derbyshire and Ufton, Warwickshire, the latter having been commissioned in November 2009.

The facilities are based around a reception building, feedstock preparation area, a number of concrete composting tunnels, a compost maturation pad and air handling equipment.

Incoming wastes are sorted, shredded to achieve a maximum particle size, mixed to achieve the correct "recipe" and then loaded into composting tunnels in turn. Air is drawn through the feedstock with the rate of air flow being controlled to ensure an optimum treatment temperature. Following completion of an initial composting period the material is removed from the tunnel, remixed and returned to another tunnel for the composting process to be completed. The treated material is then stored for a period of 4 to 8 weeks to enable final maturation to occur whereupon the product is screened and sieved to remove any contamination and to achieve a variety of material grades depending on the final end-use.

View contract of supply guidelines for use and safe handling of Biffa compost.

Lady in hard hat in front of large pile of soilMan in overalls at composting plant