Capturing the carbon benefits of bio-compost

Trials for Leicestershire City Council have shown that spreading bio-compost on land increases soil carbon capture while providing a valuable source of organic matter for agricultural application. Paul Gibbs outlines the benefits

Under increasing regulatory pressure to reduce the quantity of waste sent to landfill, mechanical biological treatment (MBT) is an attractive waste treatment option to many local authorities. Co-mingled municipal solid waste (MSW) is separated after collection, during which any recyclable materials are removed, organic materials are recovered and then only the residual material is sent to landfill.

Such was the appeal to Leicester City Council. Under a 25-year waste management contract with Biffa Waste Services, the council plans to meet minimum recycling and composting recovery targets of 60% through a strategy based on MBT. As part of this treatment, the organic fraction of the waste from the council is washed and pasteurised to meet the standards of the animal by-product order and then subjected to both anaerobic and aerobic treatments to stabilise the material.

This produces biogas, along with a stabilised bio-compost which the council had hoped to sell on to the farming community for use on agricultural land growing food crops. However, under current legislation any material used in this way must be source-segregated by the waste producer prior to treatment. As a result, the council's bio-compost is excluded because the waste is separated after collection and can therefore only be used for landscaping, restoration and landfill.

Consequently, the council has been working with ADAS to undertake a comprehensive assessment of the quality of the compost, along with the risks of applying it to agricultural land. The research aimed to quantify the nitrogen release from the compost and demonstrate that it has a beneficial effect on crop yields. ADAS is the first to take advantage of an Environment Agency position on the regulation of trials of waste management.

Under Article 11 of the EU Waste Framework Directive, an exemption can be granted if the waste is recovered or disposed of without endangering human health and without processes or methods which could harm the environment. In October 2008 the EA issued a statement that research could be conducted on the bio-compost without an environmental permit because the agency was satisfied that the research is a "genuine trial of a previously untested process" in which it would be "disproportionate to require an environmental permit".

Findings are positive
The results are very positive. Thorough analysis of the nutrients, heavy metals and contaminants found in the bio-compost, along with investigation of a list of organic compound contaminants outlined by the EA, has confirmed that it does not pose a risk to the environment or human health when used at agricultural rates of application on cultivated land.

ADAS's assessment of the organic outputs from the Leicester plant has found that they are similar in composition to treated sewage sludge, or biosolids, and green compost. The mixed MSW contains comparable levels of heavy metals, with the exception of lead, which is present at higher levels. This slightly elevated concentration is most likely due to a high number of batteries going through the plant, and consequently the council is considering introducing a separate collection for them.

At typical rates of agricultural application, however, the rates of heavy metal addition would still be within the limits stipulated under the code of practice for agricultural use of sewage sludge. In terms of potential benefits on crop yields, the work to date has found that the bio-compost provides a valuable source of organic matter and slow release nutrients for agricultural land. These include nitrogen, phosphate, potash, magnesium and sulphur.

Storing up the benefits
In addition, application of the bio-compost to land increases soil carbon storage - a considerable benefit in the midst of increasing concerns around greenhouse gases and the threat of climate change. The alternative is to send the material to landfill, resulting in the release of significant quantities of methane, a powerful greenhouse gas that has far more impact on climate change than the more widely documented carbon dioxide. Sending bio-compost to landfill would also occupy the limited void space available in the UK.

The results of the research promise to revolutionise recycling processes and offer hope to local authorities striving to meet stringent landfill and recovery targets. Application of the compost to agricultural land not only helps to reduce the amount of waste sent to landfill, but also has considerable benefits for both crops and the wider environment. Leicester City Council is consequently now seeking an environmental permit to recycle all the bio-compost produced by Biffa in Leicester to agricultural land.

Other waste firms and councils have also been following the results of the experimental study closely. The BioCompost Alliance - which currently comprises Bedminster, Biffa, Global Renewables, Leicester City Council, Premier Waste, Shanks, SITA, Sterecycle and Viridor Waste Management - is currently working to develop 'fit for purpose' bio-compost products that are suitable for recycling to agricultural land, helping to complete natural nutrient and carbon cycles.

Dr Paul Gibbs is principal scientist at ADAS

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