Compost Stability: Impact and Assessment

The British Standards Institution Publicly Available Specification 100 (BSI PAS 100) for composted materials was first introduced in 2002, when the majority of composting sites were processing green waste in open windrow systems. Since then a shift in the type of compost feedstocks as well as processing conditions has been observed. The use of in-vessel composting technologies (IVC) has increased significantly in the UK as processing food waste has become more common. Factors such as feedstock type, specific operating (or process)
conditions of the in-vessel stage, as well as the subsequent stabilisation and maturation stages, may influence the effectiveness of the composting process, and hence the stability of the end product. This industry shift raises the question as to whether the current UK compost stability test method (ORG0020), as well as the compost stability limit prescribed by PAS 100, are still meaningful.

Since the introduction of ORG0020 in 2005, the upper test limit for compost stability has been set at 16 mg CO2 / g organic matter (OM) / day to cover all market areas as a baseline quality specification. A more stringent stability limit applies to certified composts used in growing media (10 mg CO2 / g OM). Around 50 % of compost is currently certified to PAS 100 and the remainder is for the most part applied to land under the various UK waste regulatory regimes.

A critical re-evaluation of the compost stability limit in the UK was proposed by the industry, particularly for composts used in agriculture. For non-PAS 100 compliant composts, the question whether stability limits should be imposed on very unstable materials when stored and used in the field was of significant regulatory concern.

The overarching objectives of this project were to design and implement a programme of desk, field and laboratory work to provide information and data regarding:

*whether the PAS 100 stability test was fit for purpose, particularly for determining the stability of composts produced by IVC processes;

*whether the PAS 100 stability baseline could be changed, particularly for PAS 100 composts used in agriculture;

*whether stability limits should be set for non-PAS composts, and if so, what test could be used, and how might the limit be selected; and

* the financial and commercial (operational) implications for PAS and non-PAS compost producers of the implementation of any of the recommendations made in this work.

This project was aimed at collating currently available information and relevant data to enable WRAP and the Environment Agency to derive their own, evidence-based recommendations. This project should be considered a "scoping study" that informs possible future more detailed investigations into specific aspects of compost stability.