The scale of plastic pollution from composts and digestates, its potential impact and opportunities for its mitigation are largely unquantified. This project has explored some of these questions to develop an understanding of the knowledge base and data gaps.

    Setting out the state of knowledge with respect to:

    • Levels of compostable and non-compostable plastics in UK source-segregated composts and digestates;
    • Methods for their detection, identification and enumeration;
    • Methods to reduce plastic levels;
    • Impacts of plastics and options to revise limit values.

    Executive summary

    Using a combination of structured evidence assessment, supplier and operator engagement this project sought to address the following questions:

    1. What evidence is there for the level of plastic contamination in UK (source-segregated) composts and digestates, and does any of this differentiate between compostable and non-compostable plastics?
    2. How can plastics be measured in composts, digestates and soils, and how can compostable and non-compostable plastics be differentiated?
    3. What process interventions are available to reduce the level of plastics in composts and digestates, how do they perform and what do they cost?
    4. How are other countries tackling this issue? How do our limits for plastics in compost and digestate compare? Is there any evidence that the levels of plastic found in UK (source-segregated) composts and digestates cause harm when applied to farmland?

    Key findings

    Data on compost and digestate contamination with plastics are rarely reported in published literature. The most significant UK dataset is held by Renewable Energy Assurance Ltd (REAL) as owners of the Compost and Biofertiliser certification schemes. It was not possible to analyse this dataset within the timeframe of this project.

    Evidence indicates that certified compostable plastics will break down during composting processes, providing the process conditions align with those specified within the standard against which plastics are certified. Non-compostable plastics can be removed before or after composting to achieve PAS100 limits. Stakeholder feedback suggests that compliance with the lower Scottish limits is challenging, requiring a focus on litter-picking from feedstocks prior to composting.

    There are no data on the presence of compostable plastics in digestates. Since they are not intended to biodegrade under anaerobic conditions, residues might be anticipated in digestate prior to final screening. Screening of whole (liquid) digestates is used where necessary to ensure compliance with PAS110 limits. Stakeholder feedback suggests that compliance with the lower Scottish limits is achievable, albeit that this creates a greater quantity of plastic-contaminated screenings for onward processing / disposal.

    Although a number of harms have been demonstrated from plastics in soils, reliance on acute dosing of environmentally unrepresentative concentrations of plastics with short exposure times, combined with inconsistencies in reported impacts and complexities resulting from responses to different polymers, particle sizes and particle shapes means that it is not currently possible to suggest evidence-based limits for plastics in composts, digestates or soils. Stakeholder feedback during a project workshop indicated that retaining the current PAS limits outside Scotland was not an option, and a series of actions has been proposed to understand how ALARP (As Low As Reasonably Practicable) or zero-tolerance / precautionary limits might be defined. The proposed actions are summarised in the full report.

    This project has highlighted an absence of strategic data to inform current and future policy relating to biowaste recycling and the need to reduce or eliminate environmental contamination with plastics. Addressing this deficit will require collaboration across policy, industry, regulatory and research communities.

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    • Plastics in Composts and Digestates final report

      PDF, 3 MB