Certified biodegradable plastics do not contribute to long-term microplastic pollution in agricultural soils and composting systems, according to a comprehensive evidence review published by BB-REG-NET, the UK regulatory science network for bio-based and biodegradable materials.

The report, published yesterday (14 October), examined evidence from agricultural mulch films, tree guards, and industrial composting facilities across Europe. It found that whilst biodegradable materials initially fragment into microplastics during breakdown, these particles continue to biodegrade and mineralise into carbon dioxide, water, and biomass. In contrast, conventional plastic microplastics persist indefinitely in the environment.

Gail Shuttleworth, lead author from Alder BioInsights, explained: "In contrast to conventional microplastics, genuinely biodegradable microplastics are transient and will undergo full mineralisation when exposed to the appropriate environmental conditions. This fundamental difference makes them a key tool in tackling microplastic pollution from plastics used in, or prone to, entering the open environment."

The report responds to UK government concerns raised in 2021 about whether biodegradable plastics simply accelerate plastic fragmentation into microplastics. The research demonstrates that certified materials meeting established standards undergo complete mineralisation rather than accumulating in the environment.

Agricultural applications

Field trials of biodegradable agricultural mulch films certified under European Standard EN 17033:2018 showed materials reach equilibrium between application rates and degradation. This standard requires at least 90 per cent of organic carbon to convert to carbon dioxide within 24 months under controlled conditions.

A five-year field trial in Mediterranean climate conditions found that biodegradable mulch degraded at the same rate it was applied, with no accumulation of visible fragments over time. Modelling indicated field degradation took between 21 and 58 months, longer than laboratory conditions but resulting in steady-state concentrations. This contrasts with conventional polyethylene mulch films, which studies showed can accumulate to concentrations of 40.35 mg/kg after 30 years of continuous use.

Dr Adrian Higson, Managing Director of Alder BioInsights, stated: "Our report demonstrates that biodegradable plastics can play an important role in avoiding persistent microplastic pollution."

An estimated 70-75 per cent of conventional mulch films are collected across Europe, with only a quarter subsequently recycled, resulting in ongoing soil contamination.

Industrial composting trials

Industrial-scale composting trials across multiple countries showed certified compostable plastics disintegrate within 22 days, with some items breaking down in 11 days. The EN 13432:2000 standard requires 90 per cent biodegradation within six months and less than 10 per cent visible residue after 12 weeks.

Trials by Envar Composting through the UKRI-funded Composting Coalition found that certified compostable materials broke down during standard industrial composting operations. James Cooper, Head of Compliance at Envar Composting, said: "In our trials for the Composting Coalition, certified compostable materials broke down cleanly when managed through standard industrial composting. Crucially, our analysis found no detectable compostable-plastic microplastics in the screened product."

A Dutch study examined nine different compostable packaging products in a full-scale organic waste treatment facility. All products broke down within 22 days, with polylactic acid plant pots disintegrating within 11 days, faster than paper and orange peels. No compostable plastics were visually identified in finished compost.

Studies from five Spanish composting facilities found microplastic concentrations of 10-30 particles per gram of dry weight in finished compost. These consisted almost entirely of conventional polymers, with no debris from compostable plastics detected.

Research cited in the report indicates that when biodegradable materials do not fully break down during composting, the process continues once compost is applied to soil. A 2024 study confirmed that biodegradable microplastics like polylactic acid undergo further biodegradation driven by soil microbial activity, with water and enzymes breaking down the polymer chains into smaller molecules that microorganisms can consume.

The report makes eight recommendations including establishing application-specific biodegradation standards, supporting research into acceptable mineralisation timeframes, and developing global monitoring frameworks for microplastics from both conventional and biodegradable sources.

BB-REG-NET reports that whilst biodegradable plastics are not a complete solution to microplastic pollution, they offer a fundamentally different trajectory from conventional plastics. With appropriate product design, use, and regulatory oversight, biodegradable and compostable materials can reduce long-term persistence of microplastics in the environment.