From sludge to supper: biosolids as a potential source of microplastics in the food chain

Microplastics are widely documented as arriving in municipal wastewater from diverse sources such as clothing fibers, tire shreds and cosmetics. Wastewater treatment works typically remove 70–99% of microplastics from the aqueous phase before discharging treated water back to the aquatic environment. However, rather than being eliminated, the microplastic burden is merely transferred to and concentrated in the sewage sludge created by the treatment process. This sludge is typically subjected to varying levels of treatment to form biosolids but the microplastics remain and can potentially enter agricultural soils when biosolids are spread to land as a nitrogen- and phosphate-rich fertilizer or soil amendment. Here, it may become part of the food chain, providing a pathway to human ingestion of microplastic via plant uptake, animal grazing and leaching back to watercourses. This potential route for human exposure to microplastics is of concern, particularly considering the additional contaminants that also concentrate to sludge and can follow the same route to reach humans. Alternative methods of sewage sludge disposal such as landfilling and incineration also have major shortcomings and governments with available landbank wish to retain land application as an option. This paper therefore evaluates where intervention is possible across the entire pathway from source, via wastewater and biosolids, to soil and the food-chain. We evaluate appropriate options to reduce the microplastic burden both upstream by reducing their inputs at source and downstream along the land-based food chain through innovative changes in the end-of-pipe handling of biosolids and sludge using techniques such as vermicomposting, thermal treatments and other emerging developments, and we investigate how current evidence can inform policy and practice.