Plastics and other extraneous matter in municipal solid waste compost: A systematic review of sources, occurrence, implications, and fate in amended soils

Municipal solid waste (MSW) composting is rapidly growing globally as a sustainable approach to valorize the organic fraction of municipal solid waste (OFMSW) into compost for agricultural use. However, MSW compost use in agriculture is threatened by physical contaminants, mainly plastics, glass, metals, and stones in the compost, exceeding the legal thresholds in some cases. This study comprehensively reviews the literature on various physical contaminants in MSW compost, focusing on sources, occurrence, environmental implications, and fate in amended soils. The review shows that physical contaminants in MSW compost are highly heterogeneous depending on waste origin, source separation, and sorting and sieving practices before and after composting. Plastics are the most widely occurring and abundant physical contaminant in MSW compost, reaching up to 15,300 mg/kg in compost, capable of inputting up to 536 kg plastics/ha/year in amended soils. Glass, stones, and metals also regularly occur in MSW compost, reaching up to 17.2%, 18.2%, and 1.5% of the compost mass respectively. Repeated application of contaminated compost increases physical contaminant accumulation in amended soils, severely impacting the soil's physical, chemical, and biological performance. Synthetic plastics in compost-amended soils tend to have a long residence time, slowly degrading and releasing small-sized plastic particles and their metabolites. Further, they may be transported from the point of application by biotic or abiotic agents, posing secondary pollution effects. Microplastics (MPs) are the most significant emerging physical contaminant in MSW compost, and present detection challenges and regulatory laxity in compost marketing. The strategies to mitigate physical contaminants in MSW compost include proper biowaste source separation, improved biowaste separation and screening before and after composting, regulatory adherence and monitoring of contaminants in compost, and the adoption of compostable biodegradable plastics in MSW biowaste collection.