Microplastics and impurities in digestates and compost: A comparative study of waste-derived soil amendments

Organic waste treatment plays a key role in sustainability by reducing greenhouse gases, minimizing landfill, and recycling nutrients. However, microplastics (MPs) are increasingly reported in organic amendments and digestates, drawing attention to their possible adverse effects on soil health. Even with proper biowaste management, impurities, including glass, metal, plastics, and MPs may still escape into the environment due to mechanical sorting limitations. This study examined impurities and MPs in digestates and organic amendments from four waste treatment plants over three months to assess material quality in a circular economy. Five materials were analysed: (i) compost from green waste and manure (CG), (ii) composted sewage sludge and wood waste (CS), (iii) digestate from separately collected biowaste (DD), (iv) compost of previous digestate and wood waste (CD), and (v) bio-stabilised organic fraction recovered through mechanical treatment (BST) of mixed municipal waste. Gravel and impurities were manually separated, and MPs were isolated via oxidation and density separation, then quantified and characterized using stereomicroscopy and Fourier transform infrared spectroscopy (FTIR). Results indicated that no samples exceeded legal thresholds for gravel/stones in compost, while only BST exceeded impurity limits. Plastic concentrations ranged from 0.03% (CG) to 2.44% (BST) dry weight, with MPs under 2 mm comprising 30-65% of total plastics, predominantly fibers, films, and fragments. The most frequently detected polymers were polyethylene, polypropylene, polyethylene terephthalate and polyester. These findings highlight the importance of effective separate collection in households, efficient mechanical sorting and polishing steps, and the quality of waste-derived amendments to prevent plastics and MPs entering the environment.