Green and biowaste, processed within large facilities into compost, is a key fertilizer for agricultural and horticultural soils. However, due to improper waste disposal of plastic, its residues often remain or even lead to the formation of microplastics (1 µm − 5 mm, MiPs) in the final compost product. To better understand the processes, we first quantified ‘macroplastics’ (> 20 mm, MaPs) input via biowaste collection into an industrial composting plant, and, then determined MiP concentrations at five stages during the composting process (before and after shredding and screening processes), and in the water used for irrigation. The total concentrations of MaPs in the biowaste collected from four different German districts ranged from 0.36 to 1.95 kg ton-1 biowaste, with polyethylene (PE) and polypropylene (PP) representing the most abundant types. The “non-foil” and “foil” plastics occurred in similar amounts (0.51 ± 0.1 kg ton-1 biowaste), with an average load of 0.08 ± 0.01 items kg−1 and 0.05 ± 0.01 items kg−1, respectively. Only 0.3 ± 0.1 kg MaP t−1 biowaste was biodegradable plastic. Compost treatment by shredding tripled the total number of MaPs and MiPs to 33 items kg−1, indicating an enrichment of particles during the process and potential fragmentation. Noticeably, a substantial amount of small MiPs (up to 22,714 ± 2,975 particles L-1) were found in the rainwater used for compost moistening, being thus an additional, generally overlooked plastic source for compost. Our results highlight that reducing plastic input via biowaste is key for minimizing MiP contamination of compost.
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