Breakdown of plastic waste into microplastics during an industrial Composting: A case study from a biowaste facility

Microplastic pollution is an increasing environmental concern, and further research into its sources is urgently needed. One potential pathway for microplastics to enter agricultural lands is the use of compost-based soil amendments or recycled organic fertilizer. While techniques exist to remove visible plastics from biowaste, microplastics present a hidden challenge. EU fertilizer regulations only account for particles ≥ 2 mm, excluding smaller ones. Research on plastics in biowaste management systems is limited. Our study investigated plastic fragmentation into microplastics and the role of microbial communities in plastic degradation. Samples were collected before sanitation, after tunnel composting, after outdoor maturation, and from a compost-based fertilizer, focusing on particles > 20 µm using Raman spectroscopy. Microbial community analysis was conducted using 16S rRNA sequencing and phospholipid fatty acid analysis. We observed a significant increase in microplastic particle counts and a decrease in size throughout composting. Mature compost contained an average of 944 ± 586 particles/g of dry weight (<0.25 mm), primarily polyethylene terephthalate, with plastics accounting for up to 0.25 % of dry weight. Only 7 ± 2 bigger plastic particles (>0.5 mm) were found from all samples. Subtle changes were observed in microbial communities during the composting process, predominantly among fungal communities, while Firmicutes remained the most abundant bacterial phylum in all samples. Our results suggest that plastics are fragmented into smaller particles during the industrial composting process and are not efficiently decomposed by microbes during the process.