Vermicomposting leads to more abundant microplastics in the municipal excess sludge

Municipal excess activated sludge is not only an important reservoir of microplastics particles, but is also a vehicle of entry of microplastics into the environments as soil amendments or organic fertilizer. Vermicomposting is a cost-effective technology for sludge valorization. However, it is not clear whether vermicomposting affects the occurrence of microplastics in residual sludge. Here, the variation of microplastics (0.05-5 mm) in sludge, including the abundance, type, size, and morphology, before and after vermicomposting by epigeic earthworms under different temperature conditions (15 °C, 20 °C and 25 °C) were investigated by micro Fourier Transform Infrared Spectroscopy (μ-FTIR) and Scanning Electronic Microscopy (SEM). More abundant (over 10⁴ particles ∙kg^-1 (dry weight)), and smaller microplastics (over 60% in total with 0.05-0.5 mm) in the treated sludge via earthworms were observed compared to the raw sludge. The increment of vermicomposting temperature was more obvious (p < 0.05) for the enrichment of the microplastics, especially for polyethylene particle. Gizzard grinding and microbial digestion in the gut of earthworms may contribute to the fragment of microplastics. The present study suggests that the sludge-sourced vermicompost is still an important hotspot of microplastics, posing a potential threat to the receiving environments.