Mesophilic anaerobic digestion of mixed sludge in CSTR and AnMBR systems: A perspective on microplastics fate

Most microplastics (MPs) end up in the biosolids produced in wastewater treatment plants (WWTPs) and can pose contamination risks when the biosolids are applied to agriculture. This study evaluated the impact of mesophilic anaerobic digestion on the fate of MPs in WWTP sludge. For this, two laboratory-scale anaerobic digesters were operated in parallel, consisting of a continuous stirred tank reactor (CSTR) and a membrane bioreactor (AnMBR) equipped with an ultrafiltration membrane to decouple the hydraulic and sludge retention times. Both digesters were continuously fed with mixed sludge from a municipal WWTP. The results showed a significant reduction in the MP concentration, with the AnMBR having the higher MP removal efficiency (88.6% vs. 62.1%) and obtaining a higher percentage of biomethanisation (58.3% vs. 43.7%). Polypropylene (PP) and polyacrylonitrile were the main polymers in the mixed sludge, while PP and polyethylene were the dominant polymers in the digested samples. The MP particles in all the samples were predominantly in the 500-104 μm size range. Microbiological analysis indicates a greater species diversity in the microbial community of the AnMBR, the results also revealed a symbiotic relationship between the Firmicutes and Patescibacteria phyla in this digester.