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Mesophilic anaerobic digestion of mixed sludge in CSTR and AnMBR systems: A perspective on microplastics fate
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Researchers compared how two types of anaerobic digesters used in wastewater treatment affect the fate of microplastics in sewage sludge. They found that while digestion reduced some types of microplastics, it also fragmented larger particles into smaller ones, and a membrane bioreactor retained more microplastics than a conventional stirred tank. The study suggests that current wastewater treatment processes may not fully eliminate microplastics and could even create smaller, harder-to-remove particles.
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.
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The occurrence and fate of microplastics in a mesophilic anaerobic digester receiving sewage sludge, grease, and fatty slurries
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Researchers analyzed microplastic loads entering and leaving a mesophilic anaerobic digester at a wastewater treatment plant serving nearly 800,000 people. They found that the digester received approximately 7,326 kg of microplastics per year, with digested sludge containing about 30% less, though this reduction was within the variability of the measurements. The study provides important data on the fate of microplastics during sewage sludge treatment processes.
Transport and accumulation of microplastics through wastewater treatment sludge processes
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Researchers tracked microplastic concentrations through each stage of wastewater treatment sludge processing — including thickening, anaerobic digestion, and dewatering — finding that each step altered MP abundance and size distribution, with digestion fragmenting some particles into smaller pieces rather than eliminating them.
Impact of micro-nanoplastics on biochemical phases of anaerobic digestion in sewage sludge treatment: mechanistic insights and future prospects
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Micro- and nanoplastics were found to disrupt the biochemical phases of anaerobic digestion, affecting the efficiency of the biological process used to treat organic waste. Understanding these impacts is important because anaerobic digestion is a common wastewater and sludge treatment method that may both receive and process microplastic-contaminated materials.
Microplastics in real wastewater treatment schemes: comparative assessment and relevant inhibition effects on anaerobic processes
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Researchers investigated microplastic occurrence and removal in an Italian wastewater treatment plant, finding 3.6 MPs/L in influent dominated by polyester fibers, with conventional activated sludge achieving 86% removal, and demonstrating that concentrated sludge-associated microplastics can inhibit downstream anaerobic digestion.
Migration and transformation modes of microplastics in reclaimed wastewater treatment plant and sludge treatment center with thermal hydrolysis and anaerobic digestion
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Researchers tracked how microplastics move and change throughout a wastewater treatment plant and a sludge treatment center that uses thermal hydrolysis and anaerobic digestion. They found that while the treatment plant removed about 98% of microplastics from the water, most particles ended up concentrated in the sludge. The study suggests that sludge disposal and reuse practices need to account for the microplastics that accumulate during wastewater treatment.
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