Effects of polyethylene terephthalate microplastics on performance of sequencing-batch membrane bioreactor for simulated municipal wastewater treatment

Municipal wastewater treatment plants (WWTPs) are recognized as key recipients of microplastics (MPs), with polyethylene terephthalate (PET) being among the most prevalent types in sewage. However, the systemic impact of PET MPs on integrated biological-membrane systems—especially their role in microbial ecology and membrane fouling—remains poorly understood. Therefore, the influence of PET MPs on the performance, microbial community and membrane fouling in a sequencing-batch membrane bioreactor (SMBR) was evaluated in this study. Based on the results, adding PET MPs decreased the MLSS from around 5000 mg/L to 4500 mg/L whereas the MLVSS/MLSS remain basically consistent. The SV₃₀ and SVI increased rapidly to 76% and 173.2 mL/g on the 3^rd day (from 64% and 128.3 mL/g on the 1^st day) of adding PET MPs, however, they could be restored in the following days. For pollutants removal, the COD and NH₄⁺-N removal were initially negatively affected but gradually recovered after several days of operation. The addition of PET MPs enhanced denitrification, resulting in a decrease in the effluent TN concentration from 15.1 ± 4.9 mg/L to 10.4 ± 4.4 mg/L. PET MPs changed microbial community structure and decreased the abundance of dominant bacteria and species diversity in activated sludge. Arenimonas and Sphingopyxis had strong relationships with PET MPs addition. PET MPs addition exacerbated membrane biofouling, and the microbial diversity on membrane at was basically consistent with activated sludge whereas the abundance changed significantly. This research provides a comprehensive understanding of how PET MPs affect the performance of integrated biological-membrane systems.