Effect of polypropylene microplastics on the performance of membrane bioreactors in wastewater treatment

Membrane bioreactors (MBRs) can effectively remove microplastics (MPs) because of their good rejection performance. However, the influence of MP concentration and particle size on the short-term and long-term operation efficiency of MBRs remains unclear. To address this issue, this study investigated the effects of short-term stress and long-term accumulation of polypropylene microplastics (PP-MPs) with different particle sizes on the operational efficiency of MBRs by running three MBR systems at four concentration stages. Variations in pollutant removal, biological activity, composition of extracellular polymeric substances (EPS), membrane fouling, and microbial communities in the MBRs were systematically investigated. The results showed that low concentrations (0.02 g/L) of PP-MPs stimulated the biological activity in the MBRs in the short term, and small particle size PP-MPs exhibited higher biological toxicity while accelerating membrane fouling. With an increase in time and concentration (0.2-2.0 g/L), PP-MPs showed significant toxic inhibition effects, increasing EPS secretion and accelerating membrane fouling. This effect was more significant for small particle size PP-MPs. In addition, at 0.02 g/L of PP-MPs, the richness and diversity of microbial communities in activated sludge tend to increase, such as the relative abundance of Bacteroidota, while the growth of Proteobacteria is inhibited. The results of this study provide a reference for understanding the effects of short-term stress and long-term accumulation of MPs on MBR efficiency. It provides reference for exploring the influence of MPs with different particle sizes on MBR reactor and the future research direction.