Microplastics-induced effects on membrane fouling and effluent quality in MBRs for landfill leachate treatment under long-term operation.

Microplastics (MPs) are pervasive in landfill leachate and tend to be retained in membrane bioreactors (MBRs). However, the long-term effects on treatment performance of MBRs remain insufficiently understood. Herein, this study presents a 210-day laboratory experiment comparing a control MBR with two MPs-added MBRs continuously dosed with polystyrene (PS) or phenolic formaldehyde (PF) particles (150-250 µm). Results showed that MPs tended to accumulate in the sludge and settle at the reactor bottom in MBRs, with a small portion incorporated into the membrane cake layer. The final concentrations of MPs in the sludge were 63.1 ± 3.2 mg/L, and 46.4 ± 3.6 mg/L in the PS MPs, and PF MPs groups, respectively. The presence of MPs reduced the removal efficiencies of chemical oxygen demand in the landfill leachate, and intensified membrane fouling. Continuous exposure to MPs stimulated oxidative stress in sludge microorganisms and likely promoted elevated production of extracellular polymeric substances (EPS), thereby forming denser, smoother biofilms with higher organic content on the ultrafiltration membrane surface. By the end of operation, the transmembrane pressure of PS MPs and PF MPs groups were 23.34 kPa, and 33.85 kPa, respectively, which were significantly higher than the Control group (13.17 kPa). Metabolomics analysis further revealed enhancement of pyruvate, citrate cycle metabolism and increased levels of metabolites such as palmitic acid, trehalose and proline. These findings demonstrate that MPs drive metabolic shifts in microbial communities and enhance EPS secretion, leading to persistent membrane fouling in MBRs for landfill leachate treatment.