Removal of mixed micro/nanoplastics from synthetic wastewaters in MBR systems: Influence on process performance and microbial community

Membrane bioreactors (MBRs) are widely recognized as an effective solution for microplastics (MPs) removal from wastewater. However, the fate of smaller micro and nanoplastics (MNPs) under environmentally relevant exposure conditions remains largely unexplored. Particles below 20 μm, including nanoplastics (NPs) that may form through fragmentation during treatment, are often challenging to quantify using routine analytical methods. Moreover, most studies focus on individual plastics and short term or single dose exposure, providing limited insight into long term effects of continuous accumulation of mixed MNPs on MBR performance. Here, we investigated the removal of mixed MNPs and evaluated the impact of their long-term accumulation impacts on laboratory scale submerged MBR, focusing on treatment performance, membrane fouling, and microbial community structure. One reactor was exposed daily to environmentally relevant mixtures of polyethylene MPs (343 μm), polypropylene MPs (141 μm), and polystyrene NPs (120 nm), while a parallel reactor served as a control. Both reactors maintained high and stable performance, achieving around 97% of organic matter removal and efficient nitrification. MPs in treated water were below the limit of quantification, while NPs were detected only in traces amounts after additional preconcentration step corresponding to an overall removal efficiency of 99.99%. MPs exposure mitigated membrane fouling, consistent with a scouring effect. Moreover, MNPs accumulation moderately influenced microbial structure and diversity, indicating gradual microbial adjustment to MNPs stress. These findings highlights the resilience of MBR systems under prolonged, environmentally relevant MNPs loads and support their suitability for high effluents quality applications such water reuse. • Both control and exposed reactors to MNPs achieved ~97% of organic matter removal. • ~99.99% removal of micro and nanoplastics was achieved in the effluent. • Nanoplastics traces in effluent were detectable only after preconcentration with membranes. • SEM revealed a less compact cake layer on membrane exposed to MNPs, consistent with MPs scouring. • Microbial communities adjusted to the continuous exposure to MNPs mixture.