Influence of polystyrene microplastics on the structural stability of activated sludge microbial flocs in sequencing batch reactors

Polystyrene microplastics are commonly found in wastewater treatment plants, accumulating in activated sludge systems. However, their specific impact on the integrity and aggregation of microbial flocs-which are crucial for effective sludge settling and pollutant removal-remains poorly understood. This study uses sequencing batch reactors (SBR) to systematically investigate how environmentally relevant concentrations of polystyrene microplastics (0, 10, and 100 mg/L) affect microbial floc structure and function. We found that microplastic exposure causes a significant 7.8 %-13.3 % reduction in floc density, and a corresponding increase in effluent turbidity (from 6.084 to 16.555 NTU, P < 0.001), indicating impaired sludge settling. This was attributed to an increase in the negative surface charge of microbial cells induced by microplastic presence, which generated electrostatic repulsion that reduced floc formation by 10.31 %-13.56 %. Additionally, microplastics suppressed the production of extracellular polymeric substances (EPS)-key for microbial cohesion-by decreasing protein and polysaccharide content, as evidenced by shifts in amide I/II bands and carbohydrate peaks. Additionally, microplastics altered microbial metabolism, evidenced by increased specific oxygen uptake rates (SOUR) alongside a 22 % decline in dehydrogenase activity, indicating stress and reduced microbial functional efficiency. We provide evidence and uncover mechanisms through which polystyrene microplastics compromise the cohesion of microbial flocs in activated sludge systems. Our findings can guide improved ecological risk assessments and inform the development of more effective microplastic management strategies to maintain wastewater treatment efficiency and protect environmental health.