Polystyrene microplastics impair nutrient removal in AAO systems by disrupting microbial communities and functional genes

Biological nutrient removal in anaerobic–anoxic–oxic (AAO) systems is central to modern wastewater treatment, relying on the stability and activity of specialized microbial communities. The growing presence of microplastics, particularly polystyrene microplastics (PS-MPs), poses emerging risks, yet their impacts on AAO performance and microbial mechanisms remain poorly understood. We exposed lab-scale AAO reactors to PS-MPs at three concentrations (10, 100, and 200 mg/L) and evaluated system performance, microbial community composition, and functional gene expression. Nutrient removal efficiency declined with increasing PS-MPs, with reductions of up to 17% for nitrogen and 5% for phosphorus. Elevated PS-MPs reshaped microbial communities, reducing the relative abundance of key taxa such as Candidatus Accumulibacter , Dokdonella , Thauera , and Nitrospira , while suppressing expression of nitrogen-cycle genes ( amoA, amoB, amoC, nxrA, nxrB, narG, narH, narI ) and phosphorus-cycle genes ( ppk1 , ppx ), leading to nitrate and ammonium accumulation. These findings demonstrate that PS-MPs destabilize the microbial and genetic machinery underpinning nutrient removal in AAO systems, providing mechanistic evidence that microplastic pollution compromises the reliability of advanced wastewater treatment and poses risks to water security in urbanized catchments. Polystyrene microplastics (PS-MPs) compromise the microbial and genetic foundations of nutrient removal in AAO wastewater treatment, particularly within oxic and anoxic zones. By suppressing key functional microbes and genes, increasing risks of nitrogen and phosphorus discharge. Given the ubiquity of microplastics in sewage, their interference with biological processes poses significant threats to urban water security and highlights the urgent need for long-term monitoring and mitigation strategies.