Differential impacts of conventional and biodegradable microplastics on treatment performance and bacterial community in sequencing batch reactors

Microplastics (MPs) are emerging contaminants in wastewater treatment plants (WWTPs), yet their polymer-specific impacts on biological treatment processes remain poorly understood. This study systematically compared the effects of conventional (Polyethylene (PE), Polypropylene (PP), Polystyrene (PS), Polyethylene terephthalate (PET)) and biodegradable (Polylactic acid (PLA)) MPs under identical sequencing batch reactor (SBR) conditions (100 particles/g VSS, 30-day exposure). Conventional MPs induced moderate microbial stress, evidenced by increased reactive oxygen species (~2-fold with PS), but had minimal inhibitory effects on COD removal (83–86 %) and ammonia oxidation (86–92 %). In contrast, PLA significantly affected system performance by reducing the removal of COD and ammonia by 4.3 % and 5.9 %, respectively. PLA also caused suppression (~0.7–0.8-fold as compared to the control experiment) in the production of extracellular polymeric substance (EPS), and abundance in Proteobacteria by ~14.6 %. PLA also led to reduced microbial richness (the Chao1 index) and diversity (the Simpson index), likely due to selective biofilm formation, with potential enrichment of pathogenic taxa. These findings raise concern over the generalized perception that biodegradable MPs are inherently safer and emphasize the need for polymer-specific risk assessments for different MPs found in WWTPs.