Microplastic-associated biofilms in wastewater treatment plants: Mechanisms and impacts

Wastewater treatment plants (WWTPs) represent critical interfaces controlling microplastic (MP) flux between urban and natural environments, with removal efficiencies ranging from 70 to 99.9 %. Within these engineered systems, MPs undergo biological transformations through biofilm formation, creating unique ‘engineered plastisphere’ that fundamentally alter their environmental fate and impact. This review comprehensively analyzes MP-biofilm formation in WWTPs, examining the complex interplay between MP properties, operational parameters, and environmental conditions governing these interactions. Our synthesis reveals that MP-associated biofilms create a paradoxical scenario: enhancing MP removal through improved settling (5-time increase in settling velocities) while simultaneously serving as reservoirs for pathogens and antimicrobial resistance, with substantially higher ARB abundance within the biofilm than surrounding wastewater. Advanced analytical techniques have unveiled distinct microbial succession patterns and community structures unique to MP surfaces in different treatment stages. This analysis identifies critical research needs: standardization of MP-biofilm characterization methods, understanding of biofilm-mediated MP transformation mechanisms, and quantification of treatment operational impacts, providing insights for optimizing MP removal while minimizing associated microbial risks. • MP biofilms in WWTPs are “engineered plastispheres” distinct from natural systems • MP properties and operating conditions govern MP-biofilm development in WWTPs • Treatment stages specifically influence biofilm development and MP removal patterns • Biofilms enhance MP removal but increase pathogen persistence and resistance transfer • Engineered MP biofilms offer new opportunities for enhanced wastewater treatment