Impacts of per- and polyfluoroalkyl substances on biological nitrogen removal processes: Performance, microbial responses and inhibition mechanisms

Abstract

Per- and polyfluoroalkyl substances (PFAS) are persistent synthetic chemicals widely detected in wastewater treatment plants (WWTPs), where they are incompletely removed and may accumulate within biological treatment systems. As major sinks and redistribution nodes, WWTPs serve as critical interfaces linking PFAS contamination to downstream aquatic environments. This review synthesizes current knowledge on PFAS occurrence in WWTPs and critically evaluates their impacts on nitrification, denitrification, and anammox processes, together with the mechanisms underlying PFAS-induced functional instability. Available evidence indicates that PFAS effects on nitrogen removal are concentration-dependent and structurally selective, influenced by carbon chain length, functional groups, exposure intensity, and duration. Autotrophic pathways—particularly anammox—generally exhibit greater sensitivity than heterotrophic denitrification. Mechanistically, PFAS can disrupt membrane integrity, alter extracellular polymeric substance composition, interfere with substrate transport and electron transfer, and induce oxidative stress, collectively impairing process performance. Molecular analyses further suggest that long-chain PFAS display stronger binding affinity toward key nitrogen-transforming enzymes, potentially influencing nitric oxide and nitrous oxide dynamics. Beyond mechanistic insights, this review integrates process-level sensitivity patterns and identifies key uncertainties related to exposure realism, mixed-contaminant interactions, and long-term system resilience. Future research should prioritize environmentally relevant exposure scenarios, underexplored treatment matrices, and integration of multi-omics approaches with engineering performance indicators. This synthesis provides a framework to support risk assessment and development of more resilient nitrogen removal systems under PFAS stress.