Insights into impact of tire additives on activated sludge systems: Treatment performance, extracellular polymeric...

This study systematically investigated the impacts of seven common tire additives (TAs) and their derivatives on the performance and microbial ecology of an activated sludge system exposed to environmental concentrations (0.2-20.0 μg/L) over 160 days. While most individual TAs showed minimal effects, the mixture of TAs (MIX) and 2-((4-Methylpentan-2-yl)amino)-5-(phenylamino)cyclohexa-2,5-diene-1,4-dione (6PPD-Q) significantly inhibited nitrogen removal efficiency. At 20.0 μg/L, the abundances of key nitrification and denitrification genes (amoA, nirK, nirS, nosZ) were markedly suppressed, leading to reductions in NH₄⁺-N (12-14%) and total nitrogen (18-23%) removal efficiencies. The impairment was associated with elevated oxidative stress level, as indicated by a sharp increase in reactive oxygen species (0.9-1.1 fold) and lactate dehydrogenase (1.5-2.1 fold) release, alongside suppressed adenosine triphosphate (ATP) synthesis. Concurrently, sludge settleability deteriorated and mixed liquor volatile suspended solids (MLVSS) decreased, which correlated with reduced sludge hydrophobicity and enhanced hydrogen bond intensity (3435 cm^-1). Microbial community restructuring was observed, with tolerant genera (e.g., Comamonadaceae, Rhodobacteraceae) increasing, while key nitrogen-removing genera (e.g., Nitrosomonas, Thauera) decreased. Three-dimensional fluorescence spectroscopy analysis revealed a decline in tryptophan and tyrosine like proteins in tightly bound extracellular polymeric substances. Molecular docking demonstrated that 6PPD-Q exhibited the strongest binding affinity to tryptophan and tyrosine synthases, suggesting a high potential for enzymatic interference even at low concentrations. This study demonstrated that MIX and 6PPD-Q, at environmental concentrations, significantly affect activated sludge systems and investigated the mechanisms involved, thereby providing important evidence for the ecological risk assessment of such pollutants during wastewater treatment.