New insights into long-term impacts of sulfamethoxazole on anaerobic/oxic/anoxic (AOA) processes performance, sludge properties, and microbial oxidative stress mechanisms.

The widespread detection of sulfamethoxazole (SMX) in aquatic environments necessitates understanding its impact on the anaerobic/oxic/anoxic (AOA) process. This study systematically elucidated the multidimensional mechanisms by which SMX affects AOA system performance, sludge characteristics, and microbial metabolism, integrating analyses of oxidative stress and sludge parameters. Results demonstrated concentration- and time-dependent effects. Long-term exposure to medium-high SMX concentrations (≥5 mg/L) significantly inhibited microbial metabolic activity (13.5% decrease in specific oxygen utilization rate, suppression of polyphosphate synthetase), induced oxidative damage (46.3% increase in lactate dehydrogenase leakage), and reduced removal efficiencies for ammonium nitrogen (9.1%), total nitrogen (15.9%), and phosphate phosphorus (12.5%). Concurrently, it decreased sludge biomass (13.8% lower volatile suspended solids/total suspended solids ratio), worsened settleability (12.2% increase in sludge volume index), and disrupted extracellular polymeric substance (EPS) composition (PN/PS ratio decreased to 1.6). While low-concentration SMX (0.5 mg/L) elicited a transient stress response (10% compensatory EPS increase), microbial community adaptation ensured system stability. These findings reveal novel interference mechanisms of SMX in biological nutrient removal. Proposed strategies for microbial community optimization and process regulation offer crucial theoretical support and technical pathways for efficiently treating SMX-containing wastewater, advancing practical engineering applications for antibiotic pollution control.