Long-term performance, granulation mechanism and microbial response of aerobic granular sludge for treating ultra-high concentration oxytetracycline-containing wastewater.

Aerobic granular sludge (AGS) represents a promising biological treatment technology for antibiotic-containing wastewater. However, most previous studies have focused on low-concentration antibiotic stress (μg/L to mg/L), while the long-term response of AGS to ultra-high oxytetracycline (OTC) levels is poorly understood. In this study, a laboratory-scale sequencing batch reactor was established to systematically investigate the AGS granulation, pollutant removal performance, and microbial community succession under long-term stress of 50 mg/L OTC, a concentration representative of pharmaceutical effluent. The results showed that, despite the severe inhibitory effects of OTC, AGS was successfully cultivated within 28 days, and its granulation period was comparable to that in conventional municipal wastewater without antibiotic stress. Furthermore, AGS exhibited good settling performance and stable biological activity. Pollutant removal gradually stabilized with granule maturation, resulting in removal efficiencies of 75% for chemical oxygen demand, 32% for ammonium nitrogen, 40% for total nitrogen, 54% for total phosphorus, and 56% for OTC, respectively. OTC removal gradually shifted from adsorption-dominated process in the start-up stage to a synergistic adsorption-biodegradation process in the stable stage. Spearman correlation analysis further indicated that the recovery of pollutant removal efficiency was more closely associated with microbial functional adaptation than with biomass accumulation alone. Microbial analysis revealed that Saprochaete, Bordetella, and Raoultella gradually became dominant, while carbohydrate metabolism and amino acid metabolism were significantly upregulated to mitigate OTC stress. These synergistic interactions between dominant genera and metabolic functions jointly constituted the core microbial mechanism for maintaining structural stability and pollutant removal performance of AGS under high OTC stress.