Naphthenic acids (NAs) - tolerant aerobic granular sludge: Removal performance, degradation mechanism, and microbial response.

Naphthenic acids (NAs) are toxic pollutants causing severe environmental issues that are difficult to be biodegraded. Aerobic granular sludge (AGS) has emerged as a promising biotechnology for NAs-containing wastewater treatment, though the mechanisms underlying effective removal and microbial response remain unclear. This study systematically compared degradation mechanisms and microbial response in cyclohexane carboxylic acid (CHCA)-tolerant AGS (T-AGS) versus unacclimated AGS (C-AGS). The results showed that T-AGS demonstrated stronger tolerance and faster removal rates for high CHCA concentrations, achieving a 99 % removal rate for 70 mg/L CHCA, and biodegradation accounted for 80.4 % of total CHCA removal. T-AGS maintained higher biomass content, sedimentation performance, and particle stability by secreting more EPS, particularly tryptophan- and aromatic protein-like substances. T-AGS also exhibited higher TN and PO-P removal rates, which was 12-18 % and 25 % higher than C-AGS, respectively. Prolonged CHCA exposure imposed strong selection pressure on microbial community, favoring Zooglea and Azoarcus as dominant genera. PICRUSt analysis revealed enhanced pathways for amino acid metabolism, membrane transport, and signal transduction, along with upregulation of CHCA degradation and denitrification genes. This study highlighted the promising potential of AGS for NAs removing and the critical role of sludge domestication, offering valuable insights for NAs-containing wastewater treatment.