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

Naphthenic acids (NAs) are a type of pollutant with high ecological toxicity that are difficult to biodegrade and cause serious environmental problems. Aerobic granular sludge (AGS) has been proven to be a promising biotechnology for the treatment of NAs-containing wastewater. However, little is known about the effective removal strategy, degradation mechanism, and microbial response of AGS system towards NAs. This study systematically evaluated the differences between cyclohexane carboxylic acid (CHCA) - tolerant AGS (T-AGS) and conventional AGS (C-AGS) on degradation mechanism and microbial response. The results showed that compared to C-AGS, the T-AGS system has stronger tolerance and faster removal rate for high concentrations of CHCA, and the removal rate of 70 mg/L CHCA could reach 99%. The T-AGS system mainly removed CHCA through biodegradation, with a proportion of 80.4% in total removal amount for 80 mg/L CHCA. In terms of the AGS system, T-AGS showed a stronger resistance to the increasing concentration of CHCA in terms of biomass content, sedimentation performance and particle stability by secreting more extracellular polymeric substances (EPS) (mainly tryptophan and aromatic protein-like substances) as a block. For pollutant removal, the removal rates of TN and PO43--P in the T-AGS system were 12-18% and 25% higher than those in C-AGS system, respectively. The long-term presence of CHCA exerted a strong selection pressure on the microbial community, especially with Zooglea and Azoarcus being the dominating genus. PICRUSt analysis further revealed that T-AGS system enhanced the pathways of amino acid metabolism, membrane transport and signal transduction to resist toxic stress of CHCA, while enhancing CHCA degradation and denitrification functional genes to improve removal efficiency. This study elucidated that AGS qualified promising application prospects in the removal of NAs present in wastewater, and emphasized the importance of sludge domestication, which may provide reference on the treatment of NAs wastewater using AGS.