Abstract:Antibiotic resistance genes (ARGs) pose a serious threat to public health. Wastewater treatment plants (WWTPs) are hotspots for ARGs, and their operational conditions significantly influence the removal of ARGs from wastewater. However, research on the effects of hydraulic retention time (HRT) in different wastewater treatment units on ARGs remains limited. In this study, two pilot-scale AAO (Anaerobic-Anoxic-Oxic) systems with advanced treatment and disinfection processes were designed with different HRTs to investigate the concentrations and distributions of ARGs at various treatment stages. The correlations among ARGs, water quality parameters, and microbial communities were also analyzed. The results showed that in the long-HRT group, the ARG concentrations in the secondary sedimentation tank effluent were lower than those in the short-HRT group. When considering the concentrations of ARGs in the activated sludge, the retention of ARGs by sludge was found to be similar in both groups. Therefore, it was speculated that the sludge in the long-HRT group had a longer contact time with the ARGs, promoting their biological degradation. The concentrations of ARGs in the sand filtration effluent of both groups showed no significant decrease compared to those in the secondary sedimentation tank effluent. After disinfection, the concentrations of blaTEM, sul1, tetX, ermB, intI1 and 16S rDNA in the long-HRT group decreased to 4.26×103, 2.31×105, 7.15×103, 7.29×103, 4.64×104, and 3.61×105 copies/mL, respectively, all of which were lower than those in the short-HRT group. Overall, the removal rates of blaTEM, sul1, tetX, ermB, intI1 and 16S rDNA in the long-HRT group reached 2.55, 2.60, 3.23, 2.99, 2.77, and 2.97 log, respectively, indicating that the long-HRT group exhibited higher removal efficiencies of ARGs than the short-HRT group. However, it is worth noting that the ARGs/16S rDNA ratio after disinfection in the long-HRT group was significantly higher than that in the filtration effluent. This indicates that although prolonged disinfection reduces ARG concentration in the effluent, it also significantly increases the ARGs/16S rDNA ratio. This phenomenon may be due to horizontal gene transfer of ARGs, whereby excessive disinfectant dosage damages bacterial cells, increasing the frequency of ARG transfer to non-resistant bacteria. This may affect the microbial community in the receiving water body. In addition, both the biological and disinfection stages reduced the abundance of pathogenic genera such as Aeromonas, Desulfovibrio, and Pseudomonas. However, in both the long and short-HRT groups, the relative abundance of these genera was higher in the disinfection effluent than in the filtration effluent. Redundancy analysis revealed that genera such as Nitrospira, Dechloromonas, and Ferruginibacter had a high correlation with ARGs, and both 16S rDNA and suspended solids (SS) were highly correlated with ARGs. This study provides theoretical support for WWTPs to better control the risks associated with the release of ARGs. Close-