The factors controlling antibiotic resistance genes in different treatment processes of mainstream full-scale wastewater treatment plants.

The alteration of antibiotic resistance genes (ARGs) in wastewater has been less studied in wastewater treatment plants (WWTPs), making it difficult to assess ARGs' spreading risk comprehensively. Therefore, this study investigated the distribution and reduction of ARGs in the main process (Anaerobic-Anoxic-Oxic with Membrane Bio-Reactor (A/O + MBR), Oxidation Ditch with sedimentation (OD), and Cyclic Activated Sludge System (CASS) with sedimentation) and disinfection process (Ultra-violet and Chlorination) of full-scale WWTPs. The wastewater was sampled before and after the different main process and disinfection process; then, the diversity and abundance of ARGs and mobile genetic genes (MGEs, helping the horizontal transfer of ARGs) in wastewater of different treatment stages were determined by a real-time high-throughput quantitative PCR (HT-qPCR) system. It was found that similar influents would result in similar ARGs in wastewater samples, independent of the treatment processes used. The main process could effectively reduce the abundance of ARGs and MGEs by 1.80-2.12 and 1.46-2.18 logarithm units, respectively. The main factors affecting ARGs were mainly wastewater quality index, especially COD, and MGEs like transposase and insertion sequences which were significantly associated with 66 and 48 subtypes of ARGs, respectively. Moreover, disinfection was more effective than the main process in inactivating antibiotic resistance bacteria (ARB), and the removal rate of ARB by disinfection reached 43.53 %-100 %. However, there are still risks of ARB regeneration (up to 4.22 log units) in the effluent of WWTPs. In the future, nutrient removal and disinfection process improvement is necessary to benefit ARG and ARB removal.