Fate of antimicrobial resistance genes and non-tuberculous mycobacteria across a full-scale wastewater biological treatment plant with effluent ozonation

Wastewater treatment plants (WWTPs) are primarily designed to prevent the discharge of highly contaminated water to aquatic environments. While wastewater has been identified as an important niche of antimicrobial resistance genes (ARGs), little is known about their removal in a full-scale treatment process. This study investigated the abundance of 16S rRNA, non-tuberculous mycobacteria (NTM), intI1 and 22 ARGs in a full-scale WWTP using an activated sludge process and, during summer, ozonation of the final effluent. Samples from the influent, the secondary effluent, the final effluent (ozonated or not) and dehydrated sludge were collected, and associations between genetic markers and treatment progression were evaluated. Activated sludge was responsible for 88% to 100% of the overall ARG removal. While ozonation significantly reduced suspended solids and chemical oxygen demand, its impact on removal of 16S rRNA, NTM, intI1 and ARGs varied from 0 to 0.7 log. Variable responses and sensitivities to the different stages of the treatment process were observed for bacteria, integron and ARGs, but increased relative abundance in sludge were noted for NTM, intI1 and 7 ARGs. The genes bla SHV and ermB , both classified as high risk for public health, were still detected at concentrations >100 GC/mL in ozonated effluents, highlighting the risk of dissemination of critical ARGs in the environment. These results contribute to a better understanding of the fate of ARGs in WWTP and emphasize the need of future research and actions to mitigate ARGs release from WWTP into soils (sludge) and aquatic environments (effluents).