The balance between treatment efficiency and receptor quality determines wastewater impacts on the dissemination of antibiotic resistance

This study investigated the balance between treatment efficiency and impact caused by urban wastewater treatment plants (UWTPs) on the dissemination of antibiotic resistance. Four full-scale UWTPs (PT1-PT4) and the receiving river were sampled over four campaigns. The 16 S rRNA gene, two mobile genetic elements (MGEs), eight antibiotic resistance genes (ARGs), and culturable bacteria were monitored over different treatment stages and in hospital effluent. The bacterial and antibiotic resistance load was not significantly different in the inflow of the four UWTPs (p > 0.01). Biological treatment promoted ARGs reduction values up to 2.5 log-units/mL, while UV (PT1, PT2) or sand filtration/ozonation (PT3) led to removal values < 0.6 log-units/mL. The final effluent of PT3, with the highest removal rates and significantly lower ARGs abundance, was not significantly different from the receiving water body. Emerging ARGs (e.g., blaVIM, blaOXA-48, and blaKPC) were sporadically detected in the river, although more frequent downstream. Hospital effluent might contribute for the occurrence of some, but not all these ARGs in the river. A major conclusion was that the impact of the UWTPs on the river was not only determined by treatment efficiency and final effluent quality, but also by the background contamination of the river and/or dilution rate.