Assessing treatment technologies for reducing antibiotic resistant gene abundance and diversity in domestic wastewater treatment effluents

Abstract

Antibiotic resistance is natural, but the wide use of antibiotics in anthropogenic activities has accelerated the rate of evolution and dissemination of antimicrobial resistance (AMR) strains around the world. One location of interest relative to AMR is domestic wastewater treatment plants (WWTPs). WWTPs are very effective at reducing the discharge of water contaminants and pathogens to the environment, and AMR levels are generally lower in places where wastewater is treated. However, understanding the relative value of different treatment options is less clear, which is central to this work. Here, four full-scale biological wastewater treatment technologies (trickling filter, granular activated sludge, activated sludge, and membrane bioreactor) were compared in their relative ability to reduce antibiotic resistance genes (ARGs) in treated effluents. Further, two advanced oxidation processes (AOPs; i.e., ozonation and hydrogen peroxide coupled with UV) and different combinations, and also a Fenton-like system using Fe-bearing clay minerals were assessed as additional steps to biological treatment. Data based on quantitative polymerase chain reaction (qPCR) and high-throughput qPCR showed that all biological options effectively reduced ARG abundance and diversity in final effluents, and enrichment of ARGs was not evident. The efficacy of each WWTP at reducing ARGs and overall bacterial loads was highly related to the treatment technology applied, with secondary treatment contributing > 0.73 to the total ARG removal in all WWTPs, while, membrane bioreactor was the most effective technology achieving up to a log removal of 6.1. Ozonation (3 g/m3) as an additional step to an activated sludge system showed potential to reduce further abundances and diversities of ARGs by 1.25 log unit, achieving a total removal of 3.61. Using propidium monoazide (PMA)-qPCR to differentiate viable vs non-viable cell carriage of ARGs, we showed that ARGs were often carried in non-viable cells, which has implications to downstream gene exchange related to AMR spread. Finally, removal rates up to log 2.94 of target ARGs achieved by using iron-bearing clay minerals, which shows promise as an alternate to AOPs for ARG reduction in WWTPs. With a few AOP exceptions, all treatment technologies removed ARGs from domestic wastewater, although the membrane bioreactor was the most effective overall.