Critical review of antimicrobials, antimicrobial resistance, and their associated genes in decentralized wastewater treatment systems

The pervasive use of antimicrobials across human health care, veterinary, and agricultural sectors has led to their widespread environmental occurrence. Centralized and decentralized wastewater treatment systems often fail to completely remove these antimicrobials, driving the emergence and propagation of antimicrobial resistance (AMR), resistant microbes, and antimicrobial resistance genes (ARGs). Decentralized wastewater treatment systems (DWWTSs), which serve over 30% of the global population, are emerging as overlooked reservoirs of AMR and ARGs. This review provides a critical overview of the occurrence and fate of antimicrobials, AMR, and ARGs within DWWTSs and their role in spreading AMR and ARGs. We highlight the persistence of antimicrobials in DWWTSs and the mechanisms facilitating the proliferation and dissemination of AMR and ARGs. Particular attention is given to septic tank conditions, such as prolonged sludge retention times, intermittent loading, anaerobic-dominated conditions, and oxidative and nitrosative stress, which may promote the selection and proliferation of resistant microbial populations. We further examine how trace antimicrobials and co-selective agents interact to exacerbate AMR. This review reveals crucial research gaps, including the overlooked role of reactive oxygen species in AMR proliferation and the need for laboratory studies isolating the impact of individual factors. Current studies often target specific bacterial taxa, leading to overgeneralizations about the broader microbial community involved in AMR dynamics. This review underscores the overlooked role of DWWTSs in AMR spread and calls for improved design, operation, and monitoring strategies to transform them from passive conduits into active barriers against AMR. • Septic tanks are overlooked reservoirs for AMR and ARG dissemination • Septic tank conditions create selective pressures that promote AMR development • ROS play a critical but understudied role in AMR proliferation within septic tanks • Trace antimicrobials, co-selective agents, and ROS exacerbate AMR proliferation • Design, operation, and policy upgrades are needed to mitigate AMR in septic tanks