Insight into on-site sewage facilities as an overlooked contributor to antimicrobial resistance: Environmental impacts and existing mitigation strategies

Antimicrobial chemicals and resistance genes (ARG) drive antimicrobial resistance (AMR) proliferation. Decentralized, on-site sewage facilities (OSSF), usually with small serving capacities, are commonly exempted from secondary treatment requirements. To date, dissemination of AMR contaminants from OSSF to the environment remains underexplored. This review aims to critically examine the environmental impacts of OSSF on AMR and the effectiveness of existing mitigation strategies, with a comprehensive synthesis of literature on the fate and mitigation of AMR contaminants in OSSF. The typical design of septic tank followed by soil infiltration often poorly remove AMR contaminants, especially in winter, with high leaching potential into groundwater and occasionally posing AMR selection and ecological risks. Additional treatments (e.g., constructed wetlands, aerobic systems) can overall provide better mitigation. We further perform a meta-analysis of AMR selection risk, ecological risk, and environmental hazards to result in a list of priority AMR chemicals in OSSF-impacted waters, with erythromycin-H₂O, ciprofloxacin, triclocarban of top concerns. Despite limited literature on ARG, those of clinical relevance are highly abundant in OSSF systems. AMR dissemination could be influenced by different factors. Particularly, chemical diversities tend to increase with OSSF serving capacities and sampling methods can influence their detection. Our review highlights the overlooked role of OSSF in environmental AMR dissemination, and the limited research suggests a poor understanding of this issue, that needs future studies, especially on ARG and broader geographical context including low-/middle-income countries. This review also urges for improving existing mitigation strategies at OSSF to better manage and control AMR dissemination globally.