First nationwide survey of the distribution and seasonal variation of antimicrobials in effluents from wastewater treatment plants in Japan

• First nationwide survey of residual antibiotics in WWTP effluents across Japan. • 43 WWTPs sampled in 8 regions during summer and winter campaigns in 2024–2025. • Widespread detection of antibiotics highlights national-scale contamination risks. • Seasonal variation observed, with higher loads in winter at several regions. • Findings provide baseline data for AMR mitigation and wastewater management. The widespread occurrence of antimicrobial residues in aquatic environments has emerged as a pressing environmental and public health concern due to their contribution to the development and spread of antimicrobial resistance (AMR). Despite medical advancements, nationwide data on antimicrobial discharges from wastewater treatment plants (WWTPs) into receiving waters remain scarce, particularly in Japan, among other Asian countries. This study presents the first nationwide, year-round assessment of residual antimicrobials in WWTP effluents across Japan that considers seasonal variations. The survey encompasses 43 WWTPs representing all major geographic regions. Seventeen antimicrobials from five major classes were quantified using high-throughput LC-MS/MS analysis. Levofloxacin, vancomycin, and azithromycin were the most frequently detected compounds, exhibiting pronounced seasonal variability with peak concentrations observed in winter. The results revealed substantial variability in removal efficiencies across treatment configurations, with advanced biological processes demonstrating higher removal performance than conventional systems. Correlation and multivariate analyses further identified significant associations between antimicrobial concentrations and key operational parameters, particularly hydraulic retention time (HRT). Overall, these findings elucidate the environmental dissemination patterns of antimicrobials and highlight the need for optimised wastewater treatment strategies to reduce AMR-related risks. By improving understanding of antimicrobial behaviour in wastewater systems, this study supports the development of targeted pollution control measures and effective AMR mitigation strategies.