Fate and Behavior of Antibiotic Resistance Genes in Rapid Sand Filtration Drinking Water Treatment System and Analysis of Potential Influencing Factors

Antibiotic resistance genes (ARGs) are increasingly recognized as a concern in drinking water, yet the factors influencing their persistence from raw to finished water in drinking water treatment plants remain poorly understood. This study investigated the occurrence, removal, and potential factors associated with the persistence of ARGs (sul1, sul2, and tetG) in a full-scale rapid sand filtration drinking water treatment system with intermediate and post-chlorination. ARGs were detected in raw water at a median total concentration of 106 copies/L and remained detectable in finished water at 104 copies/L. Relative ARG abundance increased after treatment despite substantial absolute reductions (2.1–3.6 log). Intermediate chlorination achieved the greatest ARG log reduction value (0.53–2.4 log), likely due to higher chlorine dose and lower pH favoring HOCl formation. By contrast, post-chlorination at higher pH provided limited additional removal, possibly due to predominance of less reactive OCl− and survival of chlorine-tolerant bacteria. Multivariate analyses showed a shift from particle-bound ARGs in raw water to dissolved organic matter (DOM) and fine-particle-associated fractions along the treatment train. These findings suggest that reducing fine particles and DOM, together with optimized disinfection, may help lower ARG-associated risk in finished water.