Occurrence and reduction of antibiotic resistance genes in conventional and advanced drinking water treatment processes.

Antibiotic resistance is extensively detected in drinking water sources, threatening its safety and human health, which deserves further attention to the removal of antibiotic resistance genes (ARGs) in the drinking water system. In this study, the occurrence and reduction of integrase gene intI1 and forty-one ARG subtypes, which confers resistance to six antibiotic classes (β-lactam, aminoglycoside, macrolide, tetracycline, sulfonamide, and quinolone), were investigated in a drinking water treatment plant (DWTP). Seventeen ARG subtypes with absolute concentrations ranging from 1.4 × 100 to 7.3 × 105 and 3.9 × 104 genes/mL (intI1) were detected in the raw water; and sul1 and sul2 were the two dominant ARG subtypes. Overall, the whole DWTPs achieved 0.03-2.4 log reduction of ARGs compared with those presented in raw water. The reduction efficiencies of sul1, strA, and intI1 were the highest (1.0-2.4 log) in both conventional and advanced processes. However, the levels of sul1, sul2, and ermC still remained high (1.3 × 100 -1.9 × 104 genes/mL) in finished water. The treatment units, including pre-flocculation/sedimentation/sand filtration, and ozonation units, were beneficial for the reduction of ARGs, which was mostly ascribed to the decline in biomass and the strong oxidizing properties of ozone. However, the reduction effect was subsequently counteracted by the granular activated carbon and chlorination units. This study provides basic data for ARG pollution in the drinking water system, and suggests that ARGs persist in drinking water, even after conventional chlorination or advanced treatment processes, highlighting the need for new and efficient water purification technologies.