Removal of Targeted Antibiotic Resistance Genes and Coselectors in a Bench-Scale Modified SBR Treating Domestic and Hospital Sewage in India

Although various measures are being implemented to address antimicrobial resistance (AMR) at its sources, wastewater treatment plants (WWTPs), major point sources, continue to contribute to the spread of AMR in ambient environments. Research on the efficacy of existing treatment systems in removing antibiotic resistance genes (ARGs) remains limited. The current study investigates the removal of six ARGs, namely, sul1, czcA, tetA, acrA, qnrS, and blaTEM, along with one integron integrase gene (intI1), the yccT gene (a bacterial marker for Escherichia coli), and the 16S rRNA gene (a general bacterial DNA marker) using a 10-L lab-scale modified sequencing batch reactor (SBR). The reactor treated a mixture of hospital and domestic wastewater, with equal cycle durations for aerobic and anoxic phases. The influence of ciprofloxacin, triclosan, chromium, zinc, and arsenic on ARG proliferation was also assessed. The SBR demonstrated efficient removal of most ARGs, with sul1 reduced by three orders of magnitude and tetA by over five orders of magnitude between the inlet and outlet. Notably, blaTEM levels increased in the effluent, likely due to hospital wastewater inputs and selection for β-lactam resistance during settling phases. The yccT and intI1 gene markers showed strong positive correlations with mixed liquor suspended solids, indicating that biomass influences ARG persistence. Ciprofloxacin and triclosan were removed with efficiencies exceeding 86%, while chromium and zinc were removed at more than 83% (arsenic removal was limited to 26%). The significant Spearman correlation analysis revealed a negative correlation between the targeted genes and the coselectors, contradicting classical coselection theory paradigms and highlighting the complex interactions between the ARGs, coselectors, and environmental behavior in actual WWTPs. These findings suggest that variations in design parameters significantly improve ARG removal without structural overhauls or chemical disinfectants. The study offers valuable insights into optimizing existing SBR-based WWTPs, contributing to the reduction of emerging contaminants and minimizing public health risks associated with AMR dissemination into the ambient environment. This research also emphasizes the potential of modifying biological treatment systems to mitigate the environmental impact of AMR, especially in low- and middle-income countries like India, which face infrastructural and economic challenges.