Removal of Antibiotic Resistant Bacteria and Genes by Conventional and Nature-Based Municipal Wastewater Treatment Systems

Abstract The presence of antibiotic-resistant (AR) E. coli , antibiotic resistance genes (ARGs), and intI1 was investigated in different treatment stages, comparing a conventional activated sludge wastewater treatment plant (WWTP) with a nature-based system in a decentralised area comprising upflow anaerobic sludge blanket reactors (UASBs), constructed wetlands (CWs), and an ultraviolet (UV) disinfection unit. Furthermore, the efficacy of UV irradiation and chlorination in the removal of AR E. coli , ARGs and intI1 from the effluent of CWs and the secondary treated effluent of the conventional WWTP was investigated. The study found that the natural system decreased AR E. coli by 3.7–4.1 log and ARGs by 0.80–2.3 log. In contrast, the conventional WWTP reduced AR E. coli by 2.5–2.8 log and ARGs by 1.9–3.2 log. CWs enhance ARB removal through longer hydraulic retention times, plant–microbe interactions, and diverse ecological processes, offering advantages over conventional WWTPs. UV treatment with doses of 70 mJ/cm 2 and 100 mJ/cm 2 effectively removed AR E. coli but increased their percentage, indicating higher resistance to UV. However, these UV doses were ineffective in removing ARGs. Chlorination at doses of 35–50 mg Cl₂/L·min effectively inactivated antibiotic-resistant bacteria (ARB) in wastewater. However, inadequate doses increased the prevalence of antibiotic-resistant strains and facilitated horizontal gene transfer via natural transformation. A high chlorine dose of 220 mg Cl₂/L·min was insufficient to remove ARGs from secondary treated and CW effluents. These results highlight the persistence of ARGs despite aggressive chlorination and high UV disinfection doses, emphasizing the need for complementary or advanced treatment methods.