Efficiency of activated sludge process for reduction of antibiotics from municipal wastewater

Abstract

With time, a lot of change in the nature of impurities, especially a surge in emerging contaminants in urban wastewater has been observed due to changing lifestyle, uncontrolled and mismanaged urban sprawl, increasing pollution and disease burden, and easy access to antibiotics. Conventional sewage treatment plants have thus faced challenges in treating emerging pollutants such as antibiotics, with variable success as reported in few studies.  Antibiotics are persistent in the environment and result into development of antimicrobial resistance. The concentration of antibiotics reportedly varies from µg/L to ng/L in raw/treated sewage which is generally dependent upon differences in environmental/social factors as well as treatment technology. The present study was conducted with the purpose of identifying the role of activated sludge process (ASP) in a standalone mode as well as in a hybrid mode duly integrated with upflow anaerobic sludge blanket reactor (UASB) in removal of antibiotics from the raw sewage. The antibiotics were analysed with a Liquid Chromatography Mass Spectrometer (LCMS), and the removal efficiencies were compared for both the treatment systems. The concentration of selected antibiotics in raw/treated sewage of the hybrid UASB-ASP varied in the range of 0.92-79025.9µg/L and 0.03-3439µg/L respectively. It was observed that the concentration of erythromycin was very less inspite of being used as a wide spectrum antibiotic against gram positive/gram-negative bacteria causing upper and lower respiratory diseases. An apparent reason could be that it is mainly metabolised by human liver and only 5% is excreted in active form. Also, low concentration of sulfamethoxazole and enrofloxacin were detected in the ranges 0.04-0.92µg/L and 0.03-0.94µg/L respectively in the raw/treated sewage. Notably, even these concentrations could also inhibit bacterial growth by altering microbial production of folic acid and induce antimicrobial resistance at sub lethal concentration. The removal efficiency for UASB-ASP for selected antibiotic was between 51.09% to 95.87% indicating an efficient reduction. Low concentration of sulfamethoxazole and enrofloxacin was observed within the range of 0.15 – 0.21 g/L and 0.007 – 0.01 µg/L in the raw/treated sewage of ASP. Negative removal (increased concentration in the treated sewage) was observed for erythromycin and ciprofloxacin, apparently because of resistance to degradation. The reduction of sulfamethoxazole, enrofloxacin, tetracycline was 27%, 52%, 65% where trimethoprim demonstrated maximum removal of 88% in ASP.  The hybrid UASB-ASP performed better than the standalone ASP with respect to reduction of all antibiotics, indicating that ASP can perform more efficiently when integrated with other technologies alongwith addition of a proper dosing of chlorination. Risk associated with the selected antibiotics from sewage treatment plant to the receiving environment (both water/soil) was quantified employing hazard quotient (HQ) using predicted no effect concentration (PNEC) values derived from literature. HQ for sulfamethoxazole was calculated to be above 1, and higher values were observed for trimethoprim (in the range of 589-628), and tetracycline (in the range of 405-722) indicating potential environmental concern for aquatic environment/soil, whichever may be of concern. No risk seemed to appear for indirect human exposure to enrofloxacin as indicated by the calculated values of HQ (0.004-0.02).