Evaluation of the occurrence of antibiotics at different treatment stages of decentralised and conventional sewage treatment plants

Antibiotics are widely used in human society and are consequently found in sewage treatment plants. However, information regarding the fate of antibiotics in decentralised sewage treatment plants is limited. Therefore, this study monitored and compared four frequently prescribed antibiotics (ampicillin, ciprofloxacin, erythromycin and sulfamethoxazole) in each stage of four conventional treatment plants (three extended aerations and one conventional activated sludge) and two decentralised treatment plants (two Imhoff tanks) in Johor Bahru district between December 2018 and August 2019. The findings revealed that ampicillin, ciprofloxacin, erythromycin and sulfamethoxazole were detected in most wastewater samples, including influent, secondary effluent, final effluent and sewage sludge samples. The antibiotic concentrations in the aqueous and sludge phases ranged from 4.2 to 2690 ng/l and 1.7 to 317.4 ng/g, respectively. Ampicillin and sulfamethoxazole were effectively removed by extended aeration and conventional activated sludge plants (> 75% reduction), erythromycin was effectively removed in IT plants (> 85% reduction) and ciprofloxacin could not be removed effectively by any systems (< 30% reduction). Statistical analysis showed that the approximate relationships between the removal of antibiotics and several wastewater parameters existed. A detailed antibiotic mass flow was conducted in extended aeration and Imhoff tank plants. An estimate of 115.72 g, 202.25 g, 170.55 g and 213.21 g of ampicillin, ciprofloxacin, erythromycin and sulfamethoxazole, respectively, were discharged annually. The mass balance analysis indicated that biodegradation was the major route for the removal of all antibiotics studied. Meanwhile, sorption was only responsible for minor removal of ciprofloxacin, erythromycin and sulfamethoxazole.