Prevalence and fate of antibiotics and its derivatives in sewage treatment in Durban and the receiving environment

Abstract

Antibiotics are released to the environment either directly in an unchanged form or partially metabolized. The discharge is usually through untreated waste or through wastewater treatment effluents. The stable antibiotics in reduced amounts persist through the wastewater treatment processes and end up in receiving waters, where they may impact crops through irrigation or affect drinking water intakes. Antibiotics in the waste and sludge fractions may similarly impact crops and arable land through their use as fertilizers. Conventional wastewater treatment plants are not designed for the removal of antibiotics but may to a varying extent reduce their concentrations. Their quantitative occurrence within the water matrices depends on the frequency and quantities of use for therapeutic purposes or as growth promoters in animal production. Additional inputs may emanate from individual waste discharges. Antibiotics present in sub-inhibitory concentrations may predicate for resistance among the resident bacteria in the water matrix, biofilms or in humans and animals. In South Africa antibiotics are extensively used both in human therapy and in animal husbandry without clearly followed regulations and are sometimes readily available. The available studies have focused on the presence of antibiotic resistant bacteria in wastewater influent and effluent but there is a paucity of information relating to these antibiotics as emerging contaminants in South Africa wastewater. In this thesis a rapid and sensitive analytical methodology was initially assessed and applied, based on the use of HPLC/diode array UV detector for six antibiotics (ethionamide (ETI), metronidazole (MET), trimethoprim (TRI), ciprofloxacin (CIP), sulfisoxazole (SUF) and albendazole (ALB). Validation of the method was performed by screening assessment in selected wastewater treatment plants (WWTPs) with the aim of determining the sensitivity of the equipment (Shimadzu 2020), assess the limit of detection, optimize the extraction procedure (solid phase extraction) and screen for the most prevalent

antibiotics. The percentage recovery for the optimized method using wastewater sample was above 65 % for all antibiotics of interest. The limit of detection, which ranges from 0.03 to 0.48 mg L-1, enables the determination of a range of concentration of antibiotics in polluted sample such as the wastewater influent sample. Furthermore in this thesis, a more advanced, online solid phase extraction – high performance liquid chromatography mass spectrometry (SPE-HPLC-MS) method, was applied to measure the concentration of these and an additional seven antibiotics, norfloxacin (NOR), ofloxacin (OFL), clindamycin (CLI), sulfamethoxazole (SUL), erythromycin (ERY), clarithromycin (CLA), azithromycin (AZI) and roxithromycin (ROX) in ng L-1 concentrations. The quantity and occurrence of the selected antibiotics was assessed in untreated wastewater in four wastewater treatment plants in Durban, KwaZulu-Natal (KZN), at different treatment stages and in the effluent and recipient surface water environment. In the influent the additive concentration of the antibiotics associated to the separated sediment fraction through centrifugation and in the supernatant of samples collected were accounted for and analyzed. The limit of detection (LOD) and the limit of quantification (LOQ), ranged from 0.07 – 0.33 ng L-1 and 0.23 – 1.09 ng L-1, respectively for the 13 assessed antibiotics and the percentage recovery were in the range of 51 to 111 %. The percentage of antibiotics recovered from the sediment (centrifuged) samples, which would have been lost to filtration if not analyzed in parallel, were in the range of 2.6% – 97% (n = 32), while the frequency of detection in the influent samples for the sampling period ranges from 62.5 – 100 % (n = 32). All the studied antibiotics were detected in the influent of each WWTP and the concentration was in the rage of 1.3 ng L-1 (AZI) – 81748 ng L-1 (CIP). The antibiotics with the highest concentrations (median) detected in the receiving water (downstream) for each of the four WWTPs in KZN, were TRI (217 ng L-1), SUL (239 ng L-1), CIP (708 ng L-1) and ALB (325 ng L-1) respectively. The overall percentage removal efficiency for the four WWTPs ranged from 21 % - 100 %. The most effective treatment steps were assessed

with the focus on activated sludge filter and trickling filter. Within these, it was actually the sedimentation treatment stages (secondary clarifier), after these steps that played the most vital role in the reduction of antibiotics where > 70 % of the antibiotics was removed. Finally, the impact of post chlorination was analyzed for the effluent of the WWTPs. The presence of transformation product as a result of post chlorination was examined in a parallel study using a controlled experiment and full scale analysis. The efficiency of chlorine in the reduction of antibiotics was more of transformation of antibiotics than degradation. The oxidative ability of chlorine enhances its reaction with antibiotics thereby transforming the antibiotics. The percentage reduction of antibiotics in relation to chlorination was >85 % (pilot experiment) and ranged between 14 % - 97 % in the field experiments. Likewise, UV was effective in the degradation of antibiotics, with longer exposure time producing higher degradation. Future research should focus on determining the toxicological impact of these transformation products. The concentration of the antibiotics in the downstream samples were generally low when compared to their influent concentrations.