Determining the extent of de facto water re-use in South Africa : the case of wastewater treatment plants in Gauteng, KwaZulu-Natal and Western Cape provinces

Abstract

The water quality of South African rivers is greatly impacted by insufficiently treated wastewater effluents (de facto reuse). Although de facto reuse serves as an alternative water supply it poses potential threats to human health and the environment. In this study therefore, the contribution of de facto reuse was determined for 6 wastewater treatment plants (WWTPs). Two methods were used to determine de facto reuse, viz. wastewater tracers (caffeine (CAF), lamivudine (LAM), and sulfamethoxazole (SULF)) and a geographic information system (GIS) based method. The wastewater tracers were selected based on their abundant use in food and medicine. Initially, the wastewater tracers were identified using ultra-high pressure liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) by their fragmentation patterns. After identification, the method was optimized and validated and then used to quantify de facto reuse. Subsequently, the wastewater traces were used to validate the GIS model results. The GIS model was developed using stream flow data and wastewater treatment locations to do spatial analysis for the WWTPs and the rivers they discharge to. Consequently, mass balance calculations were conducted based on the volumetric flow of the WWTPs and the stream flows thereby determining de facto re-use. In addition, the operation and maintenance (O&M) costs were predicted for the three Kwa-Zulu Natal WWTPs based on population equivalent (PE). The target analytes were successfully identified by their fragmentation patterns. The obtained fragments corresponded with the fragments recorded in the United States Environmental Protection Agency’s (EPA’s) Estimation Program Interface. According to the optimization results, methanol (MeOH) is the most suitable solvent because it yielded higher signal-to-noise ratios for the analytes compared to acetonitrile (ACN) resulting in better sensitivity of the method. Solid phase extraction (SPE) efficiency results for CAF showed high recovery % in HLB cartridges compared to C-18 cartridges (103.75 and 56.98% respectively). In contrary, LAM had high recovery % in C-18 cartridges compared to HLB cartridges (100.71 and 32.91% respectively). In addition, low recoveries were obtained for SULF in both cartridges (31.74 and 20.05% respectively). Method validation results showed that the method was linear because the correlation coefficients (R2) of the calibration curves for all the analytes ranged from 0.9921-0.9984. Further, the results for matrix effect revealed that the sample matrix suppressed the ions of the target analytes because the matrix effect percentages were less than 100%. The method was also sensitive because of low limits of detection (LODs) (0.34, 0.06, and 0.04 μg/L) and limits of quantification (LOQs) (1.03, 0.17, and 0.14 μg/L) were obtained for CAF, LAM, and SULF, respectively. The results for repeatability and reproducibility demonstrated that the method is precise because the %RSD of the peak areas were < 4% and < 11% respectively. Additionally, the results proved that the method is precise because the mean recovery percentages were between 99.3% and 101.4%. In addition, the method was robust because the %RSDs of injection volumes and mobile phase flow rates were less than 7%. Method application results demonstrated that the concentrations of the target analytes were higher in winter (11.8-912.1 μg/L) compared to spring (0.5-10.6 μg/L). The results for de facto reuse quantification proved that LAM is a more suitable tracer for quantifying de facto reuse than CAF and SULF because it yielded more reliable results. This is because LAM has a lower rate of degradation compared to CAF and SULF. De facto reuse trends were determined for WWTP1, WWTP2, WWTP3, WWTP4, and WWTP6 using a GIS model over a period of 10 years. The data was selected from 2009 to 2018 based on availability of monthly stream flow data. Out of all the WWTPs, WWTP1 had the highest percentages for de facto reuse (62.75-107.94%) throughout the 10 years due to its large design capacity (4.63 m3/s). Consequently, the GIS-model and tracer method results were compared, and the results obtained using both methods followed a similar pattern (4.04-85.49 and 16.55-77.32 respectively). In contrary, the results obtained for WWTP3 (using the tracer method) were very high because of seasonal streamflow variations. A case study was conducted for the Jukskei river (one of the rivers mostly impacted by de facto reuse) and the assessment results demonstrated that the high levels of de facto reuse are a result of the large population served by WWTP1. Also, O&M costs were predicted for WWTP3, WWTP4 and WWTP5 and the results revealed that the O&M costs are influenced by the economies of scale (R 171.34, 5.53 and 1.69 respectively).