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).
