Water quality and potential toxicity assessment of desalinated seawater for drinking purpose in the City of Cape Town, South Africa

Abstract

The Western Cape is progressively becoming threatened by resultant water shortages caused by the frequent drought conditions, necessitating the need to explore alternative water supplies through seawater desalination to produce reliable drinking water to meet demand. Desalination involves the removal of dissolved salts from seawater to generate saline free drinking water to meet various human needs. The study investigated the water quality levels and potential toxicity of seawater desalination processes from intake water, to the final treated water intended for drinking, with the purpose of ascertaining its fitness for consumption. The discharge effluent from these plants was also assessed to determine its potential toxicity on the environment using aquatic test organisms. The microbiological and physico-chemical water quality of the raw and final treated water samples of the Strandfontein and Monwabisi desalination plants, Cape Town, South Africa, and their efficiency were investigated. The raw, final treated water and brine effluent of the Strandfontein and Monwabisi desalination plants were analysed for ecotoxicity using the test organisms, namely: marine algae (Phaeodactylum tricornutum), marine crustacean (Artemia franciscana) and marine bacterium (Vibrio fischeri). The monitoring studies were conducted over a 12 months period from December 2018 to November 2019. The raw and treated final water quality from seawater samples were determined and assessed against the South African National Standard (SANS) 241: 2015 limits for drinking water pertaining to microbiological, physical, aesthetic and chemical determinants related to long-term consumption. The study findings showed trends of highest bacterial counts for Escherichia coli (E. coli) and enterococci in the raw water from these two desalination plants during the winter period, which may be associated with rainfall periods within the City of Cape Town that flushes faecal contaminants from wastewater effluents into the rivers and ultimately into the sea. Higher trends of E. coli in the raw water from Monwabisi were also observed during the summer period which may be associated with increased recreational use of this beach during the hot summer months and favourable temperatures for bacterial growth. Enterococcus and E. coli were determined in the raw water from both desalination plants and the t-test results for the bacteria showed a p value > 0.05, thus there was no significant difference for E. coli and enterococcus in the raw water samples. Increased heterotrophic plate counts (HPC) of 324 CFU/mL for Monwabisi and 175 CFU/mL for Strandfontein were observed during the summer period in the treated water. The HPC CFU/mL from the two desalination plants was less than the set standard limit of SANS 241: 2015 of ≤1 000 CFU/mL for treated water. The compliance of HPC by both desalination plants indicates the effectiveness of the reverse osmosis treatment process and the adequacy of the residual chlorine used. Also, highest E. coli bacterial populations of 1 CFU/100 mL for Strandfontein and 6 CFU/100 mL for Monwabisi were observed during summer period, which may be associated with proliferation of bacteria during warmer conditions. There was a significant difference p < 0.001 in E. coli between the raw and treated water for both plants showing treatment efficiency in removal of E. coli initially found in the raw water sources, and also indicating absence of faecal pollution in the treated water. Increased bacterial counts of total coliforms (TC) in the treated water from both plants were detected during warmer periods of spring and summer when compared to other periods. High TC counts of 201 CFU/100 mL in Strandfontein could have resulted from localized run-off hard surfaces and ablution facilities at the beach. In Monwabisi, high TC counts of 201 CFU/100 mL were suggested to have emanated from storm-water detention pond located near the plant and had an influence on the presence of these bacteria in the treated water. High significant variation (p < 0.001) was observed for pH, total dissolved solids, conductivity, alkalinity, nitrates, and chlorides from the raw and treated water from Strandfontein and Monwabisi desalination plants. A significant reduction to acceptable levels of these parameters from the raw to the treated drinking water samples is regarded as an indication of the effectiveness of treatment process applied at the two desalination plants. These physico-chemical parameters were mostly all compliant with the standard guideline limits throughout the study period. In terms of potential toxicity of the raw and treated water as well as the brine effluent, the raw water samples from both plants showed the least toxicity with the growth inhibition (algae) and mortality (crustacean) test compared to the treated water samples and brine effluent. The treated water and brine effluent showed some toxicity to P. tricornutum and A. franciscana. The addition of chemicals during the desalination treatment process was suggested to have influenced the detected toxicity on the treated water and the brine effluent. The V. fischeri bioluminescence test results for the three matrices (raw, treated and brine water samples) showed some bacterial stimulation, indication of no toxicity presence. Overall, the results of the study showed that the final treated water product from both plants was of high quality and in compliance to SANS 241: 2015 and depicting limited toxicity against test organisms. Findings suggest that regular water quality monitoring of the desalination plants is an essential component. In conclusion, the desalination technology offers a great benefit in the augmentation of water supplies and narrowing the gap of diminishing freshwater resources.