Sustainable drinking water supply service and development in the face of different resource challenges : a case study of Midvaal Water Company, South Africa

Abstract

Water quality of resources in large parts of South Africa is declining. The Vaal River is heavily polluted when it reaches its mid-section at Midvaal Water Company in the North-West Province. Midvaal, a water service provider, abstracts and treats water from the Middle Vaal to supply bulk potable water in compliance with the South African National Standard 241:2015 for drinking water. The main objectives of the case study were to investigate water treatment challenges at Midvaal due to changes in source water quality, to evaluate wastewater recycling at the plant and the effect thereof on sustainable drinking water supply and to determine the impact of the Koekemoerspruit on the Middle Vaal River by an integrated study of phytoplankton assemblages and water physico-chemistry. The dissolved air flotation process, since 1997, had the most significant impact since it accounts for almost 70% total chlorophyll removal. Surface water samples were sampled on a monthly basis at sites located in the Koekemoerspruit and on a daily basis from the Middle Vaal River intake as well as sites within the plant for the different sections of this study. Sampling frequency, durations and required analytical methods were based on monitoring programs for each system. Samples were analysed at Midvaal Water Company Scientific Services. The phytoplankton identification and enumeration were performed at the North-West University, Potchefstroom campus. The yearly average total chlorophyll concentrations of the source water gradually increased from 33 μg/L (1984) to 133 μg/L (2014). The treatment facility suffers from severe taste and odour episodes during summer due to the presence of 2-methylisoborneol (MIB), released by Cyanophyceae. Concentrations of > 300 ng/L MIB were recorded. The processes of the wastewater recycling system did not compromise final water quality. Total chlorophyll concentration was identified as the principal risk during wastewater recycling, especially after filtration. Results from the Koekemoerspruit indicated that target water quality objectives for orthophosphate, nitrate and nitrite and ammonia were exceeded during 2014 and 2015, indicating severe organic pollution. Colour, ammonia and total chlorophyll concentrations displayed significant increasing trends over time and increased drastically after 2012. Average phytoplankton concentrations of 1 410 069 cells/ml and 417 931 cells/ml were determined for the Middle Vaal and Koekemoerspruit respectively. A total of 86 phytoplankton genera were collectively identified. A redundancy analysis confirmed that water quality had a definite effect on the phytoplankton assemblages (p-value of 0.08). The treatment process changes enabled the plant to manage the increasing phytoplankton load in the source. Wastewater recycling was both operational- and cost-effective. The Koekemoerspruit, suffering severe organic pollution, did not impact significantly on the water quality of the Middle Vaal, except for total chlorophyll. The Chlorophyceae taxon (48%) and Scenedesmus spp. genus dominated in the nutrient enriched Middle Vaal River. The Cyanophyceae taxon (45%) and Nitszchia spp. genus were dominant in the Koekemoerspruit. This study highlighted how the eutrophication of water resources and associated chlorophyll concentrations escalate to have multidimensional effects on sustainable drinking water supply. Stricter regulation by authorities concerning wastewater discharges are recommended to protect South-African water resources