Midvaal, a case study : the influence of ozone on water purification processes
This study investigated the influence of ozone on water purification processes at Midvaal Water Company. The utility is situated on the banks of the Middle Vaal River in the North West Province and supplies potable water to the local municipality of Matlosana as well as surrounding mining companies. The Middle Vaal River is hypertrophic and the only raw water source for the treatment works. Midvaal usually experiences taste and odour problems during summer periods when the cyanoprokaryote, Oscillatoria, occurs in the raw water and produces geosmin and methylisoborneol (MIB). These compounds do not pose a health risk to consumers but degrade the aesthetic quality of the water. The earthy, muddy tastes and odours are detected at extremely low concentrations and are not effectively removed by conventional treatment methods which therefore necessitate the application of an advanced method such as ozonation. Midvaal has been using ozonation in their treatment process since 1985 and upgraded the ozonation plant in 2007. The aim of pre-ozonation at this plant is to improve dissolved air flotation (DAF) by inactivating the algal cells while intermediate ozonation is applied for oxidation of iron and manganese as well as colour improvement. Samples were collected weekly for a one year period from the raw water, after intermediate ozonation and after pre-ozonation and were measured for pH, electrical conductivity, turbidity, chlorophyll-a, total chlorophyll, dissolved and total organic carbon, manganese, iron, aluminium, SAC 254, geosmin, MIB as well as algal identification and enumeration. SAC 254, chlorophyll-a, total chlorophyll, total algal cells (of which the Bacillariophyceae and Chlorophyceae dominated) and turbidity declined significantly after intermediate ozonation which was also supported by effect sizes. Intermediate ozonation had variable influences on pH, DOC, TOC, as well as manganese, iron and aluminium concentrations. Conductivity was the only variable to increase after intermediate ozonation together with MIB concentrations. The study showed that increased ozone dosages together with and optimum DAF process may alleviate taste and odour problems during Oscillatoria blooms and the associated occurrences of MIB in the raw water. Even though the water purification process did not rely heavily on ozonation for the oxidation of manganese, iron and aluminium concentrations in the raw water during the study period, it remained an essential step for improving colour. Other than the influence of ozonation on the water purification process, the study also demonstrated the crucial role of the DAF process in the overall success of the plant by removing a great number of algal cells intact. The study also showed that the Ozone Quicktest could successfully be applied to determine the ozone concentration in the process gas. Even though pre-ozonation did not have a significant effect on the chlorophyll concentrations of the raw water as was both desired and expected, the influence of pre-ozonation on the water purification and more especially the DAF process could not successfully be investigated during this study as the time was insufficient. The information obtained from this study is of value to Midvaal Water Company as well as other water utilities making use of or planning to apply ozonation as a means to increase the quality of potable water supplied to South African consumers.