Abstract
The aim of this study was to determine the efficacy of ozone in water purification processes at the Midvaal Water Company, which uses the hypertrophic Middle Vaal River for source water. It was found that pre- and intermediate ozonation had no significant effect on pH, conductivity, dissolved organic carbon (DOC) and total organic carbon (TOC). Chlorophyll-a, total chlorophyll, spectral absorbance coefficient (SAC 254) and total algal cells were not influenced by pre-ozonation (as desired) but were greatly reduced after intermediate ozonation. The dissolved air flotation step which occurs after preozonation and prior to intermediate ozonation contributed to an average total chlorophyll removal of 74%. The effect of ozonation on the removal of manganese, iron and aluminium could not be determined during this study since these elements were present in relatively low concentrations in the source water. Intermediate ozonation had variable effects on the removal of Cyanophyceae, Dinophyceae, Euglenophyceae and Chlorophyceae, but Chrysophyceae, Bacillariophyceae and Cryptophyceae were greatly reduced after this stage.
Highlights
Water is a scarce resource in South Africa and this situation may very well intensify over time, forcing purification companies to utilise every available water source, even a river, reservoir or wetland with water that looks like pea soup, or one clogged from bank to bank with aquatic plants (Davies and Day, 1998)
Due to the efficiency of water treatment processes, water for potable supply need not be of the highest quality (Mason, 1991); if a water source of better quality is not available, advanced treatment methods have to be implemented and developed
The ecological state of the source water (Table 1) compared favourably with trends identified in other studies, such as those of Pieterse and Janse van Vuuren (1997), Kruskopf (2002) and Carrim (2006), and no shift in environmental variables over time was detected
Summary
Water is a scarce resource in South Africa and this situation may very well intensify over time, forcing purification companies to utilise every available water source, even a river, reservoir or wetland with water that looks like pea soup, or one clogged from bank to bank with aquatic plants (Davies and Day, 1998). Midvaal Water Company (Fig. 1) is situated on the banks of the Middle Vaal River (26o 48’ 1.4” East and 26o 56’ 4.5” South) in South Africa and abstracts between 95 and 180 Ml of water from the Vaal River per day. Tributaries in the catchment of the Vaal River contribute to the deteriorating water quality of Midvaal Water Company (midvaalwater.co.za) 1 = Water abstraction (not shown on diagram) 2 = Pre-ozonation and flocculation channels 3 = Dissolved air flotation 4 = Intermediate ozonation 5 = Control room 6 = Sedimentation 7 = Filtration 8 = Disinfection with chlorine gas 9 = Water recovery = Storage and distribution = Office (not shown on diagram) = Laboratory the Middle Vaal system (Anon., 2008) by introducing various pollutants into the system at times
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