Coagulation and sedimentation of algal cells and associated material in Vaal River water

Abstract

In the research of drinking water treatment by various researchers, attention was given to the coagulation of inorganic particles (Gregory, 1989). Processes of coagulation and the removal of natural organic matter and suspended organisms received less research attention. Experience at Sedibeng Water showed that the lack of applicable information on removal requirements of organic matter, results in costly treatment processes (Basson and Pieterse, 1993). Experiments were therefore, conducted to determine the optimum Fe3 + concentration for removal of dissolved organic matter, suspended algae and suspended particles, the removal efficiency by different iron salts, the pH adjustment chemical most suitable for the coagulation, and the effect of different algal concentrations as well as associated organic substances, on removal efficiencies. Water from the Vaal River at Balkfontein (Sedibeng Water) were used in the experiments. Jar tests were conducted to optimise chemical dosage and to determine removal efficiencies of dissolved organic matter and suspended particles and algal cells and colonies. The three algal species used in this investigation were Monoraphidium minitum, Cyclotella meneghiniana and Pandorina morum. Results showed that it is recommended to use lime instead of NaOH, in accordance with practice, because lime is a cheaper alternative and more effective than NaOH. When FeCl3 and Fe2(SO4)3 was compared, no significant differences were observed. FeCl3 concentrations between 8 and 10 mg r1 were determined to be the optimum concentration for the removal of dissolved organic matter and suspended particles. High pH conditions (pH 11 .5) also improved removal efficiencies. C. meneghiniana cells and P. morum colonies were effectively removed at pH 11.5. C. meneghiniana cells apparently

enhanced the removal of suspended matter at pH 6.4. Organic substances excreted by M. minutum improved the removal of cells at pH 11.5, but algal biomass removal was inhibited at pH 6.4 by the organic substances. Organic matter excreted by M. minutum apparently improved the removal of suspended matter present in Vaal River water, but reduced the removal of dissolved organic carbon (DOC). In addition, results showed that algal cells, especially of M. minutum and C. meneghiniana (at pH 11.5) and P. morum colonies themselves, apparently assist in the removal of DOC. It can, in general, be concluded that increases in algal biomass together with the organic substances excreted by the algal cells, affected the removal of biomass, suspended matter and dissolved organic carbon.