Seven phytoplankton groups were recorded in the source water supplied to South Africa's largest conventional drinking water treatment plant (DWTP). Two phytoplankton genera, Anabaena and Ceratium were identified as the problem-causing phytoplankton due to their ability to interfere with the water treatment process and negatively impact on water quality. The objectives of this study were to identify problem-causing phytoplankton genera and investigate the efficacy of unit processes in removing phytoplankton genera and associated organic compounds. Phytoplankton and organic compound data were obtained from four different sampling localities throughout the treatment plant and statistically analysed to evaluate the removal efficiencies of unit processes. The highest percentage removal for the Cyanophyceae average seasonal concentration (> 1 000 cells/mℓ) was recorded at 98%, while the highest percentage removal for the Dinophyceae average seasonal concentration (± 9 cells/mℓ) was recorded at 100%. Microcystis and Anabaena were removed by the processes of coagulation, flocculation and sedimentation (> 95%), while Ceratium cells were removed by sand filtration (> 80%). Ineffective removal of Ceratium by coagulation, flocculation and sedimentation (and subsequent penetration to the sand filtration step) will negatively impact on filter run times when these phytoplankton genera are present in high concentrations in the source water. Total photosynthetic pigments (TPP) were removed effectively by all the different water treatment processes. Not enough statistical evidence could be displayed to suggest effective removal of geosmin in this conventional water treatment plant. With good removal of intact cyanobacteria cells during coagulation, flocculation and sedimentation, geosmin concentrations in the final water could be kept to accepted organoleptic levels of 5-10 ng/ℓ in the final water. Optimising conventional drinking water treatment processes can effectively remove problem-causing phytoplankton as well as their associated organic compounds and thereby reduce the potential risk to drinking water consumers.

The dinoflagellate, Ceratium hirundinella (C. hirundinella) cells are known to cause many problems when source water due for purification contains relatively high concentrations. The objective of this study was to investigate strategies for the effective and simultaneous removal of turbidity and C. hirundinella cells using turbidity and total photosynthetic pigments (TPP) as indicators of appropriate coagulant dosages during conventional drinking water purification. Source water samples with low turbidity, and high number of C. hirundinella cells were collected. A laboratory-scale conventional water purification plant was used to simulate coagulation, flocculation, sedimentation and sand filtration. Various coagulant options were dosed as part of conventional coagulation. The coagulant option Ca(OH)2–organic polymer achieved the best removal of both turbidity (50%) and C. hirundinella cells (75–82%) after sedimentation using TPP as an indicator. Ca(OH)2–SiO2 and organic polymer alone achieved better removal of C. hirundinella (57–75%) and turbidity (33–50%) respectively when TPP was used as an indicator rather than turbidity. Sand filtration removed the remaining turbidity and C. hirundinella cells from the supernatant completely. Implementing new purification strategies may increase treatment costs, but the focus of drinking water purification utilities should always be primarily the production of safe and aesthetically acceptable drinking water.

Unit processes of a conventional water purification facility are designed to remove suspended material from source water (both inorganic and organic impurities). Organic substances in source water include phytoplankton species (algae and cyanobacteria) that are generally negatively charged on the surface of the cells. The zeta potential (ZP) of algal cells needs to be destabilized in order to enhance removal thereof during water purification. The aims of this study were to investigate the ZP changes of Ceratium hirundinella (C. hirundinella) cells and the ability of cells to restore their natural ZP during the water purification process. C. hirundinella cells (>500 cells per mL) were collected from the Middle Lake in respectively. However, after extended settling times (120-240 minutes), more metabolically active cells were observed in the supernatant of samples containing higher cell concentrations. The findings showed that the ZP of C. hirundinella cells changes as a result of adding coagulants to form flocs, but may be restored when water purification facilities employ poor optimization practices and allow extended settling periods or retention times.

Total photosynthetic pigments (TPP) or chlorophyll-a analysis can be useful in selecting coagulant treatments that will improve phytoplankton removal and reduce treatment costs. The objectives of this study were to compare the efficacy of phytoplankton and turbidity removal when using TPP and turbidity as indicator parameters of appropriate coagulant treatments as well as to evaluate the cost impacts thereof. During seven different sampling occasions, source water samples with substantially different TPP and turbidity contents were collected from two South African freshwater sources (Benoni Lake and Vaal Dam) to conduct jar stirring tests. After sedimentation, TPP and turbidity analyses were performed to assess the efficacy of coagulant treatments (Ca(OH) 2 -SiO 2 , Ca(OH) 2 -organic polymer and organic polymer). Results showed that TPP analysis is indeed a useful indicator parameter to consider purifying source water enriched with phytoplankton. Ca(OH) 2 -organic polymer treatment was more costly than Ca(OH) 2 -SiO 2 and organic polymer, but the only coagulant treatment that met the removal criteria after sedimentation. Benoni Lake source water (TPP: 34.29 μg/l; 4.29 NTU) was more costly to treat than Vaal Dam source water (TPP: 2.29 μg/l; 80.29 NTU). Findings made from this study confirm that high phytoplankton concentrations in source water due for treatment will increase the treatment costs.