Total photosynthetic pigments in addition to turbidity during the selection of coagulant treatments: a drinking water treatment perspective

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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-SiO2, 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-SiO2 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.

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