Physical and chemical impacts of conventional unit processes and coagulants on the dinoflagellate, Ceratium hirundinella

The freshwater dinoflagellate, Ceratium hirundinella (C. hirundinella) with its complex morphology and robust thecal plate cell covering, is responsible for extensive problems during drinking water production. To have a better understanding of these problems, knowledge of what happens to the integrity of the cells after each step of the conventional water treatment process is essential. Therefore, the aim of this study was to investigate the physical and chemical impacts of conventional unit processes (prior to sand filtration) on the morphology of C. hirundinella cells and the appearance of cells in aggregation or in flocs. Source water samples enriched with C. hirundinella cells (>500 cells/ml) were used to conduct jar stirring experiments. Samples for scanning electron microscopy (SEM) were collected from raw water, after flash mixing and from the sediment that formed when dosing various coagulant chemicals. The coagulant options included hydrated lime and activated silica (Ca(OH)₂-SiO₂) which increase the pH to levels above 10, hydrated lime in combination with organic polymer (Ca(OH)₂-poly) that increases the pH to levels of approximately 9 and organic polymer (poly) alone which has no effect on the pH of the water. Results obtained from SEM investigations revealed significant damage to the cells due to flash mixing, as well as due to the dosing of Ca(OH)₂-SiO₂. When dosing organic polymer alone, no further impacts on the cell integrity were observed after flash mixing, but it resulted in poor cell removal. Ca(OH)₂-poly caused less damaging effects to the cells when compared to Ca(OH)₂-SiO₂, but resulted in moderate removal of C. hirundinella cells. Treatment plants that experience algal-related problems, especially during coagulation should consider using SEM to select appropriate coagulant dosages in order to avoid further cell damage that may occur during floc formation.