Laboratory-scale simulations with hydrated lime and organic polymer to evaluate the effect of pre-chlorination on motile Ceratium hirundinella cells during conventional water treatment

Algal genera such as Carteria, Chlamydomonas, Chlorogonium, Cryptomonas, Ceratium, Peridinium and Euglena are motile and may disrupt unit processes and cause water treatment problems. Algal  species belonging to these motile algal genera are known to interfere with coagulation and flocculation unit processes which are the main processes for algal removal. These cells are well adapted, by means of their motile structures, morphological shapes and storage products, to remain in the supernatant (by swimming or floating) until it is carried over to sand filters, where cells may cause filter-clogging  problems. When organic material is released from algal cells as a result of physical-chemical impacts on the cells, it may result in tasteand odour-related problems or the formation of harmful organic products such as trihalomethanes (THM). The aims of this study were to: (i) determine chlorine concentrations required to immobilise C. hirundinella cells; (ii) determine the removal efficiencies of pre-chlorination; (iii) investigate the integrity of C. hirundinella cells; and (iv) identify trihalomethanes that are formed. Source water samples enriched with C. hirundinella cells were exposed to a pre-determined chlorineconcentration range (0.05–0.45 mg/L). This study found that the half-maximal inhibitory concent ration (IC50-values) for chlorine < 0.20 mg/L is sufficient to render C. hirundinella cells immobile, while cells remain intact. Pre-chlorination did not have an impact on C. hirundinella removal when hydrated lime was used as a coagulant or coagulant aid. However, when organic polymer only was used as coagulant, removal efficiencies were improved by 20%. Chlorine by-products were measured, but posed no specific health risks to drinking water consumers due to the low concentration levels measured. Algal removal challenges that occur in water treatment plants when dosing organic polymers can be resolved by implementation of effective pre-chlorination strategies.Keywords: algae, coagulation, dinoflagellate, pre-treatment, trihalomethanes (THM

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)$_2$-SiO$_2$) 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)$_2$-SiO$_2$. 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)$_2$-SiO$_2$, 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 formatio