Direct filtration of Chlorella and Scenedesmus suspensions for potable water treatment

Algal growth in raw water sources results in two components in the water; a particulate fraction made up of the algal cells, and a soluble fraction made up of algal extracellular organic matter (EOM). During potable water treatment, both these components govern the demand for treatment chemicals, and the subsequent removal of the algal cells;Laboratory monocultures of Chorella and Scenedesmus were cultured under continuous lighting with artificial nutrients. Suspensions were drawn from these concentrated cultures and diluted with tap water for use in laboratory-scale sand filtration experiments. A total of 98 filter runs were performed. Four types of coagulation were used; no coagulants, ferric chloride, aluminum sulfate and cationic polymer. For all coagulation modes, the effects of prechlorination were tested. Filtration runs were supplemented with analyses on the algal EOM to determine molecular weight distribution, electrical charge concentration, chlorine demand and cationic polymer demand;Without any coagulants, algal removal was poor and erratic. The metal coagulants caused removal which was mostly good, but with a very high head loss development rate (HDR). Cationic polymer caused fairly good, but inconsistent removal, with an acceptable HDR. Prolonged rapid mixing after cationic polymer addition led to better initial removal and a lower HDR;Prechlorination caused poorer removal in the absence of coagulants. Together with the metal coagulants, prechlorination caused little effect on removal and on the HDR. With cationic polymers, prechlorination led to substantially better removal, but only if cationic polymer was dosed in excess of that demand by the algal EOM. Prechlorination caused an initial period of filter ripening, a phenomenon not observed in the absence of chlorine