The effects of ozone and chlorine on algae were examined with respect to cell surface characteristics, lysis and coagulation ability. Two algae were studied: a green alga (Scenedesmus quadricauda) and a diatom (Cyclotella sp.). Cell properties were characterized using scanning electron micrographs, particle size distributions and electrophoretic mobility measurements. Jar tests were used to evaluate the coagulation of the algal suspensions with a polyaluminium chloride (PACl). The results showed that changes in the characteristics of the algal cells from ozone or chlorine yielded an improvement in removal of Scenedesmus through a combination of lysis and improved coagulation ability with PACl. Cyclotella removal was not enhanced by preoxidation. Additionally, preoxidation increased the organic carbon concentration of the settled water, which could lead to increased tastes and odours and production of disinfection by-products.

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