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In this study, the zeta potential of Microcystis aeruginosa (cyanobacteria), Synechococcus sp. (picocyanobacteria) and Chlorella vulgaris (algae) was investigated in order to determine the zeta potential range for optimum cell removal. Algae and cyanobacteria species were treated by coagulation-sedimentation using 0-500 mg/l polyaluminum chloride (PACl). Analyses included zeta potential measurement, cell counts and turbidity removal. The role of pH on the zeta potential also investigated in this study. The pH of coagulation system was adjusted to pH 6.5 and 7.0. At pH 6.5, the optimum zeta potential bands were between -2.1 mV and +20.45 mV for Microcystis aeruginosa, +3.45 mV and +8.71 mV for Chlorella vulgaris and +7.41 mV and +13.33 mV for Synechococcus sp. The ranges were much narrower at pH 7 than at pH 6.5. The cell removal efficiencies were 98.9%, 90.6% and 55.7% for Microcystis aeruginosa, Chlorella vulgaris and Synechococcus sp, respectively. The implications of such findings are that the charge measurement can be used for controlling coagulation of algae and cyanobacteria. In addition, the type of cell species in the system was shown to a significant factor in the coagu- lation performance. [ABSTRACT FROM AUTHOR]