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Questions about the effect of temperature on algal cell coagulation and the relationships between algae removal and temperature, alum dosage, paddle speed, flocculation time, and settling time are answered. The investigations consisted of two series of tests: Studies on algae cultured under laboratory control, and algae in wastewater stabilization ponds. The jar-test technique was used for the coagulation of algal suspensions by alum. Coagulation of algae is caused mainly by the insoluble flocs of aluminum hydroxide. Other aluminum species also have coagulative properties but are far less effective. The destabilization of algal colloids results from enmeshment within the hydroxide precipitates, or by adsorption of coagulant species. Neutralization of the pH-dependent charges of algal cells is not necessary for coagulation, but does aid coagulation. Analyses of variance of the variables and their interactions showed significantly the effect of all variables ad most of their interactions. Step-wise multiple regression technique was used for the development of mathematical models for the estimation of algal removal. Increased temperatures adversely affected the percentage removal of algal cells by alum coagulation. The effect of temperature on the removal of algae grown in the wastewater stabilization pond effluent was more pronounced than that found for the algae cultured in the laboratory. Alum was effective in removing algae from Logan wastewater stabilization ponds. However, high alum dosages are required which may not be justified economically. It also was found that at low concentrations of algal cells the straight line portion of the Langmuir isotherm describes the removal of algae with alum.