Electrophoretic Studies of Turbidity Removal by Coagulation With Aluminum Sulfate

One important factor affecting coagulation is the negative zeta potential, often referred to as the charge, that is associated with natural clay particles suspended in water. This article discusses a study of the zeta potential of clay particles that were found to be dependent on the pH and on the alum dosage. An amount of alum equivalent to several times the base‐exchange capacity of the clay suspension was required to neutralize the particle charge. Clarification was best in the range pH 7.5 – 8.5 where the particles were negative, rather than at pH values where the particle charge had been neutralized. Fair coagulation was often obtained below pH 4.5 where the particles were nearly neutral. In many instances, residual turbidities changed sharply without any accompanying change in mobility values. The eight coagulant aids used with alum all could be made to improve or inhibit coagulation by selection of dosages of alum and aid. As in the coagulation at different pH values, final particle charges could not be correlated with residual turbidities.