The objective of a conventional treatment plant is to produce floc of such size and density that the major portion will settle out in the allowed settling time, and the amount of remaining turbidity will be removed by the filter. Strength of the floc is one factor that affects the success with which suspended solids are removed from the water in the various unit operations. This paper reviews some of the factors that may affect floc strength, and describes a method for measuring the relative strength of floc formed under various conditions.

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.

When the removal of suspended solids from unsoftened surface waters is the primary objective of treatment, coagulation is all‐important. The efficiency of the plant and the quality of the treated water depend upon careful control of the process. Type and frequency of tests for controlling coagulation depend largely upon the character of the raw water and the plant design. Some parameters may be evaluated in the laboratory, while others need monitoring of the treatment equipment or the treated water at various stages of treatment. Every operator develops his favorite monitoring and control techniques that allow him to produce a satisfactory product most of the time. This article compares some of these techniques and evaluates their general applications for research and plant control.

This paper discusses some of the optical properties of suspended matter that affect turbidity measurements and suggests procedures for obtaining more representative measurements of relatively low‐turbidity waters.