Coagulant Dosage Control by Colloid
Titration Technique
SUSUMU KAWAMURA, Research Assistant
GEORGE P. HANNA, JR., Associate Professor
Civil Engineering Department
The Ohio State University
Columbus, Ohio
INTRODUCTION
Interest in the study of the mechanism of the colloid flocculation in water has
been limited primarily to colloid chemists, agronomists, and sanitary engineers.
In general, flocculation is achieved by the addition of a polyvalent salt, 01 which
alum is one of the most effective and economical for the removal of turbidity and
color from water. Both the cost and effectiveness of the overall treatment process
depend on the determination of the proper coagulant dosages.
At the present time, electrophoretic mobility studies are considered by many
investigators (1,2,3,4,5,6,7,8,9) to be the most satisfactory means of determining coagulant dosages. The subsequent calculation of zeta potentials from electrophoretic mobility measurements has also been used in this connection. However, the phenomenon of effective flocculation is very complex, being a function
not only of the electrokinetic characteristics of a system, but also of its chemical
and physical properties and is, at this time, not completely understood.
Most filtration plants still rely on the jar test for coagulant dosage control.
Dosage control through means of zeta potential measurement (2,3,4) has not been
used extensively, mainly because of the difficulties encountered in the measurement procedure. Since zeta potential is one of the determining factors in colloid
stability, it follows that it is an important factor in the controlled flocculation of
a colloid system. Zeta potential, however, must be calculated from the migration velocity of the colloid particles in an electrical field, which depends on many
factors, including the nature, shape, and size of the particles. Since wastes with
naturally occurring turbidity and color due to organic constituents are characterized by a variety of types of colloidal particles, each type being present indifferent size distributions, the calculation of electrophoretic mobility or electrophoresis requires the measurement of an "average" migration velocity. An accurate measurement of this type is not easy, and requires skill, experience, time,
and the use of specialized apparatus. Moreover, it is impossible to determine the
migration velocity for most color colloid particles by the usual apparatus because
of their submicroscopic size.
Preliminary experimental work has indicated the feasibility of a colloidal
titration technique tor flocculation control. Continuous studies have shown that
the technique appears to offer promise as a reliable method for the measurement
of charge characteristics of colloids, determination of the isoelectric point of the
systems, and for determination of effective alum dosage on flocculation treatment
of the colloid system. In this capacity, it is suggested as a replacement for electrophoretic mobility measurement or zeta potential as an applicable means of
coagulant dosage control.
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