Effect of Addition Sequence on Dual‐Coagulant Performance

Abstract

This research investigated the effect of addition sequence of cationic polymers in combination with inorganic coagulants on flocculation kinetics. A photometric dispersion analyzer (PDA) was used as the primary device to study the flocculation kinetics of kaolin clay suspensions at different concentrations and under different experimental conditions in a batch reactor. An automatic image analyzer was used in a limited number of experiments to reinforce the results obtained by the PDA. Zeta potential and residual turbidities were also measured. Results indicated that when cationic polymers are to be used with ferric inorganic coagulants, they should be added simultaneously or at the same place in water treatment plants. Simultaneous addition of coagulants produced higher flocculation index values at the end of rapid‐mixing time, suggesting a higher rate of particle growth, as well as higher resistance to shear induced by extended rapid‐mixing periods. This affected the entire flocculation period and produced lower residual turbidity values. The authors hypothesized that the improved performance produced by simultaneous addition is attributable to the matrix of the first flocculi formed in the presence of both the cationic polymer and ferric hydrolysis products.