Laboratory study of ballasted flocculation

Ballasted flocculation is applied successfully in the water treatment industry, but the concept has not yet been subject to extended experimental studies. The use of a microsand and a polymer together to increase the weight of the flocs and the rate at which they settle is radically changing the coagulation flocculation-settling methodology. The objective of this research is to study ballasted flocculation in the laboratory by means of a modified jar-test procedure. The first tests, conducted using a statistical approach, enabled us to identify those parameters that have a significant influence on the quality of settled water. The principal treatment parameters considered in this study were the chemical dosage, the contact times in the different basins, the pH of coagulation and the dosage of microsand. The parameters used to evaluate the quality of the settled water, and hence treatment performances, were turbidity, UV absorbance, TOC and membrane filterability. The results showed that the coagulant dosage and the coagulation pH have a preponderant effect on the variability of the quality of the settled water. By contrast, filterability as measured by the laboratory filter tests, was found to be more sensitive to coagulant and microsand dosages. It was dependent on the ratio of microsand to polymer dosage, and improved when this ratio increased.