Optimization of the coagulation–flocculation treatment: influence of rapid mix parameters

Abstract

Turbid waters, containing suspended and colloidal particles, are normally treated by coagulation–flocculation followed by clarification; the process usually consists of the rapid dispersal of a coagulant into the raw water followed by an intense agitation commonly defined as rapid mixing. Rapid mixing is influenced by chemical and physical parameters and the final result of the whole treatment depends on this step. The study focused on the most important parameters of rapid mix design: velocity gradient and rapid mix time. Wastewaters from a tannery processing stored ovine hides were treated with aluminium(III) or iron(III) salts. Experimental results indicate that rapid mix time has a strong influence on final results; an optimal combination of rapid mix parameters was determined. The same approach was applied to highly turbid synthetic water, obtained through dissolving kaolin in distilled water and the results compared with those obtained for the tannery wastewater. It was confirmed that high turbidity removal can be achieved both with a short mixing time (about 10 s) and a long mixing time (60–90 s).