The step of coagulation-flocculation plays a dominant role in the drinking water treatment process. The goal of this step is to agglomerate suspended particles and make easily settling composites called "flocs". The first step, coagulation, consists of the destabilization of colloidal particles upon the addition of certain chemicals, such as aluminum or iron salt, which react with the water in stirred tanks. This critical mixing stage is strongly influenced by the turbulence intensity. The particles are then flocculated in more slowly stirred tanks. The rate of floc formation is directly proportional to the velocity gradients in the stirred tanks. Floc aggregation and breakup are determined by the local shear stresses. FLUENT has been useful in simulations of the stirred tanks, providing results for the flow field, impeller performance, and RTD. The results allow researchers to determine the optimum conditions for the coagulation-flocculation step through the testing of various configurations and operating conditions.

Flow path lines in a flocculation tank
Courtesy of Veolia Environnement