Using the flocculation index to optimise velocity gradient during slow mixing in drinking water treatment

Abstract

This article aimed to study the influence of the velocity gradient on the flocculation process by aluminum sulfate (AS) in raw water samples under the action of a sweep mechanism at a temperature of 20°C and, with the help of continuous flocculation monitoring equipment (CFME), to verify the potential use of the flocculation index (FI) as a way to choose an adequate velocity gradient to achieve higher sedimentation rates and to obtain kinetic flocculation aggregation (K_A) and breakup constant (K_B) data. K_A and K_B helped to explain why the best tapered velocity gradient (G) conditions (G_{0 − 5 min} = 80 s^− 1, G_{5 − 10 min} = 50 s^− 1 and G_{10 − 15} min = 20 s^− 1: 83.4%) promoted greater turbidity removal efficiency than did the fixed gradient (G_{0 − 15 min} = 20 s^− 1: 78.6%), highlighting the compartmentalization benefits. It was also observed that floc size was the most relevant factor for selecting velocity gradients that generated greater removal efficiencies for turbidity and apparent color. Finally, the model developed through kinetic constants was applied to water treatment plant conditions, and the mean absolute errors were 1.5% and 0.1 s^− 1, considering the turbidity removal efficiencies and the best estimated velocity gradients, respectively, allowing us to improve the quality of the treated waters.