Determination of conventional velocity gradient (G) using CFD technique for a pilot-scale flocculation system

Achieving uniform mixing conditions are essential for the flocculation process to optimize floc size and avoid floc-breakup. Limited literature is available on establishing consistent operational conditions and procedures for pilot-scale flocculation systems, which have tank sizes smaller than full-scale and larger than jar-test equipment. In this study, the influence of mixing speeds on the determination of the conventional design parameter, the average velocity gradient (G), was investigated for pilot-scale paddle flocculators. The pilot-scale plant for this paper was hosted at the J.D. Kline Water Supply Plant (JDKWSP) in Halifax, Canada. Computational fluid dynamics (CFD) was evaluated as an alternative design technique and compared against traditionally used empirical-based calculations. Comparison of both approaches showed that the G-values of empirical method were substantially higher than the predicted values for rotational speeds greater than 5 rpm. In contrast, CFD predictions found that G-values used for tapered paddle flocculation process (up to 60 s⁻¹) could be achieved at lower rotational speed (around 15 rpm), which minimizes the power input required for mixing. The practical implications of operating at higher than required G-values relates to potential negative consequences such as floc break up, and the reliance of chemical additives to avoid floc break-up. These very practical outcomes could impact the interpretation of findings from pilot-scale treatment systems.