Abstract
In this work, the effect of flow fields on flocculation efficiency in a kaolin suspension using microbial flocculant GA1 (MBFGA1) was studied. The flow fields were controlled via mechanical mixing of a Rushton turbine in a fully baffled flocculation reactor and simulated by a three-dimensional Computational Fluid Dynamic (CFD) model. Good agreement between experimental and simulated results confirmed the validity and applicability of the CFD model. By integrating flocculation tests with CFD simulations, it was shown that the impeller with different speeds generated different flow fields, and hence offered different efficiencies for flocculation. Flow fields of rapid mixing at 400 rpm then slow mixing at 80 rpm, which formed the largest flocs (538 +/- 18 mm), achieved the optimum flocculation efficiency, i.e. the lowest residual turbidity (3.03 +/- 0.10 NTU) and the maximum flocculation rate (98.2 +/- 0.8%). The two-loop flow pattern associated with the distribution of velocity magnitude, local shear rate and turbulent kinetic energy provided an improved understanding of flow behaviors within the reactor. Additionally, charge neutralization and adsorption bridging were proposed as the main flocculation mechanisms of MBFGA1.