Effect of impeller clearance on floc growth behaviors in a baffled square stirred-tank reactor: Flocculation-test and CFD-aided studies

The impeller clearance (C) and shear rate (Gave) effects on aggregation and breakage behaviors in the whole process of floc growth were investigated by applying lab-scale flocculation tests with the aid of computational fluid dynamics (CFD) simulations. For a baffled square stirred-tank flocculating reactor, the recirculating times for water flow to the impeller region and intensity of turbulence occurring within this region, together with the non-uniform nature of turbulence for water flow above the rotating impeller, were found to be more or less different in all C-value cases at a constant Gave, thereby affecting the development of floc morphology. Besides, the importance of impeller clearance (C) to floc morphological evolution seemed to be largely determined by predominant growth behavior(s), likely as a result of distinct occurring degrees of either aggregation or breakage for each combination of C and Gave; and interestingly, a certain compaction for the above effect took place at a relatively high shear, possibly attributed to increasingly pronounced occurrence of floc breakage and irreversibility of this behavior. To produce fractal flocs with favorable morphological characteristics, the impeller should be placed at an appropriate distance from the reactor bottom within the flocculating system, under whether aggregation-dominated or breakage-dominated circumstances, for weakening the role of reactor bottom and water surface on floc growth behaviors during flocculation. This research would be beneficial for thoroughly understanding floc formation mechanisms and optimally designing impeller-based flocculating reactors.