Combined effect of inertial and differential transport on aggregates after ceased mechanized flocculation

Abstract

The velocity gradient can be controlled for aggregates formation during mechanized flocculation, so that aggregates can be subsequently removed in solid/liquid separation units, providing operational flexibility to the system. Batch assays help in finding the optimal coagulation and flocculation conditions, making possible to change gradient velocities and hydraulic detention time. Nevertheless, the secondary effects as result of inertial and differential sedimentation transport over aggregates after mechanized flocculation are not well known. Therefore, this paper aimed to present the combined effect of inertial and differential transport over aggregates, using nonintrusive image technique and particle image velocimetry (PIV). It was possible to verify that the transport of the aggregates formed after the flocculation step is due to the action of the inertial movement of the fluid and differentiated sedimentation, in a proportion that varies over time. The inertial movement is predominant in the influence of the formation of aggregates from the moment when mechanical agitation is stopped (Gf of 20 s-1) until 5.5 ± 0.5 minutes, when the aggregates are predominantly modified by differential sedimentation. Through the analysis of the aggregates, two moments of growth in their length were observed. The first occured up to the two first minutes, and the other between four and six minutes, when the transition between mechanisms is likely to happen.