Aggregation and breakage rates in the flocculation of estuarine cohesive sediments

Abstract

A Couette device was used to flocculate estuarine sediments from the Brisbane River, Australia. The evolution of flocculation was monitored on-line using the Lasentec Focused Beam Reflectance Method (FBRM), a particle sizer based on laser reflectance. The flocculation experiments were performed under different shears and salinities to evaluate the influence of those parameters on the flocculation and breakage rates. The experimental data was used with a population balance model to evaluate the collision efficiency and the breakage rates. The population balance considered fluid shear as the only aggregation mechanism and splitting and erosion as the breakage mechanisms. The collision efficiency and breakage rates for both mechanisms were obtained by solving the population balance model for different experimental conditions. The collision efficiency was found to decrease as the shear rates increase, while the breakage rates increase with the shear. The collision efficiency ranged between 0.2 - 0.7 with a shear variation of 8.5 - 28.35 s-1. The breakage rate coefficient ranged from 1.1-2.7 cm-1s-1 in the splitting mechanism and 0.0016-0.0072 cm-1s-1 in the erosion mechanism. The experiments using different salinities demonstrated that the collision efficiency increases slightly with salinity. The breakage coefficients are independent of the salinity suggesting that the shear strength does not depend on the salinity.