| The overall purpose of this study was to characterize aggregates in terms of fractal dimension, and assess how this fractal dimension, as well as shear rate and Fe(III) coagulant dose, affect flocculation kinetics. The combination of an electronic particle counter and microscopic imaging techniques provided an accurate means of characterizing experimental flocculation particle size distribution kinetics, as well as aggregate fractal structure. Available particle contact area was assessed on a theoretical basis for numerical models ranging from a coalesced spherical through a full fractal floc assumption. Surface contact area increased by several orders of magnitude as a function of aggregate model complexity. This resulted in a corresponding increase in the number of collisions between particles.; A controlled fluid shear dominated hydrodynamic environment was created in which flocculation of algal cells was measured by the fractal dimension of flocs as a function of Fe(III) coagulant concentration dose. The resultant surface charge was measured as electrophoretic mobility (EPM). EPM increased from -1.95 {dollar}pm{dollar} 0.33 to 0.70 {dollar}pm{dollar} 0.51 {dollar}mu{dollar}m/s/V/cm as coagulant dose increased from 0 to 1.44 mg Fe{dollar}sp{lcub}3+{rcub}{dollar}/L. The EPM related to particle concentration, or dilution, as a function of coagulant dose on a log-log basis. EPM, fractal dimension, and fractal aggregate particle density all directly related to shear rate, thus supporting a fractal floc assumption. Resulting fractal dimension values were 2.23 {dollar}pm{dollar} 0.13 for the lowest velocity gradient of 2 s{dollar}sp{lcub}-1{rcub}{dollar} and 2.39 {dollar}pm{dollar} 0.24 for the highest velocity gradient of 50 s{dollar}sp{lcub}-1{rcub}{dollar}. Therefore shear rate and electrophoretic mobility can be used as flocculation potential and floc fractal dimension indicators.; Collision efficiency was determined by a generic parameter estimation algorithm using the monodispersed, polydispersed, and coalesced fractal sphere floc collision frequency functions in a completely mixed flocculation numerical model simulating the batch experimental system. Model results demonstrated the inadequacy of non-fractal formulations to characterize flocculation of low particle concentration (9-15,000/mL) systems under moderate fluid shear (2-50 s{dollar}sp{lcub}-1{rcub}{dollar}) conditions. Overall, the flocculation kernel that incorporated fractal floc geometry substantially improved model agreement with observed data over the Euclidean monodispersed and polydispersed assumptions. |
