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Characterization of floc structure and strength: Role of changing shear rates under various coagulation mechanisms

Wang, Dongsheng, Wu, Ruobing, Jiang, Yunzhong, Chow, Christopher W.K.
Colloids and surfaces 2011 v.379 no.1-3 pp. 36-42
agitation, coagulation, colloids, fractal dimensions
The floc formation, structure and strength remain a problematic topic to be addressed clearly to guide the flocculator design. To study the relationship among floc structure, strength, size distribution, and fractal dimensions, the alum–kaolin flocs formed under different coagulation stages and mechanisms were investigated at various agitation rates. A new method based on the fractal theory was employed to analyze the strength of flocs. The relationship between fractal dimensions and floc size distribution was then discussed with controlled experiments, providing comparison between calculation results of the new method and the strength factor as well. The results show that both size and fractal dimension of flocs decrease with increasing shear rate. The floc strength follows the hierarchy: gradually increased shear rate from 40rpm to 60rpm>stable 60rpm shear rate>gradually decreased shear rate from 60rpm to 40rpm>stable 40rpm shear rate. Flocs formed under gradual shear increasing could easily re-flocculate and recover their strength after breakage.