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

Abstract

a b s t r a c tThe 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 40 rpm to 60 rpm > stable 60 rpm shear rate > gradually decreased shear rate from 60 rpm to 40 rpm > stable 40 rpm shear rate. Flocs formed under gradual shear increasing could easily re-flocculate and recover their strength after breakage.

Influence of flocs breakage process on membrane fouling in coagulation/ultrafiltration process—Effect of additional coagulant of poly-aluminum chloride and polyacrylamide

“…The large flocs will occur to break because the internal bonds of flocs are destroyed by the specific stirring condition. Based on the process of floc breakage and re-growth, the surface properties of flocs might be changed, exposure some inner cores hidden in these aggregates [51]. Fractal dimension (d pf ) of flocs, is an important parameter, owing to its effects on floc compaction and effective density [52].…”

A Comparison of Fractal Dimensions of Clay and Humic Acid Flocs under Optimum Coagulation Conditions

“…The colloidal aggregates fit in the small pores in the flocs. As a result the flocs formed are more solid, tightly packed and having higher fractal dimensions [16]. Fig.…”

Size and structure of Chlorella zofingiensis/FeCl3 flocs in a shear flow

“…Many studies using both experimental and modeling techniques have been published which describe the influence of shear flow on the fractal dimension of aggregated colloidal particles (Selomulya et al, ; Serra and Casamitjana, ; Serra et al, ; Soos et al, ; Spicer and Pratsinis, ; Spicer et al, ; Wang et al, ). In one such study, Spicer and Pratsinis () experimentally studied the evolution of floc structure with time of 1 µm diameter polystyrene particles flocculated with alum.…”

“…Flocs of the smaller particles were more prone to restructuring when compared with those made up of larger primary particles particularly at low to moderate shear rates. A dimensional analysis revealed a correlation among the particle properties (size, concentration, surface chemistry) and the steady‐state aggregate properties (size and structure), showing that “larger or less dense flocs correspond to a stronger binding force or a lower level of applied shear.” Wang et al () further examined floc strength by measuring the structure of alum‐kaolin flocs with small angle laser light scattering at various agitation rates and they concluded that strong, compact flocs play a key role in designing effective flocculation reactors and efficient water treatment.…”