A review of floc strength and breakage

Abstract

The main focus of the paper is to review current understanding of floc structure and strength. This has been done by reviewing current theoretical understanding of floc growth and breakage and an analysis of different techniques used for measuring floc strength. An overview has also been made of the general trends seen in floc strength analysis. The rate of floc formation is a balance between breakage and aggregation with flocs eventually reaching a steady-state size for a given shear rate. The steady-state floc size for a particular shear rate can, therefore, be a good indicator of floc strength. This has resulted in the development of a range of techniques to measure floc size at different applied shear levels using a combination of one or more of the following tools: light scattering and transmission; microscopy; photography; video and image analysis software. Floc strength may be simply quantified using the initial floc size for a given shear rate and the floc strength factor. More complex techniques have used theoretical modelling to determine whether flocs break by large-scale fragmentation or smaller-scale surface erosion effects, although this interpretation is open to debate. Impeller-based mixing, ultrasound and vibrating columns have all been used to provide a uniform, accurate and controllable dissipation of energy onto a floc suspension to determine floc strength. Other more recent techniques have used sensitive micromanipulators to measure the force required to break or compress individual flocs, although these techniques have been limited to the measurement of only a few hundred flocs. General trends emerge showing that smaller flocs tend to have greater strength than larger flocs, whilst the use of polymer seems to give increased strength to only some types of floc. Finally, a comparison of the strength of different types of floc (activated sludge flocs, organic matter flocs, sweep flocs and charge neutralised flocs) has been made highlighting differences in relative floc strength.

125th Anniversary Review: Yeast Flocculation and Sedimentation in Brewing

“…Loosening of cells from the flocs occurs because of hydrodynamic forces. This results in a dynamic equilibrium similar to the equilibria of ordinary chemical reactions 29,85 . Thus, dilution favours an increased proportion of free cells, which is why a sufficient concentration of yeast cells is needed for effective flocculation and one reason why some cells remain in suspension even after bulk sedimentation.…”

Characterization of floc size, strength and structure under various coagulation mechanisms

“…However floc strength is harder to characterize and there is, as yet, no straightforward technique to characterize the floc strength experimentally. Generally, there are two modes of floc rupture [18], surface erosion and large-scale fragmentation [19,20]. Researchers [20][21][22] found a correlation between floc size and the strength for a given rate of shear condition.…”

“…Linearisation of the equation allows values of γ and log C to be found from the a log-log plot of floc size measurement against the average velocity gradients [19]:…”

Characterization of kaolin flocs formed by polyacrylamide as flocculation aids

“…In the previous research, characteristics of flocs formed in the coagulation process by metal salts have been well understood (Jarvis et al, 2005a;Francois, 1987;Spicer and Pratsinis, 1996;Chakraborti et al, 2000). The accurate estimation of flocs characteristics is valuable for evaluating water treatment performance under certain coagulation conditions.…”