PIV characterisation of flocculation dynamics and floc structure in water treatment

Abstract

1Particle flocculation with chemical flocculant addition is an essential step in water treatment. 2The performance of flocculation and the property of the flocs formed affect the overall results 3 of the treatment process. In addition to particulate impurities, the presence of organic matter 4 in water, such as natural organic materials (NOM), also influence the effectiveness of 5 chemical flocculation. In this paper, the PIV system was employed to investigate the 6 flocculation dynamics for different flocculants in different model waters. With the PIV and 7 image analysis, the change in particle size distribution could be well recorded. Using the 8 sequence of flocculation, shear breakage and re-flocculation on a jar-test device together with 9 the PIV system, the rate of floc formation, the strength of the flocs, the recovery of broken 10 flocs, and the morphological and structural features of the flocs were characterized. The 11 results indicated that the adsorption of HA on the particle will stabilized the particles, hence 12 hindered the flocculation process. Sweep flocculation using a higher chemical coagulant 13 dosage was an effective means of process enhancement for the removal of particulates and 14 associated organic matter. The dynamics of A-B-R process was characterized by particle size 15 distribution (PSD) measurement with PIV setup. The particle strength and reversibility 16 capability were examined. Strength index showed the HA flocs have comparable strength, 17 while recovery index indicated a less recovery capability with the increasing of HA 18 concentration after exposure to a higher shear, especially for ferric HA flocs. It appears that 19 the bonds holding HA flocs together are not purely physical bonds given the limited regrowth 20 seen. Finally, evolution of floc structure during A-B-R process was analyzed by investigated 21 the fractal dimension D b . The results were generally consistent with previous PSD 22 measurements. It suggested that the structure of flocs in breakage became more compact with 23 little permeability. An increase in floc compaction provides a further explanation for the 24 limited regrowth for most of flocs. According to the performances of alum and ferric, it can 25 3 be noticed that HA flocs have different properties dependent on which chemical coagulant is 26 used. Alum produced larger HA flocs which endured a higher recovery capability after 27 exploring higher shear, hence, compared to ferric, it could be preferred to using in the 28 practical enhanced coagulation unit. 29 30 Keywords: Aggregation-breakage-re-flocculation; humic acid; hydrodynamics; particle image 31 velocimetry (PIV). 32 33The irreversibility of aggregates during cycled shear is most likely the result of 55 particle-flocculant bond breakage during fragmentation, hence the reorganisation and 56 restructuring can both occur. Experimental shear-induced coagulation-fragmentation 57 processes [11][12][13][14] found that the fractal dimension shifts to a larger value compared to the 58 5 initial one...

Modeling particle-size distribution dynamics in a shear-induced breakage process with an improved breakage kernel: Importance of the internal bonds

“…Based on the analysis (Fig. 4), the fractal dimension of the latex particle aggregate was 2.2, which was consistent with previous reports [7,11,28,30,31]. Activated sludge flocs had a fractal dimension of 2.5, which also was in agreement with the values reported by others [29,[32][33][34].…”

Investigation of the hydrodynamic behavior of diatom aggregates using particle image velocimetry

“…This suggests that the available curvilinear model may underestimate the collision potential between particles. Compared with our previous PIV study (Xiao et al, 2007(Xiao et al, , 2011, the reduction factors of diatom aggregates were more than two orders of magnitude higher than those of similarly-sized solid falling spheres, but close to those of aggregate flocs. It appears that the porous-fractal structure of the aggregates could permit flow through the aggregate interior.…”

“…It only showed the trajectories of the buoyant bubbles. In our previous study (Xiao et al, 2007(Xiao et al, , 2011, an advanced visualization technique PIV was successful in studying the creeping flow by analyzing the movements of tracers related to falling solid spheres and particle aggregates of microspheres. Here, based on this technique, the flow pattern, hydrodynamic profile and streamlines of diatom aggregates were also successfully obtained.…”

“…To date, PIV has not been widely applied to study flow related to water treatment processes. Most studies have only utilized this technology to identify the local velocity gradient during flocculation process (Cheng et al, 1997;Coufort et al, 2005;Xiao et al, 2011;Zhong et al, 2011aZhong et al, , 2011b.…”

Survey of treatment process in water treatment plant and the characteristics of flocs formed by two new coagulants

“…These on-line values were monitored, recorded and controlled electronically. The strength factor (S f ) and recovery factor (R f ) are calculated as follows [18]:…”