Scale-variability Of Flocs During Coagulation Process And Its Fractal Simulation

It is the key to ensure coagulation effect that forming larger flocs with compactstructure which solid-liquid separation can be easily achieved by optimizing andcontrolling the coagulation process. Therefore, the dynamic evolution law andinfluencing factors of floc structure as well as the3d DLA-DLCA analogue simulationwere discussed under the support of the National Natural Science Foundation of China.Based on the previous research results, the conditions with pH equals5and7werechosen in this thesis （pH=5is the optimal coagulation pH of humic acid; pH=7is for theconventional water treatment） to study the dynamic evolution law of flocs during humicacid coagulation process. The results proved that the fractal characteristics of flocs existin both of the two conditions. When pH=5, the2d and3d fractal dimension of flocsdecreased as the floc itself growing bigger, then the fractal dimension remained stablebasically, while the free sinking speed shows a rule of firstly increases then decreasesand ultimately remained unchanged as time prolongs. When pH=7, the2d fractaldimension of flocs also decreases as the floc size increasing while the3d fractaldimension and the free sinking speed shows the same variation process of firstlyincreases then decreases and ultimately remained unchanged basically, which shows agood correlationship. This result indicated that the natural organic materials representedby humic acid have a characteristic of scale-variability during the coagulation, whichmeans when the particle size increases its fractal dimension changes either.Based on the fractal theory, one of the characteristics is the scale invariance, whichmeans its fractal dimension stay stable with the variation of fractal size. To reveal thescale variance mechanism of flocs during coagulation, the iron powder particles withdiameter of60μm were chosen as the research object, and the structure characteristics of iron aggregates were calculated and analyzed. The results indicate that the randomlyformed fractal objects show an obvious fractal characteristic, which is to say the fractaldimension of the aggregate remains unchanged when the scale varies. Comparing theaggregates formed by natural force and the hydraulic shear condition which representedby iron powder and humic acid respectively, it can be found that the main reasoncausing the scale variability of flocs during coagulation process was the hydraulic shearforce disturbance, which destroyed the natural formation process causing by randomcollision combination. Therefore, the growing mechanism and structure characteristicsof flocs was changed, which means as long as the external force disturbance exists,scale invariance is difficult to maintain in the formation of fractal objects.In this thesis, the influences of hydraulic shear force conditions and ion conditionupon coagulation process as well as the floc structure were considered. The results showthat hydraulic agitation conditions have an obvious impact on natural organicscoagulation. Initial mixing conditions are in a core position, which play a crucial role onthe rapid mixing diffusion of coagulant and the densification of initial flocs. While slowstirring intensity mainly affects the collision and combination rate of initial flocs as wellas the morphological characteristics. Anion such as SO₄^2-and SiO₃^2-in water has anegative effect on floc structure. However, high valent cation represented by Ca²⁺andMg²⁺has a positive effect on the formation of compact and bigger flocs.Meanwhile, through the analyzing and discussing on the mechanic of humic acidfloc growth, it can be found that the process of humic acid floc formation is inaccordance with DLA-DLCA two phase aggregation model, and based on which the3dfractal simulation of flocs was conducted. The result shows the3d fractal dimension ofthe simulation floc has a relation with the proportion of the two growing phase.Therefore, the proportion of DLA and DLCA phase can be changed by controllingsimulation parameters, and then to achieve the goal of controlling the floccharacteristics. The experimental results also indicate the simulating flocs have a highmorphological similarity with the real floc of humic acid.