Experimental Research On The Coagulation For Reclaimed Water Based On Fractal Theory

As a pretreatment process in reclaimed water treatment, the effect of coagulationand sedimentation always has a great impact on the subsequent processes. Nowadays,with the rapid development of computer technology and increasingly improveddetection methods, the application of the fractal theory on the study of coagulationprocess has gradually attracted extensive attention in this field and has become afrontier hot spot, which gives us new opportunity to study on the coagulationmechanism and parameters optimization.In this paper, the flooding water of a reclaimed water plant in Tianjin was usedas raw water. Combined with small experiment and pilot scale test, the microscopiccamera device was used to shoot the morphological characteristics of the flocs and theimage processing software was used to analyze the fractal dimension, the followingexperiments have been researched.（1）Through the single-factor experiments, the influences of PolyaluminiumChloride （PAC） dosage, pH and the former chlorine concentration on the flocs fractaldimension were analyzed. The results show that the best dosing quantity of PAC is12mg/L, the optimal range of PH is6.5⁷.5, and the best former chlorine concentrationis4mg/L. By image analysis, it is known that the lnA and lnL of flocs have asignificant linear relationship, indicating that the PAC flocs is a typical fractal systemwhich has a fractal characteristic; besides, it is also found that there is an obviouscorrelation between the flocs fractal dimension and the effluent turbidity.（2）Through the orthogonal test on the pilot system, the influences of mixingstirring intensity and4flocculation stirring intensity on the flocs fractal dimensionwere analyzed. The results show that the best flocculation stirring intensitycombination is: mixed stirring G value is350s^-1, G values of the4flocculation stirringare57.5s^-1,47.5s^-1,25s^-1,15s^-1respectively. Time for mixed mixing is1minute,flocculation stirring time is15minutes, it is concluded that GT value of mixing stageis2.1×10⁴, average G value of flocculation stage is40s^-1, the total GT value is3.6×10⁴.The5factors’ influences comply with the following order: the mixed stirringintensity> the4th stirring intensity> the other3stirring intensities. It’s also found that there is an obvious correlation between the flocs fractal dimension and theturbidity removal rate.（3）The dynamic flocculation process monitoring shows that the flocs fractaldimension gradually reduces and the average particle size increases gradually in the4flocculation pools. As flocculation time increases, the flocs fractal dimensiondecreases from2.1645to1.7228, presenting a decreasing trend as flocculation timeextension, and it tends to be relatively stable after12minutes. Showing an oppositetend comparing with fractal dimension, the flocs average particle size growscontinually in the coagulation process from μm to μm the growth trend is verysignificant, which is consistent with macro-floc morphological changes.（4）Used pilot system experimental data, a mathematical model about flocsfractal dimension and the raw water turbidity, pH, PAC dosing quantity and theformer chlorine concentration was established. Significant analysis and validationexperiments indicate that the mathematical model is correct and credible.