Experimental Study On Coagulation And Sedimentation Process Of Plateau Water In Northwest China Characterized By Low Temperature And Low Turbidity

Due to its features, treatment of water characterized by low temperature and lowturbidity has always been important for the water treatment industry. Snow melt waterin northwest China is typical of water characterized by low temperature and lowturbidity. According to the practical problems of operation, in this paper, laboratoryscale experiments of coagulation and sedimentation process were carried out with rawwater of Xining seventh water plant as the research object. Selection of optimumcoagulant and coagulant aids were made. Optimum dynamic control parameters weredetermined. Study of breakage and regrowth of floc at the condition of lowtemperature and low turbidity was made. Based on the laboratory scale experiments,pilot scale model equipment was designed according to energy consumption per unitvolume and shear strength. Pilot scale experiments were carried out. Operation ofcoagulation and sedimentation process was analyzed. Shock load performance of pilotscale model equipment and effect of breakage and regrowth of floc on coagulation andsedimentation process were studied. Flocculation kinetics model of plateau watercharacterized by low temperature and low turbidity was set up.Through the laboratory scale experiments, PAC was determined as the bestcoagulant and the optimal dosage was45mg/L. The best kinetic parameters should be:①rotating speed of rapid stirring （mixing） was300r/min, time was0.5min;②flocculation process was divided into two levels, n₁=120r/min， t₁=1.5min； n₂＝70r/min， t₂＝8min. Energy consumption per unit volume and shear strength at bothlevels were h₁=0.418J/L、h₂=0.442J/L and F_r1=0.03、F_r2=0.01, respectively. Theoptimal dosage of coagulant aids was0.8mg/L. And it was better to dose2⁵minafter the start of flocculation. Results of study on process of breakage and regrowth offloc indicated that dosage and time for regrowth need to be improved to obtain sameprocessing efficiency with the increase of number of breakage.Through the pilot scale experiments, the best dosing working condition ofcoagulant and coagulant aid was to dose45mg/L PAC and0.6mg/L PAM at8#shaft.Repeated experiments found that residual turbidity of water after sedimentation couldbe stablely controlled below0.4NTU under the best condition. When load of upwardflow plate sedimentation tank was3.3mm/s, residual turbidity of water after sedimentation could be controlled below1NTU. By adjusting the best dosage, residualturbidity could be controlled below1NTU when turbidity of raw water ranged from2³0NTU and2NTU when ranged from30⁷5NTU, respectively. The pilot equipmenthad strong ability against water quality impact load. When turbidity of raw waterranged from2.5³.0NTU and treatment of water ranged from0.8¹.2Q, when45mg/LPAC was dosed, residual turbidity could be controlled below1.5NTU. When0.6mg/LPAM was dosed, residual turbidity could be controlled below0.5NTU. Impact test offloc breakage showed that residual turbidity increased as the breakage time went back.When breakage happened at the same time, residual turbidity increased with theimprovement of crushing strength. The influence of the crushing strength became moreand more important as the breakage time went back.The Smoluchowski basic principle was used in this paper, shear strength （Fr） wasintroduced to control flocculation. Flocculation kinetics model of plateau watercharacterized by low temperature and low turbidity was set up. E0=0.85was provedto be appropriate for this study. Simulation results of the evolution of the particle sizedistribution with time and the impact of operating variables on steady state sizedistribution were quite close to experimental results. The relative error can becontrolled in3%¹7%, which indicated the model could be effectively applied to thesimulation of flocculation units of plateau water characterized by low temperature andlow turbidity.This paper proposed the dynamic control method and best coagulant collocationfor Xining seventh water plant, mechanism analysis of breakage and regrowth of flocat the condition of low temperature and low turbidity and flocculation kinetics modelof plateau water characterized by low temperature and low turbidity. They couldprovide effective project demonstration and technical guidance for reconstruction andoperation of water plant with similar raw water quality. Promotion and application ofresearch results will provide technical support and scientific basis for treatment ofsurface water in northwest China, which promotes economic development andprogress.