Numerical Simulation And Experimental Study Of Flow Pattern In Mechanical Flocculating Tube Settler

High density sedimentation process was a new water treatment technology with combination of flocculation, precipitation, and sludge concentration. It was widely applied because of the advantages such as small floor area and high deposition efficiency. At present, there were fifty waterworks using high efficiency settling process, with the total water supply volume exceeding15million m3/d. However, most high density sedimentation processes in China which had been built were designed by using the form of high density sedimentation tanks in other countries, and the experience of mechanical stirring-flocculation and tube settling for reference. Now there was no corresponding standard to guide the design of the new high rate settler, sedimentation tank design was paid attention to process design, in which hydraulic condition of flocculation and precipitation were hardly considered.In this study, the mechanical flocculation and inclined-tube settler in Yangzhou first waterworks was taken as a research object. On the one hand, I used FLUENT software to carry on the simulation of particle concentration and flow field of its precipitation zone, and then put forward some advanced measures. On the other hand, a pilot study was conducted to optimize its operation parameters. The conclusions as follow:(1) Based on the simulation of particle concentration field of precipitation zone, the simulation results showed that exorbitant particles density could lead to density current worse and sedimentation efficiency would be affected if particles density was too small, so the optimum of particle density was1050kg/m3.(2) Carrying out optimization simulation for the different height of water inlets and adding a water distribution trough before precipitation zone, and the results showed that when the height of water inlets was1.2m, namely the flooding velocity was0.024m/s, flow field distribution was more uniform; adding a water distribution trough before settling zone was beneficial to particle settling and sludge thickening, through making the influent uniform distribution along direction of the width of sedimentation tank and improving the stability of current.(3) A pilot experiment with the influent flux2000m3/d was researched and developed. Then, the operation parameters such as dosages of coagulants and coagulant aids, sludge return ratio were studied to optimize its performance by jar tests and pilot study. Finally, the optimal operation parameters under water source quality could be researched. The results indicated that when the dosage of PAC was30mg/L, the dosage of PAM was0.1mg/L, and the sludge return ratio was4%, effluent quality was best, effluent turbidity was basically lower than1NTU; after continuous and steady operation of mechanical flocculation and inclined-tube settler for some time, the dosage of coagulants could be reduced appropriately, the effluent could still reach the expected effect.