Simulation And Optimization Of Flow Field In Flocculation And Sedimentation Units Of Water Treatment

Actual working conditions for flocculation and sedimentation units of water treatment have a direct effect on treated water quality and costs of effluent.Also,shortage and demand-supply contradictory of water resources become more and more serious.As a consequence,it is essential that the related constructions should require optimal design and high-efficiency operation.The Computational Fluid Dynamics(CFD)technique was used to establish three dimensional models for grid flocculation tank and rectangular sedimentation tank,and then to model the flow field in each tank.The effects of the internal structures of the two tanks(for the flocculation tank,including grid plate opening ratio,spacing and water hole size;for sedimentation tank,containing perforated wall structure,effective water depth and diversion wall)on the flow-field characteristics were analyzed,respectively.Furthermore,optimizations of the two units were performed based on uniform design and orthogonal design methods.This study would be beneficial for theoretically supporting the optimizing selection of design parameters and the highly-effective operation in an operational sense.The numerical simulation results for the grid flocculation tank show that the grid plate structure and the water hole size have a significant influence on turbulence in the tank.The existence of grid plate makes water flowing past the plate grids more turbulent,generating numerous micro-scale vortex,and the lower the opening ratio of plate,the more intensive turbulence of water flow,leading to an increase in collision opportunities among suspended particles.The spacing of grid plate mainly affects the utilization rate of turbulent kinetic energy.Water hole affects the downstream flow greatly,and the hole with a smaller size produces a greater disturbance in the tank,and the greater inertia effect.Additionally,the existence of grid plate seems to further enlarge the disturbance caused by water flow and collaborated with the water hole.The numerical simulation results of the rectangular sedimentation tank show that when water flows into the sedimentation tank from the flocculation tank,both local recirculation in the vertical direction and uneven distribution of water in the tank width occurs and these phenomena are not consistent with assumptions for the ideal sedimentation-tank model,and also indicate that the flow during operation is much more complicated than that in the design of an actual settler.For the perforated wall structure,the perforation rate and the hole shape have an obvious effect on the distribution of flow while the hole arrangement affects it negligibly.The higher effective water depth will greatly increase the area of non-uniform flow and reduce the effective sedimentation volume in the pool.Although the diversion wall significantly improve uniform distribution along the tank width,limited modification may be observed when just increasing the number of the diversion wall.Finally,the structure of the perforated wall of the grid flocculation tank and the rectangular sedimentation tank was optimized by the uniform design and the orthogonal design,respectively.After the analysis on optimization,The factors influencing the hydraulic conditions of the flocculation tank and the relevant regression equations,and the significance sequence affecting removal rate and optimizing parameter combination for the perforated wall were obtained.According to the simulation results and related literature,some suggestions on optimization of the tanks investigated in this study were proposed.