Simulation And Optimization Of Flow Field In An Oxidation Ditch Drived With Inverse Cone Surface Aerator

Oxidation ditch is an important process in the sewage treatment process, the selection of hydraulic parameters of the oxidation ditch is mainly based on the design standards, design experience or semi-empirical formula, without adequacy theoretical basis. The aerator which is chosen according to the oxygen demand and performance information of the aerator can satisfy the oxygen demand, while can not ensure the sludge staying in a state of suspension all the time. The sludge deposition phenomena in the actual operation affect the normal running of the oxidation ditch and result in water quality deterioration.In this paper, three-dimensional numerical simulation was performed using the Fluent software. Both the flow field and sludge distribution profile in the oxidation ditch were investigated, which provided a reference for the design and operation of the oxidation ditch through the simulation analysis of the actual case.First of all, the experimental data measured by FAN Long was used to evaluate the model applied in the oxidation ditch. The RNG κ-ε model was used to simulate the turbulent flow and the Mixture model was used to simulate the separation of the sludge solid and water. Besides, the rotation of the aerator was simulated by setting Moving Reference Frame. It indicated that the experimental results agreed well with the simulation results, and this proved that the model employed in this study is feasible.Secondly, taking the aerobic zone of a full-size oxidation ditch as research object, numerical simulation was performed by the use of the validated model. By analyzing the flow field and sludge distribution characters of each groove, the zone where the velocity was small and the sludge was tend to deposit were obtained. The simulation results showed that counterclockwise whirlpool occurred in the cross-sectional of the straight segment downstream of the aerator. Sludge volume fraction appeared large in these two zones:the bottom of the inner channel which was in downstream of the partition wall and outer channel which was far away from the aerator upstream.The optimization measures were put forward according to the character of flow field and sludge distribution at last.(1) Changing the local structure:including setting the baffles in the corners and adding inclined baffle in downstream of the aerator. The simulation results indicated that the optimum radius, eccentric distance, and downstream extension length were2300,150, and1000mm, respectively, when setting the baffles in the corners. The effect of inclined baffle on the flow pattern of the downstream groove of the aerator was discussed by changing the inclination. The best inclined degree of the baffle was30°.(2) Changing operating parameters of the aerators, i.e. the rotation speed and the immersion length. The energy consumption and oxygen-transfer efficiency were analyzed with the flow pattern simultaneously. It suggested that increasing the the rotation speed and the immersion length properly optimized the flow pattern in the oxidation ditch.(3) Installing flow propeller under the water. The addition of the flow propeller far away from the aerator increased the flow velocity downstream the oxidation ditch, preventing the sludge settling.