Investigation of the Influence of Operating Parameters on the Settling Performance of a Vertical Sedimentation Tank Through Computational Fluid Dynamics Simulations

The operating conditions of the sedimentation tanks are very critical for wastewater treatment. The effects of operating parameters (inlet velocity, inlet sludge concentration and reflux ratio) on the settling performance of a vertical sedimentation tank (VST) are investigated using Mixture and RNG k-epsilon numerical simulation methods. In order to verify the suitability and accuracy of the selected numerical model, the experimental values are compared with the simulated values and the results show that the average relative error is 8.63%. That means the selected numerical model are reliable for describing the flow behavior of the VST. The simulation results indicate that the inlet velocity, inlet sludge concentration and reflux ratio have significant influences on the settling performance of the VST. When the inlet velocity and the inlet sludge concentration are kept at the low level, the sludge has enough time to settle down. However, the sludge easily flows out from the tank since the inlet velocity and the inlet sludge concentration are sharply increased. Meanwhile, the reflux ratio should be kept at the appropriate level to prevent the suspending sludge. Therefore, it is recommended to maintain the inlet velocity of below 0.3 mm/s, an inlet sludge concentration of below 2056 mg/L, and a reflux ratio of below 80% to ensure the effluent quality meets the Chinese discharge standards. In the future, the recommended operating conditions can be applied for large-scale vertical secondary sedimentation tanks without flocculation.