Abstract: A 3D two-phase flow model for a radial secondary sedimentation tank was developed by employing the high Reynolds number -equation. The momentum transfer term includes the drag, virtual mass and lift forces. The volume fraction and density effects of suspended solids were taken into account. The simulation results with the tank configuration variant show that the tangent velocity is nonuniform and is influenced by the tank bottom, sidewall, baffle and weir. It significantly differs with the assumption velocity of an ideal sediment basin. The 3D model is a better one to simulate the distribution of flow field and SS. The peripheral weir gets unsatisfied results. About the 2m away from the margin, both the inboard weir and twin weir make the property of the outflow markedly improved. By comparison with the experimental data of Deininger et al, the flow field predicted by the model indicates a good agreement with a real one.