A numerical model for an idealized circular tank with no baffle is used to investigate the relative importance of inlet solids concentration and inlet flow on the removal of solids field. Effluent concentration is very sensitive to the velocities in the withdrawal zone. The densimetric Froude number relates hydraulic and solids loading. It was found that the upward velocities in the withdrawal zone increase with decreasing densimetric Froude number for a constant discharge. Under constant solids loading there is an optimum densimetric Froude number for minimum effluent concentration. The performance of the clarifier is a function of the densimetric Froude number and the return activated sludge ratio. For the clarifier considered here, the minimum effluent concentration for a constant solids loading, occurred at a densimetric Froude number of about 0.5. The Reynolds number, when in the turbulent regime, did not significantly affect the tank hydrodynamics. The predicted solids distribution was in good agreement with the field data. The flow pattern was verified by comparison with thermal density current in a scale model.