Secondary sedimentation tanks, which are used widely in wastewater treatment, require optimization to improve wastewater treatment efficiency. The flow dynamics is an important parameter for the design of reactors. However, very little literature deals with the flow in secondary sedimentation tanks. Therefore, the flow dynamics in a secondary sedimentation tank is studied in this paper. The solid-liquid two-phase turbulent flow in the tank is modeled with the three-dimensional two-fluid model. Using computational fluid dynamics (CFD), the velocity profile and distribution of solid concentration are given. The use of different baffles in the same tank is also simulated. With the inclusion of the baffle, the distribution of the solid concentration differs greatly. The significant effects of the baffle height and baffle location on the distribution of solid concentrations are also discussed. These results provide useful information for the design of secondary sedimentation tanks. (C) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.