One of the most common methods of combined sewer overflow (CSO) treatment is by conventional settling, the efficiency of which depends on the properties of solid particles and the characteristics of the flow transporting these solids. Since the geometry and hydraulics of CSO facilities are often very complex, traditional design methods based on many simplifying assumptions may not predict well the actual operational performance. Therefore, there is a growing need for new tools assisting engineers in design or operation of CSO storage and treatment facilities. In a study of such a CSO facility, a commercial computational fluid dynamics (CFD) model (FLUENT) was used to investigate flow and sediment behaviour, and to explore the ways of optimizing the overall facility performance by adding flow conditioning baffles to improve particle settling. A two-stage approach was adopted; flow patterns were simulated first by means of a volume of fluid (VOF) model and subsequently formed a basis for simulating particle transport by the discrete phase (DP) model. The simulation results for water surface, flow fields in different structure configurations, and particle capture rates in three parallel CSO storage/treatment tanks are presented for various flow conditions.