This paper presents a numerical model capable of simulating sediment transport in combined sewer systems. The main objectives of the model are to model mass transport rates at the outlet from a catchment and at the same time obtaining qualitative information on erosion and deposition going on at different locations in the sewer system. The model is conceptual but based on deterministic computations of hydraulic conditions. The formulations used in the conceptual model (STSim) are presented as well as results from a sensitivity analysis. Finally, an example is given of a calibration event from a case study.