Subsurface vertical flow constructed wetlands (CWs) have been found to be a useful system to treat combined sewer overflow (CSO). The study presented uses numerical simulation to increase the understanding of the fundamental processes of COD degradation in CWs for CSO treatment. The multi-component reactive transport module CW2D was used for the simulation study. The simulation results showed that the measured behaviour of the system can only be modelled when COD adsorption is considered as additional process. A new parameter set for CW2D for modelling CSO treatment is presented. A range of values for COD adsorption parameters, COD fractionation and bacteria concentrations were estimated by an identifiability analysis. For the simulation a step wise approach was developed. On the one hand a lysimeter study was used for calibration and validation, and on the other hand field and lab-scale experiments were used for validation. Single-event simulations as well as long-term simulations were carried out. For the single-event simulations (lysimeter and field studies) a good match between measured and simulated data could be achieved. However, the long-term simulations showed that there is a need for further investigations mainly due to the uncertainties during long dry periods between the loadings.