CFD modelling has proven to be a powerful tool for the design of UV reactors. However, the validation of the hydraulics predicted by the CFD model remains a point of attention. Using standard turbulence models such as the k-epsilon model, the CFD model often wrongly predicts local flow features around a UV lamp. Therefore, more advanced modelling approaches such as the LES model were considered. The modelling approaches were explored for a single cross-flow UV lamp. It is shown by means of comparison with experimental data that the LES model predicts the flow around a UV lamp more precisely than the k-epsilon model. The impact of differences in resolved velocity fields on the predicted disinfection was also investigated. Depending on the local geometry (presence and positioning of baffles) the disinfection results were completely different for the different modelling approaches. (C) 2010 Elsevier B.V. All rights reserved.