Computational fluid dynamics (CFD) is widely used to predict the hydraulics in environmental systems, but the validity of these predictions continues to receive attention. The hydraulics of a UV bench-scale reactor was evaluated by numerical and experimental techniques. Different experimental techniques were used to examine the hydraulics: salt injection to measure the residence time distribution, dye injection to visualize the mixing patterns, and LDA to measure the local velocity profiles. The combination of these measurements techniques provides information in unprecedented detail of the hydraulics inside UV systems. A CFD model with a standard k-epsilon turbulence model was used for comparison with the experimental results. The main flow patterns were predicted well. The largest differences were found at regions in the reactor where re circulation occurred, for example in the wake region of a lamp. Also, the CFD model predicted a larger mixing over the flow cross-section, resulting in a narrower residence time distribution. (C) 2010 Elsevier Ltd. All rights reserved.