Mixing processes and their related residence times in water storage tanks are an important factor for maintaining an adequate quality of water. These processes are normally investigated with costly experimental studies. Furthermore, much has been achieved in numerical simulation techniques in recent years. The main objective of this project is to develop a CFD (Computational Fluid Dynamics) model that could be used as a flexible modelling tool, in addition with experimental procedures, for the study, design and improvement of these tanks. Mixing and residence times in a water storage tank are investigated using Unsteady Reynolds Averaged Navier-Stokes simulations (URANS). A good agreement between the experimental dilution curve in the outlet section tank and the CFD results was achieved. The flow structure and its relation to the different phases of the dilution curve have been studied, proving a predominantly advective transport.