For our client Pidpa we investigated the mixing in a 600m³ water tower. Although water towers are 'simple' means for storage and distribution, attention must be given to optimal mixing for optimal drinking water quality. Pidpa wanted to limit the risk on water 'aging' and hence, regrowth. CFD gave the necessary insights to take a measure that will significantly improve mixing and lower the risk. We applied a multiphase model and found a fast way to estimate the mixing and water aging during 7 days of operation.