Hydrodynamic processes largely determine the efficacy of drinking water treatment systems, in particular disinfection systems. A lack of understanding of the hydrodynamics has resulted in suboptimal designs of these systems. The formation of unwanted disinfection-by-products and the energy consumption or use of chemicals is therefore higher than necessary. In drinking water engineering, computational fluid dynamics (CFD) is therefore increasingly applied to predict the performance of treatment installations and to optimise these installations. CFD uses advanced numerical models to predict flow, mixing and (bio)-chemical reactions. In this thesis, the hydrodynamics and (bio)-chemistry in ozone and UV systems are studied by means of CFD models combined with experimental techniques. This combination leads to further development of CFD modelling as a tool to evaluate the performance of drinking water treatment installations. If the CFD model is applied properly, accounting for the complex turbulent motions and validated by experiments, this tool leads to a better design of UV reactors, ozone systems and other systems dictated by hydrodynamics. This work resulted in new insights in the applicability of models in ozone and UV installations, and new insights in design aspects of these installations.