UV reactors are systems that expose flowing water to light from UV lamps. They’re widely used in treating (for instance) drinking water, wastewater, industrial process water and ballast water. However, different water processes involve a variety of operating conditions (pressures, temperatures, velocities) and they therefore sustain a diversity of organisms as well. In order to ensure that UV reactors provide sufficient decontamination rates, their design and operating regimes must be properly calibrated and optimised. A particularly useful method for this is to model combined flow and UV fields in complex geometries by means of Computational Fluid Dynamics (CFD).