De Watergroep, a Flemish drinking water utility, aims to optimize the ozonation process at the Kluizen surface waterworks in order to obtain sufficient inactivation of pathogenic micro-organisms while minimizing the formation of bromate. In literature, the inactivation of micro-organisms is described as a first order process in ozone exposure (ozone concentration multiplied by exposure time or CT value). In the full-scale plant, the ozone exposure was determined at different temperatures and ozone doses, and an empirical relation between ozone exposure, ozone dose and temperature was derived. Based on the results, the optimum ozone dose required to obtain a Log 2 removal of viruses and a Log 1 removal of Giardia was determined as a function of temperature. Bromate formation was found to be dependent on the ozone dose rather than on the CT value. Therefore, an empirical relation between bromate formation, ozone dose and temperature was established. Bromate formation was found to be far below the legal standard of 10 μg/L when the optimum ozone dose, required to achieve the disinfection goals for viruses and Giardia, is applied.