In order to minimize undesirable byproducts during disinfection the conditions (amount of chlorine, temperature, pH value) are adapted accordingly. Furthermore, disinfectant content can be determined on a weekly basis. With chlorine, for example, this content ideally lies between 0.3 and 0.6 mg/l (free chlorine). The amperometric measuring cells for free chlorine, chlorine dioxide, and ozone together with the JUMO AQUIS 500 AS transmitter and controller are particularly suitable for this task.