Abstract: A method and a fluid treatment system for treating an industrial process fluid used in a system, said method comprising the steps of: transferring a part of the industrial process fluid used in the system through a first fluid path arrangement of a fluid treatment system which is connected to an outlet and an inlet of the system; measuring a concentration of an industrial process fluid concentrate in the industrial process fluid by a sensor device provided in the first fluid path arrangement of the fluid treatment system; adding an amount of this industrial process fluid concentrate to the industrial process fluid used in the system in order to maintain a predefined concentration level, wherein said amount is calculated based on the measured concentration, whereby the industrial process fluid concentrate is provided into the system via the sensor device in the fluid treatment system; cleaning the sensor device at certain time intervals by providing the industrial process fluid concentrate into the sensor devi