The present invention relates to a method of treating water and an aqueous system, situated in a pipe, with chlorine dioxide (ClO2), the method including generating ClO2 in a reaction space such that the generated ClO2 is completely surrounded by a system to be treated, and delivering the ClO2 generated in the reaction space to the system to be treated which is situated in the pipe, wherein the system surrounding the reaction space is the system to be treated, the reaction space is a component of a mobile device and the mobile device can be introduced into the pipe, in which the system to be treated is situated, and removed again independently of the pressure state of the pipe containing the system to be treated, and the reaction space is situated in the pipe containing the system to be treated.
