BLENDING CONTROL COMPRISING DETERMINING THE HARDNESS OF WATER BY MEANS OF THE CONDUCTIVITY OF SOFT WATER AND BLENDED WATER

A method for operating a water softening system (1; 1') having an automatic blending unit (19), wherein an incoming untreated water stream Vin is divided into a first substream Vsub1 which is softened and a second substream Vsub2 which is not softened, and the two substreams Vsub1, Vsub2 are combined to give a blended water stream Vblend, wherein the proportions of the two substreams Psub1, Psub2 in the blended water stream Vblend are set by the automatic blending unit (19) so as to result in a desired hardness SW in the blended water stream Vblend (100), where the proportions of the two substreams Psub1, Psub2 that are to be established are calculated (112) from the hardness of the untreated water Hin and the hardness of the softened water Hsoft, and where the hardness of the untreated water Hin is derived (110) from the conductivity of the untreated water CDin, is characterized in that the conductivity of the softened water CDsoft is measured with a conductivity sensor (9a) in the softened first substream Vsub1, and the conductivity of the blended water CDblend is measured with a conductivity sensor (9b) in the blended water stream blend, where the proportions of the substreams Psub1, Psub2 are ascertained (102, 104, 106) in the blended water stream Vblend, and in that the conductivity of the untreated water CDin is calculated (108) from the measured conductivity of the soft water CDsoft, the measured conductivity of the blended water CDblend and the ascertained proportions of the substreams Psub1, Psub2. With the invention, it is possible to set the hardness of the blended water with high reliability and improved accuracy.