Method for operating water softening device and water softening device

A water softening device comprising:

• a pressure tank configured to accommodate a cation exchange resin bed having a depth of 300 to 1500 mm and having a ratio of the depth to a diameter being 1.8 to 3;
• a valve unit configured to switch between a water-softening process in which soft water is obtained by passing raw water downward through the cation exchange resin bed and a regeneration process in which a whole of the cation exchange resin bed is regenerated by generating an opposite flow of a regenerant by collecting the regenerant at an intermediate portion of the cation exchange resin bed while supplying the regenerant from both sides of a top portion and a bottom portion of the cation exchange resin bed;
• a regenerant supply unit configured to supply the regenerant in a volume which gives a regeneration level of at least 1 to 6 eq/L-R, to a hardness leak prevention bed having a depth of 100 mm on a bottom portion of the cation exchange resin bed, in the regeneration process;
• and
• a raw water supply unit configured to supply raw water having an electric conductivity of equal to or lower than 150 mS/m and having a total hardness of equal to or lower than 500 mgCaCO3/L, in the water-softening process after the regeneration process.