PROVIDED IS A METHOD FOR STERILIZING A WATER TREATMENT APPARATUS HAVING A PLURALITY OF TANKS. THE METHOD INCLUDES, PERFORMING A STERILIZATION PREPARATION PROCESS TO DRAIN PURIFIED WATER FROM THE FIRST TANK UNTIL A FIRST AMOUNT OF PURIFIED WATER IS LEFT, PERFORMING A STERILIZING PROCESS TO STERILIZE THE FIRST TANK BY SUPPLYING THE STERILIZED WATER SUPPLIED FROM THE STERILIZING MODULE, TO DRAIN WATER FROM THE FIRST TANK THROUGH A FIRST WATER OUTLET PIPE UNTIL A SECOND AMOUNT OF WATER IS LEFT IN THE FIRST TANK, AND TO DRAIN THE SECOND AMOUNT OF WATER REMAINING IN THE FIRST TANK INTO THE SECOND TANK THROUGH A SECOND WATER OUTLET PIPE. FIGURE 3
