My Gran used to have a water softener (In the '50s) Click image for larger version.  Name: dealer_elmgarden_full_1464366928883-6548186127 (1).jpg  Views: 396  Size: 40.4 KB  ID: 23113 and it was regenerated using salt. I wonder if there is any way of reviving spent deionising cartridges in a similar manner. They are both based on ion-exchange resins.