Conventional water softeners have two separate chambers, the resin tank and the brine tank, and use an ion-exchange process to remove the hardness ions calcium and magnesium. Softeners often are able to remove not only calcium and magnesium, but also iron, manganese and radium. Raw feed water is passed through a bed of resin “beads” inside the resin tank where the hardness ions in the feed water trade places on the resin beads with sodium ions that are electrostatically bound to the beads. Eventually, the resin exhausts its supply of sodium and must be “regenerated.” When the beads have no more room for additional calcium and magnesium ions, the unit temporarily goes off-line and the resin tank is flushed with salt water from the brine tank, the source of new exchangeable sodium ions.
Thus, the product water produced by softeners will have no calcium and magnesium, but additional sodium, about 8 mg/L per grain of hardness. Routinely, water softeners are plumbed so that the feed water destined for drinking bypasses the softener when consumers have been placed on low-sodium diets.
The regeneration process involves draining from the resin tank the saltwater solution, now with the added calcium and magnesium ions, and discharging it.