The Chemical Process of Water Softening
Salt-based water softening works on the principle of ion exchange or ion replacement, a reversible process. (Ions are atoms or groups of atoms that can lose or gain electrons in water and therefore have an electrical charge.) Calcium (Ca2+) and magnesium (Mg2+) ions in the hard water are replaced with sodium ions (Na+) during the reversible ion-replacement reactions, as sodium ions leave the ion-exchange resin (or beads, or a chemical matrix called zeolite) and go into water, while calcium and magnesium ions in the hard water are bound to the resin or beads. Essentially, the ions switched places during water softening: sodium from resin matrix migrate into the water, and calcium and magnesium move from water to attach to the resin matrix. The resin or bead matrix is itself inert and does not participate in the chemical reaction, and its physical structure is not changed during water softening.
When all the sodium in the resin or beads matrix have been replaced with calcium and magnesium, then the resin in the exchange chamber is "full" and cannot perform any more water-softening reactions and therefore needs to go through a process called regeneration. During regeneration, salt (sodium chloride) is added to the exchanger chamber; the high level of salt displaces the calcium and magnesium ions and replaces them with sodium. After regeneration, the resins or beads can soften more hard water.
For most healthy people, the amount of added sodium in the softened water poses no health problems. However, for people who are hypertensive (have high blood pressure) and must live on a low-sodium diet or a sodium-restricted diet, the sodium in the softened water can be hazardous to people's health.
Are There Health Problems Caused by Water Softening?
Water Softeners Do Add Sodium to Tap Water