Water softeners operate on a simple principle: Calcium and magnesium ions in the water switch places with more desirable ions, usually sodium. The exchange eliminates both of the problems of hard water because sodium doesn't precipitate out in pipes or react badly with soap. The amount of sodium this process adds to your water is quite small -- less than 12.5 milligrams per 8-ounce (237-milliliter) glass, well below the standard set by the Food and Drug Administration for "very low sodium" [sources: Shep, U.S. Food and Drug Administration]. If you have health concerns, discuss them with your doctor, or consider either using a different kind of softener or only softening wash water.
The ion replacement takes place within a tank full of small polystyrene beads, also known as resin or zeolite. The negatively charged beads are bonded to positively charged sodium ions. As the water flows past the beads, the sodium ions swap places with the calcium and magnesium ions, which carry a stronger positive charge.
So why do you load up water softeners with salt if the plastic beads do all the work? Over several cycles, calcium and magnesium replace all of the sodium in the beads, after which the unit can no longer soften water. To fix this problem, the softener enters a regeneration cycle during which it soaks the beads in a strong solution of water and salt, or sodium chloride. The sheer amount of sodium in the brine solution causes the calcium and magnesium ions in the beads to give way, and the beads are recharged with sodium. After regeneration, the water softener flushes the remaining brine, plus all of the calcium and magnesium, through a drainpipe. Regeneration creates a lot of salty water -- around 25 gallons (95 liters).
Most home water softeners use the plastic bead and salt approach. The main difference between them is how they decide when to regenerate. Some softeners use electric timers that flush and recharge based on a regular schedule. Others use a computer that judges bead depletion based on water use. Still others use a mechanical water meter to measure water use and kick on the recharging process only when sodium exhaustion requires it.
Each approach comes with its share of pros and cons. Electronic timer units can't dispense soft water while recharging. Conversely, some computerized systems carry a reserve resin capacity, so you can tap them for a squirt or two of soft water even during recharge cycles. Most flexible of all are mechanical systems, which come equipped with two mineral tanks. One tank can make soft water while the other recharges.
Modern water softeners run between $400 and $2,700, and are designed to be easy to install and remove, which is good news if you want to take your unit with you when you move. However, unless you are confident in your electrical and plumbing abilities, you might want to hire a plumber, which could add $100 to $600 to the initial cost of your unit. Some stores include installation with the purchase of a water softener.