Water softeners usually consist of a tank containing an ion-exchange material such as zeolite or resin beads. When water passes through, calcium and magnesium ions are exchanged for sodium ions. Water-softening capacity must be regenerated at intervals depending on the hardness of water and the capacity of softener. Water softener capacity is given in terms of the number of grains of hardness it will remove between successive regenerations. It is recommended that a softener have enough capacity to last at least three days between regenerations. The choice will depend on water requirements for the household and the peak flow rate. Regeneration of the water softener is accomplished by flushing brine (common salt solution) through the exchange material to replace collected calcium and magnesium ions with sodium ions. The flush brine is a waste and must be disposed of properly.
Many softeners are fully automatic and require only a periodic resupply
of salt. They will automatically backwash before regenerating to flush
out accumulated sediment and oxidized iron. The sodium content of the softened
water supply is directly related to the original hardness. In harder water,
more calcium and magnesium ions must be substituted with sodium during
the softening process. Some people may be concerned with the increase of
sodium in their diet; however, the quantity of sodium obtained from the
water will be relatively small. For example, suppose that the hard water
contains 10 grains of calcium and magnesium. If we assume that the daily
consumption of water is one-half gallon (2 liters) per person and one-third
of the hardness is due to magnesium salts and two-thirds to calcium salts,
then the increase in sodium in the daily diet is 0.3 g (this assumes 100%
efficiency of the exchange process). This can be a significant amount for
people limited to 0.5 g or less of sodium per day.
Types of Water Softeners
Automatic -- An automatic water softener is equipped with a clock timer that automatically starts the regeneration process at preset intervals. During the regeneration process, the hardness ions collected in the ion exchange resin must be flushed out in order to allow the continuous exchange between soft and hard ions. The entire softening process is controlled automatically by the clock timer.
Demand Initiated Regeneration (DIR) -- Demand initiated regeneration (DIR) is the most efficient type of automatic water softener. Instead of regenerating at a preset interval, a DIR unit regenerates only when soft water runs out. DIR units use sensors or valves to determine when to regenerate. Because they adjust to the amount of water actually used, DIR units consume up to fifty percent less salt and water than do preset time clock automatic softeners.
Portable Exchange -- A portable exchange unit is a type of softener with a removable tank that is rented to a homeowner, and contains ion exchange resin that is fully regenerated and ready for use. When the resin material is exhausted (that is, when it is no longer exchanging hard ions for soft ions), the tank is returned to a central plant where it is regenerated and reprocessed to be used again.