Hardness is a water quality parameter that indicates the level of alkaline salts, principally calcium and magnesium ions, and expressed as equivalent calcium carbonate (CaCO3). The hardness of your water will be reported in grains per gallon, milligrams per liter (mg/l) or parts per million (ppm). One grain of hardness equals 17.1 mg/l or ppm of hardness. EPA Primary Standards are based on health considerations and Secondary Standards are based on taste, odor, color, corrosivity, foaming, and staining properties of water. There is no Primary or Secondary standard for water hardness. Water hardness is classified by the U.S. Department of Interior and the Water Quality Association as follows:
|Classification||TDS mg/l or ppm||grains/gal|
|Soft||0 - 17.1||0 – 0.9|
|Slightly Hard||17.1 - 60||1 - 3.4|
|Moderately Hard||60 - 120||3.5 - 6.9|
|Hard||120 - 180||7.0 - 10.4|
|Very Hard||180 & over||10.5 & over|
These calcium and magnesium ions "hardness ions" cause two major kinds of problems:
- The metal ions react with soaps and calcium sensitive detergents, hindering their ability to lather properly and forming an unsightly precipitate— the familiar scum or "bathtub ring". Presence of "hardness ions" also inhibits the cleaning effect of detergent formulations.
- More seriously, calcium and magnesium carbonates tend to precipitate out as adherent solids on the surfaces of pipes and especially on the hot heat exchanger surfaces of boilers. The resulting scale buildup can restrict water flow in pipes.
Treatment - A water softener reduces the calcium or magnesium ion concentration in hard water. Conventional water-softening devices use depend on an ion-exchange resin in which "hardness" ions trade places with sodium ions that are electrostatically bound to the anionic functional groups of the polymeric resin. The water to be treated passes through a bed of the resin; negatively-charged resins absorb and bind metal ions, which are always positively charged. The resins initially contain univalent sodium ions, which exchange with divalent calcium and magnesium ions in the water. This exchange eliminates the possibility of precipitation and soap scum formation. As the water passes through both kinds of resin, the hardness ions replace the sodium which are released into the water.
Regeneration - As these resins become converted to their Ca2+ form they gradually lose their effectiveness and must be regenerated. This is accomplished by passing a concentrated brine solution through them, causing the above processes to be reversed. Herein lies one of the drawbacks of this system: most of the salt employed in the regeneration process gets flushed out of the system and may be released into the soil or drainage. Total Dissolved Solids (TDS) should not exceed approximately 500 ppm (about 8 grains of hardness) and is preferably much lower.