The term hardness refers to the quantity of dissolved calcium and magnesium in water. These minerals, which come primarily from limestone type rock formations, are found to some degree in almost all natural waters. Calcium and magnesium cause problems for two principal reasons:
- When the water is warmed, they precipitate out of solution and form a hard, rock-like scale. This scale accelerates corrosion, restricts flow, and reduces heat transfer in water heaters and boilers.
- When they combine with soap, they react to form a curd, which interferes with cleaning, dries out skin, and leaves deposits on plumbing and clothes (bathtub ring; ring around the collar).
Hardness is measured in parts per million (or the equivalent mg/L) or in grains per gallon (gpg). Note: if the water analysis is given in ppm as CaCO3 then 1 gpg = 17.1 ppm. A common aspirin tablet weighs 5 grains). There is no established limit for the acceptable level of hardness in water, but it is generally considered to start to become problematic at around 3 gpg.
Levels of hardness are referred to as follows:
|Water Hardness Level||Grains Per Gallon (gpg)|
|Soft Water||0 - 1 gpg|
|Slightly Hard Water||1 - 3.5 gpg|
|Moderately Hard Water||3.5 - 7 gpg|
|Hard Water||7 - 10.5 gpg|
|Very Hard Water||Over 10.5 gpg|
Waters which naturally contain very little hardness can also be problematic because they may be corrosive.
A water softener works on the principle called “cation exchange,” in which ions of mainly hardness but also iron and manganese are exchanged for sodium or potassium ions, reducing their concentrations to insignificant levels. The exchange occurs with in a tank using tiny synthetic resin beads made from a special plastic.
After a period of use, the sodium or potassium ions are completely exchanged and the unit has to be “backwashed” or “regenerated.” This requires the use of sodium or potassium chloride, which is loaded into a “brine tank” and dissolved to form a brine solution used to recharge the resin beads.