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Source of Hardness

Hard water is found over 80% of the United States. The hardness of a water supply is determined by the content of calcium and magnesium salts. Calcium and magnesium combine with bicarbonates, sulfates, chlorides, and nitrates to form these salts. The standard domestic measurement for hardness is grains per gallon (gpg) as CaCO3. Water having a hardness content less than 0.6 gpg is considered commercially soft. The calcium and magnesium salts, which form hardness, are divided into two categories: 1) Temporary Hardness (containing carbonates), and 2) Permanent Hardness (containing non-carbonates). Below find listings of the various combinations of permanent and temporary hardness along with their chemical formula and some information on each.

Temporary Hardness Salts

  1. Calcium Carbonate (CaCO3) - Known as limestone, rare in water supplies. Causes alkalinity in water.
  2. Calcium Bicarbonate [Ca (HCO3) 2] - Forms when water containing CO2 comes in contact with limestone. Also causes alkalinity in water. When heated CO. is released and the calcium bicarbonate reverts back to calcium carbonate thus forming scale.
  3. Magnesium Carbonate (MgCO3) - Known as magnesite with properties similar to calcium carbonate.
  4. Magnesium Bicarbonate [Mg (HCO3)2] - Similar to calcium bicarbonate in its properties.

Permanent Hardness Salts

  1. Calcium Sulfate (CaSO4) - Know as gypsum, used to make plaster of paris. Will precipitate and form scale in boilers when concentrated.
  2. Calcium Chloride (CaCI2) - Reacts in boiler water to produce a low pH as follows: CaC1, + 2HOH ==> Ca(OH)2+2HC1
  3. Magnesium Sulfate (MgSO4) - Commonly known as epsom salts, may have laxative effect if great enough quantity is in the water.
  4. Magnesium Chloride (MgCI2) - Similar in properties to calcium chloride.

Sodium salts are also found in household water supplies, but they are considered harmless as long as they do not exist in large quantities. The US EPA currently has no national policy with respect to the hardness or softness of public water supplies.

Treatment of Hardness

Softeners can remove compensated hardness up to a practical limit of 100 gpg. If the hardness is above 30 gpg or the sodium to hardness ratio is greater than 33%, then economy salt settings cannot be used. If the hardness is high, then the sodium will be high after softening, and may require that reverse osmosis be used for producing drinking water.