Hardness
Hard water and soft water are terms that have no exact meaning because water considered hard in
one region might be considered soft by inhabitants in another region. Hardness is usually
associated with what happens when soap is used. Soap does not clean as efficiently in hard water.
It leaves insoluble residues in bathtubs, sinks and clothing. In addition, hard water causes scale to
incrust water heaters, boilers and pipes thereby reducing their capacity and heat-transfer properties.
Thus, it is a technical and economic problem. (See Section 3 : Water Softener).
Hardness is commonly reported in units milligrams per litre as CaCO³, although other terms are
also used, as follows:
e.g. grains per gallon (U.S.) - gpg
1 gpg (U.S.) = 17.1 mg/L TOTAL HARDNESS as CaCO³
Grains per gallon, an ancient concept, was originally defined as the degree of hardness produced
by a grain of chalk in a gallon of water.
For the most part, hardness depends on the concentration of calcium and magnesium. Hardness is
usually expressed in terms of the equivalent quantity of calcium carbonate (CaCO³). There are no
distinctly defined levels for what constitutes a hard or soft water supply. However, a general scale
of hardness (as CaCO³) has been provided by the Water Quality Association as follows:
Concentration
_____________________________________
mg/L Description
———————————————————
0 - 8.5 Very Soft
9 - 30 Soft
31 - 60 Slightly Hard
60 - 120 Moderately Hard
121 - 185 Hard
Over 185 Very Hard
30
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