The Meaning of “Temporary” and “Permanent” Hardness

Gazette Water Wizard Pure Water Annie Explains Why You Have to Watch Out for Temporary Hardness

What we call hardness in water–the property that causes hard scale to form on appliances and inside pipes and water heaters, spots on dishes, and soap scum–is caused by the presence of calcium and/or magnesium ions in the water. The more calcium and magnesium, the harder the water. The sum of the concentrated calcium and magnesium is often called “total hardness.”

All hardness, however, is not created equal. The hardness that gives you trouble in the home is what is called “temporary” hardness, as opposed to “permanent” hardness. That’s because temporary hardness, also called carbonate or bicarbonate hardness,  breaks down when it’s heated and forms hard scale. Permanent hardness, on the other hand, does not break down when heated and does not cause problems.

The test, then, for whether hardness is “permanent” or “temporary” is how it behaves when heated. Needless to say, in the home, hot water heaters and any appliances that use hot water are very vulnerable to the effects of temporary hardness.

In general terms, temporary hardness is the predominant form. Most water hardness is either all temporary or a mixture of temporary and permanent.

If you look at a water analysis, the way to determine the type of hardness is to compare the total hardness with the total alkalinity of the sample. Most water tests report both hardness and alkalinity “as CaCO3.”  Reporting “as if it were Calcium Carbonate” is simply a way of putting the items in a common frame of reference so they can be compared, the way we find a “common denominator” when we add fractions.

If the total alkalinity of the water is greater than the total hardness, then all the hardness in the water is temporary.  However, if the total alkalinity is less than the total hardness, both permanent and temporary hardness are present and the the amount of temporary hardness is equal to the alkalinity.

Here are examples:

Hardness — 150 ppm.

Alkalinity — 250 ppm.

Result: Temporary hardness=150 ppm.  (Alkalinity exceeds hardness, so all hardness is temporary.)

Hardness — 150 ppm.

Alkalinity — 100 ppm.

Result — Temporary hardness= 100 ppm.  Permanent hardess = 50 ppm.  (When hardness exceeds alkalinity, temporary hardness is equal to alkalinity and permanent hardess equals total hardness less alkalinity.)

What does all this matter?  Not much for residential water users, since most hardness is reported as “total hardness” and both types are treated with a water softener.  It might matter, though, if you were manager of a municipal water system, since temporary hardness can be reduced by a process called “lime softening” that isn’t used for residential treatment.