Definition - What does Permanent Hardness mean?
Permanent hardness refers to the mineral content in water that is not possible to remove through boiling. The hardness is typically caused by the presence in water of magnesium sulfates and/or calcium sulfate that do not undergo precipitation at increased temperatures.
Therefore, permanent hardness is the sum of magnesium hardness and calcium hardness.
Corrosionpedia explains Permanent Hardness
There are two major types of water hardness. The first is temporary hardness, caused by increased magnesium and/or calcium hydrogencarbonate. The other is permanent hardness, brought about by the presence of magnesium and/or calcium sulfate.
Temporary hardness is less detrimental because it can be eliminated through boiling. However, permanent hardness can be removed only by means of chemical treatment.
Permanent hardness in water can damage piping, boilers as well as other pressurized systems. Hard water tends to form scale deposits that can cause clogging inside the pipe surfaces and other machinery. This type of precipitation is insoluble.
To prevent such problems, water softening is recommended. One common technique for softening water involves the utilization of ion exchange resins that replace cations by ions of potassium and sodium.