Civil Engineering Reference
In-Depth Information
Noncarbonate Hardness and Salinity
Noncarbonate hardness is that fraction of total hardness associated with noncarbonate
calcium sulfate, CaSO
4
, calcium chloride, CaCl
2
, magnesium sulfate, MgSO
4
, and
magnesium chloride, MgCl
2
. The compounds that comprise noncarbonate hardness are
not precipitated when the water is heated, but form a very hard scale in boilers when
the water is evaporated. The term
incrustants
is sometimes applied to the compounds
contributing noncarbonate hardness.
SOFTENING GOALS
General
The primary goals of softening are to produce a water that is relatively stable and soft
enough for its intended use. Other goals that can be achieved in lime softening are
the reduction of certain metal concentrations, removal of iron and manganese, and
reduction of organic compounds in the raw water.
A factor that must be established before a softening project is initiated is the desired
level of hardness in the finished water. Local needs and desires are the major consid-
eration, but other factors should also be considered. The perception of acceptable
hardness varies considerably from one location to another. In New England, where
natural waters are soft, residents might consider a hardness of 100 mg / L excessive,
while residents of the Midwest or Southwest with naturally occurring hard water might
consider higher levels of hardness to be satisfactory. The cost of water softening is
generally not considered justifiable when the source of the water supply is less than
150 mg / L. Public water supplies usually are not softened below 30 to 50 mg / L be-
cause:
The solubility of CaCO
3
is 30 mg / L.
•
Waters softer than this tend to be corrosive.
•
One goal of softening should be to reduce the magnesium hardness to less than 40
mg / L, since levels higher than this tend to form magnesium hydroxide scale in do-
mestic water heaters. As a general softening goal guidelines, most lime-soda softening
plants are operated to produce the following levels in the softened water:
Calcium hardness: 30 to 40 mg / L
•
Magnesium hardness: less than 40 mg / L
•
Total alkalinity: 60 to 70 mg / L
•
All of the above are expressed as CaCO
3
.
Health Effects
Some research has indicated that there may be some health benefits associated with
the consumption of hard water. Many studies since the 1960s have demonstrated a
fairly consistent relationship between soft water and cardiovascular disease. These
