Civil Engineering Reference
In-Depth Information
studies show that people living in soft water areas have somewhat higher cardiovas-
cular disease rates than those living in hard water areas. Although several theories to
explain this phenomenon have been advanced, no causative relationships have been
established. Other studies have indicated that long-term consumption of demineralized
water may result in the lowering of the bone calcium saturation level.
1,2
Again, the
causative factors have not been clearly established. These studies suggest that hard
water may be healthier than very soft water.
In waters of very high noncarbonate hardness, there has long been a concern that
the softening process itself may increase the sodium content of the water enough to
have significance for people with high blood pressure. Recent epidemiological studies
have raised questions about this previously postulated causative relationship.
Produce Consistent Water Quality
If the quality of the raw water is highly variable, softening of the water to a level that
can be maintained year-round may be desirable. Consistency of the quality of the water
is often important to consumers, particularly commercial and industrial users. Water
that varies in hardness from season to season may result in operational problems.
Produce a Stable Water
Water that is neither excessively corrosive nor excessively scale-forming is ideal for
many uses, including domestic uses. The hardness of the water can be an important
factor in stability.
Reduce Organic Compound Concentrations
Naturally occurring organic compounds often found in surface waters may combine
with chlorine to produce undesirable disinfection by-product such as chloroform and
other trihalomethanes. The Stage I Disinfectants / Disinfection By-Products Rule (D /
DBP)
3
establishes specific requirements for total organic carbon removal by coagula-
tion and softening processes (see Chapter 1, ''Criteria and Standards for Potable Water
Quality''). Water systems using softening must provide 15-30 percent TOC reduction,
depending on the source water TOC. If they cannot achieve these TOC requirements,
they may use the following additional criteria for compliance: reduce treated water
alkalinity to less than 50 mg / L (annual average), or use softening that removes at least
10 mg / L of magnesium hardness (annual average). A later section of this chapter
discusses removal of organics in the softening process.
Reduce Heavy Metals
Lime softening can reduce the concentrations of many heavy metals. Those that can
be reduced by 80 to 95 percent include barium, strontium, silver, arsenic, cadmium,
cobalt, copper, mercury, nickel, lead, and zinc. Hexavalent chromium is difficult to
precipitate; it can be reduced by only 20 to 30 percent. If in the trivalent form, pre-
cipitation is much enhanced.
Lime softening is very efficient for arsenic removal [As(V)] (
95 percent removal)
at pH above 10.5.
4
Softening is not as successful with trivalent arsenic [As(III)]. Lime
