Environmental Engineering Reference
In-Depth Information
DID YoU KNoW?
Reverse osmosis is suitable when water has a very high concentration
of dissolved substances such as chlorides, nitrates, and fluorides, in
addition to other contaminants (e.g., salty water).
Hardness Treatment
Two commonly used methods to reduce hardness are the lime-soda pro-
cess and ion exchange. The lime-soda process is applicable for large facili-
ties, whereas ion exchange is normally employed in smaller water works.
The lime-soda process will not remove all of the hardness and is usually
operated to produce a residual hardness of about 100 mg/L as CaCO
3
.
Greater reductions are not economical and may have adverse health conse-
quences as well (McGhee, 1991). In this text, our discussion focuses on ion
exchange. Ion exchange is accomplished by charging a resin with sodium
ions and allowing the resin to exchange the sodium ions for calcium or
magnesium ions. Common resins include zeolites—natural and manmade
minerals that will collect from a solution certain ions (sodium or KMnO
4
),
and either exchange these ions (in the case in water softening) or use the
ions to oxidize a substance (in the case of iron or manganese removal). The
negative side of using ion exchange is that, even though the process softens
water by removing all (or nearly all) of the hardness and adds sodium ions
to the water, the water may be more corrosive than before. The addition of
sodium ions to the water may also increase the health risk of those with
high blood pressure.
Disinfection
At the turn of the century, 35,000 people per 1,000,000 people did not reach 20
years of age. Today, however, the rate of births exceeds the rate of deaths, and
the average lifespan is much longer. Curbing waterborne disease through
disinfection has significantly contributed to birth rates outpacing death rates
worldwide. The Safe Drinking Water Act requires that public water sup-
plies be disinfected, and the U.S. Environmental Protection Agency (USEPA)
sets standards and establishes processes for treatment and distribution of
disinfected water to ensure that no significant risks to human health occur.
