Science of Water : Concepts and Applications

Water Treatment 303
The hardness (in mg/L as CaCO 3 ) for any given metallic ion is calculated using the formula:
50
Hardness mg/L as CaCO 3M(mg/L)
eq. wt. of M
gram molecular
weight
valence
where M is the metal ion concentration (mg/L) and eq. wt. is the equivalent weight.
Treatment Methods
Two common methods used to reduce hardness are ion exchange and cation exchange processes.
Ion Exchange Process
The ion exchange process is the most frequently used process for softening water. Accomplished by
charging a resin with sodium ions, the resin exchanges the sodium ions for calcium and magnesium
ions. Naturally occurring and synthetic cation exchange resins are available.
Natural exchange resins include substances such as aluminum silicate, zeolite clays (zeolites
are hydrous silicates found naturally in the cavities of lavas [greensand]; glauconite zeolites; or synthetic,
porous zeolites), humus, and certain types of sediments. These resins are placed in a pressure
vessel. Salt brine is fl ushed through the resins. The sodium ions in the salt brine attach to the resin.
The resin is now said to be charged. Once charged, water is passed through the resin and the resin
exchanges the sodium ions attached to the resin for calcium and magnesium ions, thus removing
them from the water.
The zeolite clays are most common because they are quite durable, can tolerate extreme ranges
in pH, and are chemically stable. They have relatively limited exchange capacities, however, so they
should be used only for water with a moderate total hardness. One of the results is that the water
may be more corrosive than before. Another concern is that addition of sodium ions to the water
may increase the health risk of those with high blood pressure.
Cation Exchange Process
The cation exchange process takes place with little or no intervention from the treatment plant
operator. Water containing hardness-causing cations (Ca 2+ , Mg 2+ , and Fe 3+ ) is passed through a bed
of cation exchange resin. The water coming through the bed contains hardness near zero, although it
will have elevated sodium content. (The sodium content is not likely to be high enough to be noticeable,
but it could be high enough to pose problems to people on highly restricted salt-free diets.) The
total lack of hardness in the fi nished water is likely to make it very corrosive, so normal practice
bypasses a portion of the water around the softening process. The treated and untreated waters are
blended to produce an effl uent with a total hardness around 50–75 mg/L as CaCO 3 .
CORROSION CONTROL
Water operators add chemicals (e.g., lime or sodium hydroxide) to water at the source or at the waterworks
to control corrosion. Using chemicals to achieve slightly alkaline chemical balance prevents
the water from corroding distribution pipes and consumers’ plumbing. This keeps substances like
lead from leaching out of plumbing and into the drinking water.
For our purpose, corrosion is defi ned as the conversion of a metal into a salt or oxide with
a loss of desirable properties such as mechanical strength. Corrosion may occur over an entire
exposed surface or may be localized at micro- or macroscopic discontinuities in metal. In all types
of corrosion, a gradual decomposition of the material occurs, often due to an electrochemical reaction.
Corrosion may be caused by (1) stray current electrolysis; (2) galvanic corrosion caused by
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