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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.9

CA TION EXCHANGE PROCESS

When only specific ions need to be removed, it is convenient to use the appropriate type
of ion exchanger operated in an acceptable ionic form. Any environmentally and eco-
nomically acceptable salt can be used to regenerate the exhausted resins. Softening is the
most widely practiced of all ion exchange processes. Calcium and magnesium ions are
referred to as hardness because they react with soap to form curds. This makes it "hard"
to wash in waters containing these substances. Ion exchange softening, of course, involves
all cations; for example, copper, iron, lead, and zinc are all exchanged along with the cal-
cium and magnesium. A strongly acidic cation exchange resin is regenerated with sodium
chloride. This places the resin in the sodium form. The resin can then exchange its so-
dium ions for calcium, magnesium, and other cations in the raw water. The result is a wa-
ter containing essentially all sodium cations.
Ions with higher valences such as Ca 2+, Mg 2+, Fe z+ or 3+, CH2+, and Pb 2+ are more
highly preferred by the resins at the TDS levels typical of potable water. During the ser-
vice cycle, the multivalent ions are removed by the resin in exchange for an equivalent
amount of sodium ion; the sodium ion is replenished by the sodium chloride used to
regenerate the resin.

Salt Selection
A good regenerant should be inexpensive and contain an exchangeable ion that can ef-
fectively displace the unwanted ions from the resin. It should be soluble so as to be de-
livered at reasonably high concentrations in order to keep waste volumes low, and its co-
ion should remain soluble when paired with the displaced ions. Most chloride salts are
soluble and chlorides are nontoxic, so it is not surprising that sodium chloride is the most
common salt used to regenerate softeners. Potassium chloride, though more expensive,
can be used with similar results.

Operating Capacity
The operating capacity of the resin is defined by the number of "exchangeable" ions
it removes from the raw water each cycle. In the case of a water softener, calcium,
magnesium, and other divalent ions constitute the exchangeable ions. There are sev-
eral ways to express this. The most common expression of capacity in the United States
and the rest of North America is as kilograins (as CaCO3) per cubic foot of resin. The
higher the concentration of exchangeable ions, the lower the volume-based through-
put capacity.
The operating capacity of the resin varies according to the regenerant level. The re-
generant level is normally stated in terms of pounds of regenerant per cubic foot of resin.
Higher dosages give higher operating capacities and lower hardness leakage (higher qual-
ity) but at reduced salt efficiency. Generally, the selection of regeneration levels is based
on using the minimal amount of salt that will operate the softener at the required degree
of hardness leakage.
Operating efficiency depends on several factors including the choice of resin, regen-
eration level, method of regeneration, flow rate during the service cycle, and quality level
used to determine start and end of the service cycle.
There are a variety of process schemes to maximize regenerant effectiveness. These
usually require more complex and costly equipment, trading reduced chemical operating