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CHAPTER TWELVE
12.14

Strong
i Strong
Acid Base
Salt
Salt
Cation Anion

L
FIGURE 12.1 Strong base dealkalizer.

Although the sulfate has a higher affinity for the resin during the exhaustion cycle, the
sulfate/chloride affinity relationship is affected by the concentration and is reversed at the
regenerant concentrations. Approximately 5 lb of salt per cubic foot of resin, applied at
4% to 6% concentration, is sufficient to fully regenerate the resin bed to the chloride form.
Higher dose levels offer very little gain.
Since the bicarbonate ion is the least strongly held of the common ions by the resin,
the operating capacity depends on the total concentration of ions. The operating capacity
for most gel type II anion exchange resins can be estimated at 12,500 gr/ft 3 resin. Through-
put capacities are calculated based on all the anions in the inlet water including chlorides
as being exchanged. First, calculate the sum in milligrams per liter of all the anions as
their calcium carbonate equivalents, and divide that by 17.1 to convert the concentration
to grains per gallon. Bicarbonates will be reduced by about 80% to 90% throughout most
of the service cycle, and the treated water pH will be about 5. When alkalinity leakage
reaches 30%, the service cycle is terminated. The operating capacity can be increased to
about 15,000 gr/ft 3 and the effluent pH increased to above 7, by adding about 0.25 lb of
sodium hydroxide to each 5 lb of salt used for regeneration.

pH Effects of Dealkalization. The pH of ordinary tap water is determined by the equi-
librium between carbon dioxide, bicarbonate, and carbonate between pH values of 5.0 and
10.6. In the absence of hydroxides, the removal of the bicarbonate and carbonates auto-
matically reduces the pH of tap water to 5.0, which is the pH of carbon dioxide in water.
This is what happens when salt is used alone as the regenerant. Carbon dioxide is only
slightly ionized, so it is not removed by the resin. This scenario is changed by adding a
small amount of caustic to the regenerant salt, usually 0.25 lb of NaOH to each 5.0 lb of
salt. This places a very small amount of hydroxide on the resin. Its main effect is that it
converts the nonregenerated bicarbonate left on the resin to carbonates. The carbonates
on the resin will combine with carbon dioxide to form bicarbonates, which are held by
the resin. By this mechanism, CO2 is removed and the pH is shifted back to 7.0. The ef-
fluent is free of bicarbonates and carbon dioxide.

Dealkalization with Weak Acid Resins Regenerated with Acid. Weakly acidic resins
operated in the hydrogen form are, by their nature, unable to react with cations except in
the presence of a neutralizing agent such as alkalinity. They exchange hydrogen for cations
but are only effective in removing hardness and other divalent ions that are associated
with alkalinity. Therefore, weakly acid resins are able to work effectively as dealkalizers,
but only on certain waters. Because they are weakly acidic, they have extremely high