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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.11

rant termination of the service cycle. Therefore, barium removal by ion exchange is
achieved by ordinary softening, and the process is designed as an ordinary softener.
When sulfate is present, barium solubility is limited to the parts per billion range. If
sulfates are present in the raw water, it is probable that barium is present only as a sus-
pended solid and therefore cannot be removed by ion exchange at levels above a few parts
per billion.
The barium form of the resin is more difficult to regenerate than the hardness form.
Therefore barium will tend to accumulate on the resin, especially at lower salt dose lev-
els. During the initial (first time use) exhaustion cycle, barium continues to load on the
resin after hardness breakthrough by displacing previously exchanged hardness (calcium
and magnesium) ions. Later, when the resin bed is regenerated, the barium is less effi-
ciently displaced from the resin than the hardness ions. The ratio of barium to hardness
left on the resin after regeneration will be substantially higher than that in the influent
water. The barium will be pushed toward the bottom of the resin bed, where it could cause
leakage in the next service cycle. The regenerant dose level should be high enough to pre-
vent barium buildup in the resin bed.
If a significant amount of barium is exchanged onto the resin during the service cy-
cle, there is the potential for barium fouling from precipitation of barium sulfate. Ap-
preciable amounts of sulfate either in the dilution water or in the regenerant salt itself
will cause precipitation of the barium in the resin, which will foul the resin. Barium-
fouled resins can be cleaned, but the cleaning process is slow and involves corrosive
chemicals such as hydrochloric acid. It is usually necessary to use external vessels to
treat the fouled resins to avoid corrosion of the softener. In any case, the cleanup of
barium-fouled resins is difficult and not usually practical. One method of cleaning
barium-fouled resins is to soak the resin bed in 10% hydrochloric acid. This converts
sulfates to bisulfates, which are more soluble. The process usually requires several hours
and vigorous agitation before giving measurable improvement. It is often less expen-
sive to discard and replace the resins when performance drops below acceptable levels
due to barium fouling.

Combined Softening and Decationization. Waters containing hardness and appreciable
levels of alkalinity can be partially demineralized and fully softened by having two
columns of strong acid cation resins operated in parallel. One column is in the hydrogen
form, and one is in the sodium form, the two effluents are blended at a ratio determined
by the untreated water composition. The overall effect is partial demineralization, alka-
linity reduction, and softening. Through proper control of the blend ratios, the pH is main-
tained at acceptable levels.

Barium Removal by Hydrogen Form Weak Acid Dealkalizers. Barium can also be re-
moved by using a weak acid cation resin. The resin can be used in either the hydrogen
or the sodium form. The use of weak acid resins in the hydrogen form for softening is
limited to removing hardness associated with alkalinity. This process is usually employed
for alkalinity reduction and limited to waters that have high hardness-to-alkalinity ratios.
The effluent will have a reduced and somewhat variable pH. Initially the resin will re-
move all cations in exchange for hydrogen and will produce a very low pH. Very quickly
the resin will cease exchanging for monovalent ions such as sodium. Then more gradu-
ally, some alkalinity will start to slip through, eventually followed by hardness, and then
barium. The pH will rise gradually. The reaction of the hydrogen form weak acid resin is
usually limited to divalent cations associated with the alkalinity content of the water. Al-
kalinity in the water will be converted to carbon dioxide, which is a gas, by the hydro-
gen ions. The carbon dioxide will have to be removed. The most common methods of
COz removal are air stripping and deaerating heaters.