TM 5-813-8
Then, the water travels through a weak base anion
-The ion-exchange resins should operate at
resin, where the free mineral acidity is adsorbed but the
high capacities.
carbonic acid passes through unaffected. The water is
-The ion-exchange resins should be
then degasified, which removes the dissolved carbon
regenerated close to the stoichiometric
dioxide. The weak acid cation and strong acid cation
equivalence capacity.
resins are regenerated with either sulfuric or
-The acid and base regerants should be
hydrochloric acid, first through the strong acid cation
low cost.
resin and then through the weak acid cation resin. The
-The waste regenerants should be rinsed
strong base anion and weak base anion resins are
from the ion-exchange resins with a
regenerated in series with sodium bicarbonate, first
minimum of water, so that the capacity of
through the strong base anion resin and then through
the resin is not exhausted significantly.
the weak base anion resin. The RDI Process is shown
-Regenerant waste volumes should be
in figure 8-2.
minimized, and unused regenerants
d. Three-unit variation. In the three-unit variation,
should be recovered and reused to reduce
the waste disposal volume.
the strong acid anions in the water, such as chloride,
b. Limitations. The use of ion exchange in the
sulfate, and nitrate, are replaced with the bicarbonate
ion from a weak base anion resin in the bicarbonate
desalination of brackish water has several limitations.
form. The process then employs a weak acid cation
The volume of water treated is inversely proportional to
resin that replaces the calcium, magnesium, and sodium
the ionic concentration in the water.
Regenerant
in water with the hydrogen ion from the resin. The
consumption per unit volume of treated water is high
carbonic acid that is formed is adsorbed by a second
and becomes higher as the salinity of the brackish water
weak base anion resin in the free-base form. When the
increases. The size of the ion-exchange equipment
system has exhausted its treating capability, the lead
follows the same rationale-the more saline the water,
weak base anion resin is regenerated with ammonia,
the larger the ion-exchange equipment.
caustic, or lime, the weak acid cation resin is
A low salinity water, usually product water, is required
regenerated with sulfuric, hydrochloric, nitric, or
for regeneration of the ion-exchange resins.
sulfurous acid, and the tail-end weak base anion is not
c. Treatment processes. The treatment processes
regenerated. The lead weak base anion resin is now in
employed have either been on a pilot plant scale or
the free-base form and the weak acid cation resin in the
have been used in a limited number of full-size
hydrogen form. After its adsorption of carbonic acid in
installations. The processes have generally utilized
the previous service cycle, the tail-end weak base anion
weak acid cation and weak base anion resins. These
is in the bicarbonate form. The service flow direction is
resins have higher capacities and require less acid and
reversed for the next service cycle, with the former tail-
base regenerants than strong acid cation and strong
end weak base anion in the lead position and the former
base anion resins. Two ion-exchange desalination
lead weak base anion in the tail-end position. The
treatments that have been developed are the Desal
direction of service flow is reversed on each succeeding
Process and the RDI Process.
service cycle after regenerating only the weak acid
(1) Desal process. The Desal Process has
cation and the former lead weak base anion. This three-
several variations, but the main thrust of the process is
unit variation of the Desal Process is shown in figure 8-
the use of the weak base anion resins in the bicarbonate
3, with the following sequence of operation: Service-A
form.
followed by Regeneration-B, Regeneration-B followed
(2) RDI Process. The RDI Process is a three-
by Service-C, Service-C followed by Regeneration-D,
unit system using four different resins. The water first
Regeneration-D followed by Service-A, Service-A
passes through a strong base anion resin where the
followed by Regeneration-B, etc., in a repeating
strong acid anions, such as chloride, sulfate, and nitrate,
sequence.
are replaced with the bicarbonate ion from the resin.
e. Two-unit variation. In the two-unit variation,
The water then moves through a layered ion exchange
carbon dioxide is fed to the raw water. The carbon
unit of weak acid cation and strong acid cation resins,
dioxide in the water (carbonic acid) converts the weak
where the calcium, magnesium, and sodium are
base anion resin in the lead unit to the bicarbonate form
removed, the bicarbonates are converted into carbonic
and the strong acid anions in the water, such as
acid, and the neutral salt leakage from the previous
chloride, sulfate, and nitrate, are replaced with the
anion unit is converted into free mineral acidity, i.e.,
bicarbonate ion from the resin.
sulfuric, hydrochloric, and nitric acids.
8-3
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