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12.8 CHAPTER TWELVE

The most common regenerant used in potable water applications of ion exchange is
sodium chloride. It is used in softening, dealkalization, barium, radium, uranium, sele-
nium, arsenic, and nitrate removal. Potassium chloride, though more expensive, is also
used with very similar results in cases where low sodium levels are desired.
In demineralization, the resins are regenerated with acids and bases. Cation exchange
resins are regenerated with acid, the most common acids being sulfuric and hydrochloric.
Strongly basic anion resins are usually regenerated with caustic soda (sodium hydroxide),
but potassium hydroxide, though more expensive, can also be used. Weakly acidic cation
resins can be regenerated with weak acids such as carbonic, citric, acetic, or spent acids
from acceptable sources such as the regeneration of strong acid resins. Weakly basic resins
can be regenerated with weak bases such as sodium carbonate or ammonia or spent caus-
tic such as leftover from regeneration of strong base resins. In most cases the weak acid
and weak base resins are regenerated with the same strong acids and bases used to re-
generate strong cation and strong anion resins, especially when they are used together.
However, it is not unusual to use the waste regenerants from the strongly ionized resins
to regenerate the weakly ionized resins. In this manner, regeneration efficiencies can ap-
proach 100% whereas 10% to 50% is the norm for strongly ionized resin systems alone.
In general, for the ion exchange process to be effective, the volume of treated water
must be greater than the volume of waste generated by the regeneration process, i.e., the
backwash, regeneration, and rinse cycles. As the total ionic strength of the solution in-
creases, the exhaustion (service) cycle throughput is reduced proportionately. When the
ionic concentration is greater than 500 mg/L, ion exchange may become impractical or
less attractive than other processes. However, even at 1,000 mg/L, ion exchanger is still
an effective technology for softening and removal of selective ions. At 10,000 mg/L, it is
usually not practical for most applications except trace ion removal. The removal of trace
ions usually involves resins that are specially formulated and that are highly selective for
the ions to be removed.
Ion exchange resins are generally limited to processing waters that are relatively free
of oxidants, physical contaminants, or oily substances that could coat the resin beads. The
organic polymers used to make ion exchange resins have upper temperature limits in the
neighborhood of 300 ° F (149 ° C). That establishes the maximum temperature level for
any organic-based adsorbent, ion exchanger or not. The functional groups in strongly ba-
sic anion resins are thermally less stable than the polymer and have lower temperature
limits whereas the functional groups in cation exchanges are more stable. The ionic form
of the resin affects its chemical stability. The functional groups of anion resins are less
stable in the alkaline form. The functional groups of the cation resins are less stable in
the acidic form. Sodium form cation resins are routinely used at temperatures approach-
ing 300 ° F (149 ° C). Salt form anion resins are sometimes operated at temperatures up to
175 ° F (79 ° C) and occasionally as high as 200 ° F (93 ° C). In the alkaline form, the type
I strong base resins are best operated below 140 ° F (60 ° C); the type II resins should not
be operated over 100 ° F (37 ° C). The weakly basic resins are more stable and can be op-
erated at 70 ° F (21 ° C). Strong base resins based on acrylic polymers should not be op-
erated at temperatures above 95 ° F (35 ° C).

Types of Ion Exchange Processes
In municipal and domestic applications, ion exchange resins are normally used in single
beds for the removal of specific substances, such as hardness, nitrates, naturally occur-
ring organics (color), and alkalinity. In most cases the resins are regenerated with salts;
therefore the total dissolved solids and pH of the treated water are not significantly al-
tered. Acid and caustic regenerations are used in deionization processes, which are pri-
marily used in industrial applications.