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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.51

tle as 50% of the time. Even ion exchange systems with multiple exchange tanks and
spare tanks seldom exceed 98% availability.

Resin Deterioration
Ion exchange resins are in general very stable polymers, and their chemical deterioration
occurs at a very slow pace. Anion resins are generally less stable than cation resins. The
acid form cation resin and hydroxide form anion resin forms are less chemically stable
than their salt forms. Ion exchange resin beads are also susceptible to physical damage.
In systems that have high pressure drop, the hydraulic force of the water, especially in
conjunction with other stresses, can crack the resin beads. In general, cation resins are
stronger than anion resins. As a general guideline, cation resin beds should not be sub-
jected to pressure drops in excess of 25 psi, and anion resin beds should not be subjected
to pressure drops in excess of 20 psi. Pressure drops in excess of 50 psi across the resin
bed may lead to rapid physical deterioration of the resin beads. In poorly designed ion
exchange systems, where the water flow is so turbulent that it causes resin movement,
physical damage will also occur. The movement of the beads causes surface erosion as
the beads collide against one another and chips are broken from the surface of the resin.
Another type of physical stress is due to osmotic changes. This together with physical and
thermal stresses causes much of the damage that occurs during regeneration because the
resin bead changes size rapidly when subjected to differing ionic concentrations and tem-
peratures and pressures. The very worst effect occurs in acid base cycling because heat,
osmotic, and physical stresses all act at the same time. For example, when an acid form
anion resin is be exposed to sodium hydroxide, this type of cycling causes internal cracks
within the resin that ultimately cause the resin beads to break apart. Weakly acidic and
weakly basic exchange resins are more prone than their strongly ionized counterparts due
to the larger volume change that occurs with these resins when they are regenerated from
the exhausted to the regenerated forms. The consequence of physical deterioration of a
resin bed is increased pressure drop and erratic quality and run lengths due to maldistri-
bution and channeling. There is also a problem with the small fragments from the broken
resin beads clogging the underdrain or escaping the underdrain collector and contaminat-
ing the treated water.
By far the largest cause of deterioration of ion exchange resin is oxidation. The vast
majority of drinking water supplies are chlorinated. Chlorine is a strong oxidant that at-
tacks the polymer structure of ion exchange resins. This causes the resin bead to swell,
to increase in moisture, and to become softer as the oxidant attacks the polymer structure
of the resin. It can also attack the functional groups and cause loss of capacity. Capacity
loss is limited to the anion exchangers. Eventually the oxidant can cause the resin bead
to begin to dissolve, which contributes organic contaminants to the treated water. The ef-
fects of oxidation are variable and depend strongly on the degree of cross-linkage used
in the polymerization of the resin bead. Polymer structures with high levels of cross-
linkage are more resistant to oxidation. Oxidation occurs more rapidly at elevated tem-
peratures and in the presence of transition metal catalysts such as iron or copper. Although
the most common oxidant is chlorine, hydrogen peroxide, ozone, and even oxygen in the
feedwater, especially at elevated temperatures, can contribute to the oxidation deteriora-
tion of ion exchange resins.
Although oxidation is the prime cause of loss of resin functionality, there are several
other causes. Because ion exchange resins have different selectivities for varying ions,
and some ions exchange much more slowly than others, some ions with extremely high
selectivity or slow diffusion rates can build up and foul the resin. Such is the case with
ions such as barium on cation resins, and naturally occurring organic acids, and organic