FOULING AND DEGRADATION
Resin can become fouled with contaminants that
hinder the exchange process.Figure shows a resin fouled with iron. The resin
can also be attacked by chemicals that cause irreversible destruction. Some
materials, such as natural organics (Figure), foul resins at first and then degrade
the resin as time passes. This is the most common cause of fouling and
degradation in ion exchange systems, and is discussed under "Organic
Fouling," later in this chapter.
of Resin Fouling
Iron and Manganese.
Iron may exist in water as a ferrous or ferric inorganic salt or as a sequestered
organic complex. Ferrous iron exchanges in resin, but ferric iron is insoluble
and does not. Ferric iron coats cation resin, preventing exchange. An acid or a
strong reducing agent must be used to remove this iron. Organically bound iron
passes through a cation unit and fouls the anion resin. It must be removed
along with the organic material. Manganese, present in some well waters, fouls
a resin in the same manner as iron.
Aluminum is usually present as aluminum hydroxide, resulting from alum or sodium
aluminate use in clarification or precipitation softening. Aluminum floc, if
carried through filters, coats the resin in a sodium zeolite softener. It is
removed by cleaning with either acid or caustic. Usually, aluminum is not a
foulant in a demineralizer system, because it is removed from the resin during
a normal regeneration.
Hardness precipitates carry through a filter from a precipitation softener or
form after filtration by post-precipitation. These precipitates foul resins
used for sodium zeolite softening. They are removed with acid.
Sulfate Precipitation. Calcium sulfate precipitation
can occur in a strong acid cation unit operated in the hydrogen cycle. At the
end of a service cycle, the top of the resin bed is rich in calcium. If
sulfuric acid is used as the regenerant, and it is introduced at too high a
concentration or too low a flow rate, precipitation of calcium sulfate occurs,
fouling the resin. After calcium sulfate has formed, it is very difficult to
redissolve; therefore, resin fouled by calcium sulfate is usually discarded.
Mild cases of calcium sulfate fouling may be reversed with a prolonged soak in
Barium sulfate is even less soluble than calcium sulfate.
If a water source contains measurable amounts of barium, hydrochloric acid
regeneration should be considered.
Oil coats resin, blocking the passage of ions to and from exchange sites. A surfactant
can be used to remove oil. Care must be exercised to select a surfactant that
does not foul resin. Oil-fouled anion resins should be cleaned with nonionic
Microbiological Fouling. Microbiological fouling can
occur in resin beds, especially beds that are allowed to sit without service
flow. Microbiological fouling can lead to severe plugging of the resin bed, and
even mechanical damage due to an excessive pressure drop across the fouled
resin. If microbiological fouling in standby units is a problem, a constant
flow of recirculating water should be used to minimize the problem. Severe
conditions may require the application of suitable sterilization agents and
Silica fouling can occur in strong base anion resins if the regenerant
temperature is too low, or in weak base resins if the effluent caustic
from the SBA unit used to regenerate the weak base unit contains too much
silica. At low pH levels, polymerization of the silica can occur in a weak base
resin. It can also be a problem in an exhausted strong base anion resin. Silica
fouling is removed by a prolonged soak in warm (120 o F) caustic soda.
of Irreversible Resin Degradation
Oxidizing agents, such as chlorine, degrade both cation and anion resins.
Oxidants attack the divinylbenzene cross-links in a cation resin,
reducing the overall strength of the resin bead. As the attack continues, the
cation resin begins to lose its spherical shape and rigidity, causing it to
compact during service. This compaction increases the pressure drop across the
resin bed and leads to channeling, which reduces the effective capacity of the
In the case of raw water chlorine, the anion resin
is not directly affected, because the chlorine is consumed by the cation resin.
However, downstream strong base anion resins are fouled by certain degradation
products from oxidized cation resin.
If chlorine is present in raw water, it should be
removed prior to ion exchange with activated carbon filtration or sodium
sulfite. Approximately 1.8 ppm of sodium sulfite is required to consume 1 ppm
Oxygen-saturated water, such as that found following
forced draft decarbonation, accelerates the destruction of strong base exchange
sites that occurs naturally over time. It also accelerates degradation due to
Thermal degradation occurs if the anion resin becomes overheated during
the service or regeneration cycle. This is especially true for acrylic resins,
which have temperature limitations as low as 100 o F, and Type II strong base
anion resins, which have a temperature limit of 105 o F when in the hydroxide
Organic fouling is the most common and expensive
form of resin fouling and degradation. Usually, only low levels of organic
materials are found in well waters. However, surface waters can contain
hundreds of parts per million of natural and man-made organic matter. Natural
organics are derived from decaying vegetation. They are aromatic and acidic in
nature, and can complex heavy metals, such as iron. These contaminants include
tannins, tannic acid, humic acid, and fulvic acid.
Initially, organics block the strong base sites on a
resin. This blockage causes long final rinses and reduces salt splitting
capacity. As the foulant continues to remain on the resin, it begins to degrade
the strong base sites, reducing the salt splitting capacity of the resin. The
functionality of the site changes from strong base to weak base, and finally to
a nonactive site. Thus, a resin in the early stages of degradation exhibits
high total capacity, but reduced salt splitting capacity. At this stage,
cleaning of the resin can still return some, but not all, of the lost operating
capacity. A loss in salt splitting capacity reduces the ability of the resin to
remove silica and carbonic acid.
Organic fouling of anion resin is evidenced by the
color of the effluent from the anion unit dur-ing regeneration, which ranges
from tea-colored to dark brown. During operation, the treated water has higher
conductivity and a lower pH.
The following methods are used, either alone or in combination, to reduce
Prechlorination and clarification. Water
is prechlorinated at the source, and then clarified with an organic removal
Filtration through activated carbon. It
should be noted that a carbon filter has a finite capacity for removal of
organic material and that the removal performance of the carbon should be
Macroporous and weak base resin ahead of
strong base resin. The weak base or macroporous resin absorbs the organic
material and is eluted during regeneration.
Specialty resins. Acrylic and other
specialty resins that are less susceptible to organic fouling have been
Inspection and Cleaning.
In addition to these preventive procedures, a program of regular inspection
and cleaning of the ion exchange system helps to preserve the life of anion
resin. Most cleaning procedures use one of the following:
Warm (120 o F) brine and caustic. Mild
oxidants or solubilizing agents can be added to improve the cleaning.
Hydrochloric acid. When resins are also fouled
with significant amounts of iron, hydrochloric acids are used.
Solutions of 0.25-0.5% sodium
hypochlorite. This procedure destroys the organic material but also
significantly degrades the resin. Hypochlorite cleaning is considered a last
It is important to
clean an organically fouled resin before excessive permanent degradation of the
strong base sites occurs. Cleaning after permanent degradation has occurred
removes significant amounts of organic material but does not improve unit performance.
The condition of the resin should be closely monitored to identify the optimum
schedule for cleaning.