Water Treatment III, Sodium Zeolite Softening
Steam Generation Systems, Inc.
Water Treatment III PDF
exchange is the process in which materials exchange one ion
for another, hold it temporarily, and release it to a regenerating
solution. These materials are widely used to treat raw water
supplies that contain dissolved salts. Today, the most commonly
used material is an ion exchange resin. Resins are plastic
beads to which a favorable ion has been chemically attached
which can be exchanged for unwanted ions dissolved in the
raw in the water supply. Once the resin has given up or exchanged
most of its favorable ions, it is said to be exhausted and
needs to be regenerated by coming in contact with a strong
solution of ions called the regenerant.
zeolite softeners historical used natural zeolite materials
but have now been replaced with exchange resins made of polystyrene.
These resins have sodium ions loosely attached and will readily
give up the sodium for a more desirable ion such as calcium
and magnesium. This exchange is only for cations of positively
charged ions. This is why sodium zeolite resin is referred
to as a cation exchange resin. .
primary exchange reactions occurring at the surface of a sodium
cation exchange resin are
...where "R" represents the solid resin.
water to be softened is passed through the vessel containing
resin. Calcium and magnesium ions are exchanged for the sodium
ions in and on the resin beads. The sodium then takes the
place of the calcium and magnesium in associating with the
anionic species in solution to provide charge balance.
sodium zeolite treated water is nearly free of all hardness,
some hardness leakage normally does occur. The amount of leakage
depends primarily on the raw water hardness, sodium concentration
and softener regeneration level.
plot of the softener effluent profile shows a low, nearly
constant effluent hardness level until the ion exchange resin
nears exhaustion. At this point, the hardness level usually
increases quite rapidly and resin regeneration is required.
resin bed is regenerated by reversing the softening reactions.
Exhausted sodium zeolite resin is regenerated by exposure
to a concentrated brine solution - sodium chloride. Analogously,
strong acid cation resin is regenerated by exposure to a strong
brine used for sodium zeolite regeneration is 26% by weight
sodium chloride. However, because of the dilution of the brine
as it is forced through the softener vessel, the actual application
strength of the brine is closer to 10% when it reaches the
cations are exchanged at resins to different degrees. The
order of preference or selectivity for a specific cation to
be exchanged is a complex function of charge and strength
of solvation. In dilute solutions, the general rule is that
ions with the highest charge and the lowest strength of solvation
are preferentially exchanged. A partial list of relative affinities
for ion exchanged at cation exchange resins are contained
in the adjacent table.
systems consist of a tank, valves and a means of transporting
the brine to the softener tank. Usually the softener tank
is a steel, vertical shell, pressure vessel with dished heads.
The interior of the tanks are sometimes lined with corrosion
resistant coatings to prevent attack by the
brine regenerating solutions. The tank includes a service
and rinse water inlet distributor, freeboard or the headspace
from the top of the resin tank, a regenerant distribute, a
bed of ion exchange resin, and sometimes a supporting medium
or outlet distribution system.
inlet distributor located in the top portion of the tank,
consists of a baffle plate or hub and lateral system similar
to the spokes on a wagon wheel. Strainers are frequently incorporated
as parts of the lateral design. Special care is taken to avoid
directing the water against the tank wall that could cause
channeling of the flow around the tank perimeter.
inlet assembly is designed to distribute the incoming water,
preventing it from impinging in the resin bed and hollowing
out cavities that would cause the flow to channel, reducing
and effluent quality. The distributor also acts as a collector
for the backwash water that goes into the sewer.
freeboard space allows the resin to expand without loss to
drain during backwashing and should be at least 100% of the
resin bed volume.
regenerant distributor, which is normally located about 4
to 6 inches above the resin bed, usually consists of a header
lateral system that spreads the brine uniformly over the resin.
bed of resin, operating in the sodium cycle, softens the water.
The quantity of water to be treated per regeneration, flow
rate, and the regenerant level employed. A minimum bed depth
of 24 inches is recommended for all systems.
underdrain system, located in the bottom of the softener,
includes the media, which support the resin bed, and/or underdrawn
strainers. It collects the treated water, waste brine and
rinse water, and distributes the backwash water. The underdrawn
system must collect water evenly from all portions of the
bed during the service, brine and rinse operations. If the
collection is not uniform, channeling may occur which will
lower capacity and increase hardness leakage. Resin waste
may occur either through carryover with the backwash water
or as the result of an upset in the support material.
sodium zeolite softener operates through two basic cycles:
the service cycle, which produces soft water, and the regeneration
cycle, which restores the exhausted resin to capacity:
the service cycle, raw water enters the softener through the
inlet distributor, flows through the resin bed, is collected
by the underdrawn system and is transferred to the point of
use. The flow to the softener should be as constant as possible
and sudden large surges or frequent on-off operations should
softening cycle is conducted most efficiently at a flow rate
of approximately 6 to 8 gallons per minute per square foot
of resin surface area, with all parallel units on line. Equipment
manufacturers generally design zeolite softeners for normal
operation within this range, but provide for periodic flow
rates as high as 15 gallons per minute per square foot. This
allows total softened water requirements to be met while parallel
units are regenerated.
operational or above 15 gallons per minute per square foot
may result in channeling bed compaction, leakage, or premature
hardness breakthrough. Channeling is a very involved subject;
it can generally result in poor softener operation and loss
in total throughput .
which can contribute to channeling include poor backwash flow
rates, which do not adequately fluff up the resin bed to prepare
for regeneration, as well as broken or partially obstructed
inlet headers which "channel" the water flow through
one particular area of the softener. Operation significantly
below normal service flow rates (3 gallons per minute per
square foot or less) can also produce difficulties such as
decreased capacity and leakage. This potential exists because
at low flow rates, water is not forced through the inlet distributor
at a velocity high enough for uniform distribution across
the whole bed.
regeneration consists of four steps: backwash, brining, slow
rinse and fast rinse.
During the exhaustion or service cycles, the downward flow
of raw water caused suspended matter to accumulate on the
resin bed. The resin is an excellent filter medium. Backwashing
is an upward flow of water which passes through the underdrain
system, up through the resin bed, and out the service water
distributor to waste. This reverse flow lifts and expands
the resin bed by placing each bead in motion. In this manner,
the bed is regraded while particulates and resin fines are
or classification of the zeolite resin brings smaller beads
to the top of the unit; larger beads go to the bottom. This
enhances proper brine distribution. Expansion releases material
accumulated within the resin bed and fluffs the bed to allow
for efficient brine-resin contact. Particulate matter and
resin fines must be removed to prevent channeling, high pressure
drop and poor kinetics.
should be carried out for a minimum of 10 minutes or until
the backwash water effluent is clear. The backwash water flow
rate should be sufficient to produce a minimum of 50% bed
expansion, yet not excessive enough to cause loss of resin.
percent bed expansion resulting from a set flow rate is a
function of the backwash water temperature. At a given flow
rate, the lower the temperature, the more the bed is expanded.
Due to increased viscosity of the water, adjustments in backwash
water flow rates should be made as water temperatures vary
seasonably. Backwash rates usually vary from 4 to 8 (ambient
temperature) and 12 to 15 (hot service) gpm per square foot
of tank area, but each manufacturer's recommendations should
be carefully followed.
The brine regenerant stream enters the softener through the
regenerant distributor, flows downward through the resin bed,
is collected by the underdrain system and then discharged
potential for scale and deposit build in boilers has created
a large demand for softened water. The ability of the sodium
zeolite softener to satisfy this demand economically makes
its use appealing for preparing boiler feedwater, and many
types of chemical process waters. Compared to other softened
methods, sodium zeolite units offer many advantages:
treated water has a very low scaling tendency because this
method reduces the hardness level of most water supplies
to less than 2 ppm.
Operation is simple and reliable; automatic regeneration
controls are available at reasonable cost.
Regeneration is accomplished with inexpensive, easy-to-handle
Waste disposal usually presents not problem.
Within limits, variations in the water flow rate have little
effect on treated water quality.
Efficient operation can be obtained in almost any size unit,
making sodium zeolite softeners suitable for both large
and small installations.
sodium zeolite softeners efficiently reduce the calcium and
magnesium content of a raw water supply, they have little
effect on silica or dissolved solids content and no effect
on alkalinity Sodium zeolite softeners will not function efficiently
on turbid waters. City and well water supplies are usually
quite suitable for use but surface water often must be clarified
contaminants such as iron or aluminum
in raw water supply can foul the resin surfaces and be detrimental
to the exchange process. Aluminum may be present in the raw
water supply, but problems associated with it generally arise
from the use of aluminum compounds for flocculation. Whenever
iron or other metallic contaminants are present, the softener
must be backwashed thoroughly and a suitable resin cleaner
applied during regeneration.
strong oxidizing agents in the raw water will degrade
ion exchange resin and shortens its total service life. Chlorine
is a powerful oxidant and if present, should be eliminated
(de-chlorination) with a reducing agent such as sodium sulfite.