Softeners: Why is the Brine
Diluted?
By James McDonald, PE, CWT
Originally Published: CSCN – July 2006
I
t is reasonable to expect that the higher the brine concentration, the greater
will be the driving force to push the hardness off the ion exchange groups on
the resin and replace or exchange the hardness ions with the sodium. This is
a reasonable expectation, but some potential problems may be encountered.
The highest sodium chloride concentration that can be obtained at normal
temperatures is approximately 26% by weight or 100 salometer degrees.
A high-concentration salt solution tends to dehydrate (remove water from)
any substance that has water in it with a lower salt content. The substance
with the lower salt concentration will try to pass water to the solution with
the higher salt concentration. This process is called osmosis. The greater the
difference in salt content, the greater the pressure against the solid surface on
the ion-exchange resin bead, and the greater the driving force by the water to
pass to the other side and dilute the high salt content solution.
As dehydration of the resin occurs at the outer skin of the resin bead first, the
surface of the bead will shrink or try to shrink. Like the drying of a mudflat
during a long drought, the surface will crack as the water is removed, and the
resin beads can split or crack. This phenomenon is called osmotic shock.
There is also some shock experienced with the rehydration of the resin beads
after brining, when the resin is rinsed to remove the brine. In this case, the
resin beads swell as the salt is rinsed from the beads, causing another
physical strain on the beads. In spite of the potential problem of osmotic
shock with concentrated brine regeneration, it is used when the maximum
capacity is required and resin replacement costs can be justified.
Reducing the concentration of the brine does reduce the osmotic shock
problem, and in the normally used concentration range of 8% to 12% by
weight (30 to 40 salometer degrees), there is very little problem with reduced
resin life. Most industrial and domestic water softeners are designed to
operate in this dilute brine concentration range.
Another reason for dilution of the brine is to have a sufficient volume of
regenerant solution to uniformly contact all the resin within the distribution
system, especially at the lower regenerant levels.
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Reference: Practical Principles of Ion Exchange Water Treatment, Dean L.
Owens, Tall Oaks Publishing, Inc., 1995, pp76-77.
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