UNITED WATER INC.
AquAtek AtWS-SerieS uSer Guide
6
PRINCIPLES OF ION EXCHANGE IN THE SOFTENING PROCESS
1. ION EXCHANGE SOFTENING PROCESS
In order to understand what happens in the ion exchange softening process, it will first be necessary
to understand the meaning of the terms that are used in the explanation. HARD WATER, CATION EX-
CHANGER, and BRINE are therefore defined below and then used to show how the ion exchange pro-
cess works.
A. Hard Water - All natural waters contain much the same dissolved impurities, but in widely varying
amounts. There are always enough ANIONS (-) present to balance the CATIONS (+), but anions have
no effect on the ion exchange softening process. Water will be HARD, if it contains large amounts of
Calcium (Ca++) and/or Magnesium (Mg++) ions.
B. Brine - water in which SALT has dissolved. SATURATED brine contains as much salt as it is possible
to hold in solution (approx. 26% to 27%).
SALT - SODIUM CHLORIDE (NaCl). When dissolved in water it splits up (ionizes) into Sodium (Na+) ions
and Chloride (Cl-).
SATURATED BRINE - contains a very great number of Na+ and Cl- ions (concentration is over 200,000
ppm). When used to regenerate a CATION EXCHANGER, only the Sodium ions (Na+) are put to use.
The Chloride ions (Cl-) do not work in the process.
C. Cation Exchanger - a solid material that has a very large number of “REACTION POINTS”. These
reaction points have NEGATIVE (-) electric charges, and are able to attract and hold CATIONS, which
are POSITIVELY (+) charged (much the same way as the way opposite poles of a magnet attract
each other).
D. The Softening Process - When Ca++ or Mg++ ions have occupied most of the reaction points,
hardness will begin to slip through the bed in increasing amounts. This rise in hardness in the effluent
is an indication that the effective capacity of the CATION EXCHANGER has been reached. The CATION
EXCHANGER must then be regenerated to restore it to its original condition.
E. Regeneration - A solution of NaCl is applied to the CATION EXCHANGER at a controlled rate and the
softening process is reversed. The Ca++ and Mg++ ions are driven off of the CATION EXCHANGER and
replaced with Na+ ions. At the end of regeneration, the “spent” brine is rinsed away and the REGEN-
ERATED CATION EXCHANGER, with its reaction points again occupied by sodium ions -is again able to
soften HARD WATER.
2. QUALITY OF EFFLUENT
If the hard water contains less than 500 ppm (about 30 grains) of Calcium, Magnesium and Sodium
salts, all expressed as CaCO3, it will be found that the effluent from a softener will contain an average
of not more than 30 ppm actual total hardness (Zero hardness by the SOAP TEST). However, as the
total CATION concentration in the hard water increases above 500 ppm, the average hardness in the
effluent will also increase proportionately.
The reason for this is that when the sodium salt - those present in the raw water plus those formed by
the exchange reactions - are present in high enough concentrations, they cause a “back-regenera-
tion” effect at the same time as the softening process is taking place. This effect prevents as complete
a removal of calcium and magnesium as would otherwise be possible.
It is often possible to reduce the average hardness in the effluent below normally expected concentra-
tions, by using a greater amount of salt than usual for regeneration.
Normal Softening Cycle - At the start of a normal softening cycle, the hardness in the effluent drops
rapidly as the residue of hardness ions left in the bed at the end of the rinse are forced out. The effluent
hardness reaches a certain minimum value and remains at approximately this concentration for the
major part of the softening run.
