The resin in a water softener has limited exchange capacity,
which must be restored periodically through a process known as
regeneration. The regeneration
process reverses the normal exchange reaction of calcium sodium by
subjecting the resin to the high concentrations of sodium found in sodium
+ 2Na+ ¢ 2RNa + Ca+2
softeners often initiate regeneration of the resin automatically. The frequency of regeneration is controlled by time or quantity
of produced water. Separation of
brining cycles and service flow is essential to avoid passing salt into the
Brine Making. Applying a high concentration of sodium to the resin reverses
the ion exchange reaction. The high
concentration of sodium is provided by a saturated sodium chloride (common
table salt) solution.
(solid) + water
¢ Na+ + Cl- (aqueous)
brine tank is a plastic or fiberglass tank with a perforated salt platform
about 12 to 18 inches off of the bottom. Pellet or granular salt is added to
a brine tank. The brine valve keeps
the tank filled with water to a preset level several inches above the salt
platform. The solid salt is in
contact with the water above the platform and thus dissolves in the water. A reservoir of saturated brine is created
in the brine tank. Over a period of time, the solution will reach saturation
implying that no more salt will dissolve in the water. Saturated brine is 26% salt by
Although saturated brine would give the
most complete regeneration, experience has shown that using a 10% brine
solution minimizes salt usage. To
create a 10% brine solution, the brine is mixed with fresh water through an
eductor. The eductor is a venturi-type device or aspirator, which draws a
vacuum as the flow, passes through an orifice or throat. The vacuum is used to draw brine into the
eductor and mix it with fresh water creating brine that is less than 26%
saturation. The injector on a water softener is designed to dilute saturated
brine, initially at 26%, down to 10 to 14%.
The water to brine ratio is 1 to 1.6 parts water to one part brine.
The eductor must be matched to the softener size to create the correct flow
rate of diluted brine.
Brine Application. The regeneration reaction is relatively
slow and requires a significant contact time between the resin and the brine.
Purolite suggests a brine flow rate of
0.25 to 0.9 gpm/ft3. This is about a tenth of the service
flow rate. They suggest a duration for brining of 15 to 60 minutes applying 4
to 10 lbs of salt per cubic foot (ft3) during that time.
such low flow rates, the brine must be evenly distributed over the surface of
the resin bed. The brine enters softeners through the inlet distributors. On
some larger units, the brine is applied through a separate brine distributor
located only a few inches above the bed. The brine is allowed to flow slowly
through the bed so that the brine travels as a plug or creates a front. Turbulence is avoided. The conversion efficiency is constant so
the amount of resin regenerated is dependent upon the amount of brine or
pounds of salt applied.
The quantity of salt applied is controlled by
limiting the amount of saturated brine solution drawn from the brine
tank. The Accumatic system works on a
level float, which closes the brine valve after the specified amount of
saturated brine has been drawn.