UFC 3-240-13FN
25 May 2005
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Hardness
Sodium
Sodium
Hardness
Zeolite
Chloride
Zeolite
Chloride
The completeness of the regeneration is dependent upon the strength of the
salt solution (brine) used and the length of time the solution is in contact with the resin.
After the resin is regenerated, it can be used again and again (after regeneration) to
continue to remove hardness from water. The following procedure is typical for an ion
exchange resin regeneration process.
2-2.5.1.2.1 Backwash. Before the exhausted resin bed is regenerated, it must be
backwashed by flowing water from bottom to top. The flow rate must be adequate to
remove any solids that have been caught on top of the bed. The resin bed volume will
also be expanded by about 50% due to the backwash flow; the volume of expansion will
be dependent upon the flow rate. The backwash flow rate should be controlled so that it
will not sweep ion exchange resin out of the softener to the waste collection area. A
backwash rate of 2.72 to 4.07 liters per second per square meter (4 to 6 gallons per
minute per square foot) of bed surface for about 10 minutes is normal, but the
manufacturer's recommendations should be followed.
2-2.5.1.2.2 Brining. Next comes the addition of salt, a process known as "brining." A
10% (by saturation) solution of sodium chloride salt brine is slowly added with a down-
flow rate of 1.11 to 2.22 liters per second per cubic meter (0.5 to 1 gallon per minute per
cubic foot) of bed volume for about 30 minutes. Rock salt is preferred to granulated salt
as a brining salt because it is equally as effective, less expensive, and less prone to
cake. Some installations may start with a solution of concentrated or saturated brine,
which must be diluted before use in the brining step. The salt required and the capacity
regenerated is shown for a typical resin in Table 2-3; however, the manufacturer's
instructions should be followed, if available.
Table 2-3. Salt Required for Regeneration of a Commonly Used Cation Resin
Salt Use per liter of Ion Exchange
Capacity of Ion Exchange Material
Material
kg/m3 (lb/ft3)
80 (5.0)
43,450 (19,000)
120 (7.5)
55,130 (24,000)
160 (10.0)
61,615 (27,000)
240 (15.0)*
73,290 (32,000)*
* Practical upper limit for exchange capacity. This upper limit and the actual dose per capacity
relationship may vary with the resin; the manufacturer's instructions should be followed.
2-2.5.1.2.3 Slow Rinse. A slow rinse follows the brining step and is performed at the
same rate as the brining step. The rinse is performed with downflow through the
softener. One to 3 bed volumes of fresh water are used to remove most of the excess
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