Civil Engineering Reference
In-Depth Information
salt delivery. Large plants can take advantage of the lower cost of bulk delivery by
either truck or rail; smaller plants can use salt delivered in bags. Some large plants
use unrefined rock salt, at a significant cost savings. However, unrefined rock salt
contains sand and silt, which require removal. Sand precipitates during brine produc-
tion, whereas silt removal usually is accomplished in a separate holding basin. Periodic
basin cleaning is necessary to remove sand and silt.
Saturated brine is removed from the basin by either pumping or a hydraulic eductor.
Maintaining the brine at a consistent concentration is essential to assure that the proper
salt dose is used during each regeneration. A sampling tap or a meter should be
provided.
Water addition to the brine tank should be done with an air gap to prevent any
possible cross-connection. Also, an overflow from the brine tank to a sewer should be
provided. The density of salt is approximately 70 lb / cu ft.
Product Water Blending
Product water from the exchanger will have slight hard-
ness (about 1 percent of influent hardness), due to bed ''leakage.'' Because a product
water hardness concentration of about 100 mg / L as CaCO
3
is usually desirable, blend-
ing should be done to reduce operating costs. This may be accomplished by blending
low-hardness product water with unsoftened water in the correct proportion to achieve
the desired final water hardness, or by running an exchanger past the point where the
normal background leakage concentration is exceeded. The latter technique requires
sufficient storage capacity for softened water.
Nitrate Removal Applications
Nitrate ions can be removed from water supplies using strong-base ion-exchange resins
operating in the chloride cycle. Resins commonly used are Duolite A-101D and A-
104, Dowex SAR, Ionac A-550, and Amberlite IRA-900 and IRA-910. These resin
types can be regenerated using sodium chloride brine.
These resins will remove a number of anions in addition to nitrate, including sulfate,
nitrate, chloride, and bibcarbonate. The order of affinity for these anions is:
↑
Sulfate
•
Nitrate
•
Increasing
affinity
Nitrite
•
Chloride
•
Bicarbonate
•
Because sulfate ions are preferentially removed over nitrate ions, and nitrate ions
are preferentially removed over nitrate, chloride, and bicarbonate ions, as the resin
becomes exhausted, bicarbonate breaks through first, followed by chloride, then nitrate,
then nitrite. The sulfate ion is the most strongly sorbed, and hence will displace pre-
viously removed nitrate ions at the end of the service cycle. This is an undesirable
situation, and can result in a higher nitrate concentration in the exchanger product
water than in the exchanger feedwater. The service cycle must be terminated before
this point.
