Page 194 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.30 CHAPTER FOUR

to the raw water (for very large volumes) or passing the raw water through an ion exchange
process.

Lime-Soda Addition. The lime-soda ash method uses either hydrated lime or quicklime
along with soda ash. When added to water, these chemicals react with the dissolved calcium
and magnesium carbonate to form insoluble compounds. These compounds precipitate out
of solution and are passed through a filter to remove them. They are then discharged to
drain. This process is usually carried out during the clarification process rather than as a
separate process and is reserved for large volumes of water.

Ion Exchange The ion exchange process is a cation exchange used to remove insoluble
and scale-forming iron, calcium, magnesium carbonate, and other multivalent cations,
which are the primary causes of hardness, and replace them with sodium ions, which do not
contribute to hardness. The removal of these impurities prevents the buildup of insoluble
scale precipates on the piping and reverse osmosis membrane. Ion exchange is accom-
plished by passing the water through a bed of granular sodium cation exchange resin. This
process is commonly called sodium cycle ion exchange.
The resin bed typically occupies about two-thirds of the tank. The other one-third is
needed for expansion of the resin bed during backwash. A generally accepted range of 0.4
3
to 3 gpm/ft of resin is used to determine the volume of resin and cycle time of the unit.
Microbial growth inside the unit is a concern in softening systems used for pharmaceuti-
cal and some laboratory purposes. The water softener is regenerated with a brine solution
that does not destroy bacteria. The liquid brine solution storage and regeneration equipment
also allow microbial growth in storage tanks that are exposed to the atmosphere. An alterna-
tive is to use a dry storage system, which generates a salt solution from water mixed with
salt pellets only when necessary for regeneration. The dry storage system controls micro-
bial growth better than wet systems, but constant maintenance is required to monitor the
brine tank. The quality of the salt in all systems should be checked periodically to ensure
that there are no added substances present.
The regeneration cycle is similar to that previously discussed. The difference is that salt
is used to regenerate the resin bed. Industrial water softeners use rock salt for economy.
Rock salt, because of its high mineral content, requires a special tank called a dissolver to
dissolve the rock salt in water prior to use. The water softener is similar to the schematic
single-bed ion exchanger illustrated in Fig. 4.6.

Ion Exchange System Design Considerations

One of the major decisions that must be made when selecting an ion exchange system is
how much leakage to allow. Leakage is the presence of unwanted ions in the final treated
water. The amount of leakage is a function of the completeness of the resin regeneration.
For water softening, generally accepted leakage amounts range from 0.1 to 1 ppm. Since
total regeneration of the resin bed is impractical, most water softeners operate at one-half to
two-thirds of their ultimate capacity. There is sodium leakage from cation exchangers and
silica leakage from anion exchangers. Normally, mixed-bed units have negligible leakage.
In general, adequate purification of the water stream for high-purity applications is
not possible in one pass through the system, so a polisher is necessary. A mixed-bed ion
exchange system has an initial cost about 74 percent less than a dual-bed system when used
as a polisher. A single pass RO system is about equal to a dual-bed ion exchange system.
Usually, if the water demand for a facility is less than about 40 gpm, the greatest benefit
will be derived from simpler and less costly equipment at the expense of higher operating
costs. For this water demand, it is usual to have a mixed-bed unit without a degasifier, which
is not required. For systems with a requirement of 200 gpm or more, most of the systems

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