Page 192 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.28 CHAPTER FOUR

The quality of the chemicals used for regeneration has an important effect on the main-
tenance of exchange capacity. Although chemically pure ingredients are not required, some
contaminants found in these chemicals collect on the resins and eventually will cause dif-
ficulty in operation.
Technical grade acids, free of oils and other organic materials, are acceptable for regen-
eration of cation resins. They should be 66 degrees Baume (°Bé), free of suspended matter
and light in color. They should mix freely with water and not form any precipitate. Acid-
containing inhibitors should not be used. Sulfuric acid is usually the most economical for
large-scale use. Hydrochloric acid should be technical grade, a minimum of 30-percent HCl
by weight (18°Bé) and shall not contain excessive amounts of iron and organic materials.
HCl obtained by the salt-aid or the hydrogen-chlorine process has been found satisfactory.
HCl obtained by the hydrolysis of chlorinated organic chemicals should be avoided, par-
ticularly if used to treat potable water.
Anion exchange resins are regenerated with 76-percent sodium hydroxide, which must
be low in iron, chlorides, and silica to avoid fouling the strong-base anion exchangers.
Weak-base anion exchangers are most economically regenerated with technical grade flake
sodium hydroxide. Strong base exchangers are best regenerated using nylon or rayon grade
sodium hydroxide, also 76 percent. If purchased in a 40 percent solution, use the grade
previously indicated. All caustics must have a maximum of 2-ppm chlorates.
The flush cycle is the shortest. It is a cocurrent process whose purpose is to flush away
any remaining residue of the regeneration liquids and to repack the bed in preparation for
the new run.
The entire regeneration cycle typically takes about 1 h. If the process requires continu-
ous operation, a duplicate set of equipment is installed so that one is in use while the other
is being regenerated.
To estimate the frequency of regeneration, first, find from the manufacturer the exchange
capacity in grains of the selected resin bed. Next, from the analysis of the raw water find
the average TDS level and convert this figure into grains per gallon. Dividing the flow rate
in gallons into the grain capacity of the resin bed will give the time it takes to saturate the
resin bed before regeneration is required.

Service Deionization
Service deionization is not another form of deionization but rather, a different type of
equipment arrangement. In the regenerable type, the DI equipment is permanent and the
regeneration is done on site by operations or maintenance personnel, who must handle and
store the chemicals used for regeneration.
The service DI system uses individual cartridges or tanks for the anion, cation, and
mixed beds. When the individual cartridges are exhausted, the supplier replaces them with
recharged units on site and the exhausted cartridges are removed to be regenerated at the
supplier’s premises. This arrangement considerably reduces the initial cost of the equip-
ment, eliminates the need to store chemicals, and frees the operations or maintenance per-
sonnel from the job of regenerating the units. In addition, it saves water that does not have
to be used for backwash. However, the operating costs are higher than for the permanent
bed type.

Continuous Deionization
Continuous deionization (CDI), also known as electrodeionization (EDI), is a continuous
water purification process that uses direct current, an alternating arrangement of cation- and
anion-permeable ion exchange membranes that form parallel flow compartments (concentrat-
ing compartments) on either side of an additional flow compartment containing a thin layer of

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