Page 191 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.27

FIGURE 4.8 Typical mixed bed-ion exchanger.

must be neutralized prior to discharge into a public sewer system. It is common practice to
combine the acid and caustic waste streams to neutralize the effluent to the greatest extent
possible. Additional acid or caustic may have to be added to the final effluent to produce a
pH acceptable to the local authorities.
Backwashing is a countercurrent operation that accomplishes two purposes. The first
is to remove any particulates that have accumulated in the resin bed and on the beads. The
second is to regrade the resin beads so that new beads are on the top of the bed, which
is where the heaviest duty from the beads is required. This is done by having the resin
bed expanded by the reverse flow of water from its normal packed condition during use.
Individual manufacturers have established the required flow rate of backwash that should
be maintained. Too high a flow rate will blow resin out of the tank and into the drain. The
flow rate of water should be high enough to scrub the beads together to increase the clean-
ing action. Higher than recommended flow rates will only waste water and provide no
additional benefits.
The two chemicals used to regenerate cation resin beds are either a 93 percent solution
of sulfuric acid or a 30 to 32 percent solution of hydrochloric acid, also called mureatic acid.
As they flow through the columns, they replace the retained cations with hydrogen ions
from the acid. Hydrochloric acid is used most often because it has the greatest efficiency;
the amount of hydrochloric acid used is one-fourth the amount of sulfuric acid. Sulfuric
acid is much lower in cost and is used when there is a large quantity of resin to be regener-
ated, making its use practical. The chemical used most often for regenerating anion resins
is a 40 percent mixture of sodium hydroxide, which replaces the retained anion ions with
hydroxyl ions. For mixed-bed units, the resins must be separated prior to regeneration.

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