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12.42 CHAPTER TWELVE

Acid and Caustic Cycles. For acid base exchanges, the concentrations used are gener-
ally somewhat lower than for salt regenerations. For acids the typical concentration range
is 2% to 10% for strong acids, but concentrations of 0.5% to 1.0% are used with sulfuric
acid regeneration of weak acid resins in high-calcium waters. Hydrochloric acid is usu-
ally injected at concentrations of 5% to 10%. For caustics the typical concentration range
is 4% to 6%.
Sulfuric acid concentration must be controlled when calcium is present. Higher con-
centrations favor more complete regeneration, but calcium sulfate will precipitate if the
concentration is too high. Multiple steps with varying concentrations are sometimes used
with sulfuric acid to reduce waste volumes and maintain regenerant efficiency.
While contact times as short as 10 to 15 min have been employed successfully, in most
cases a contact time of 30 to 40 min is used for cation resins and weakly basic anion
resins. Strongly basic resins may require 60-min injection time and heated regenerant
where silica removal is critical. Even longer contact times are necessary for organic traps,
and systems experiencing organic fouling.
In many units following the introduction of the regenerant chemical, there is a slow
rinse or displacement rinse that is used to help push the regenerant chemical through the
resin and out at approximately the same flow rate as that used during the chemical intro-
duction. This slow rinse displaces the regenerant, ensures adequate regenerant contact
time, and decreases the overall rinse requirement. The final rinse is at the service cycle
rate and is used to purge the last traces of chemical from the ion exchanger and prepare
the system for the next service cycle.
Rinse volume requirements vary considerably depending on the type of resin used, the
adequacy of the internal design, the age of the resin, and the presence of foulants. Most
systems require 7 to 10 bed volumes of final or fast rinse. Rinse requirements can vary
from a minimum of about 2 bed volumes to over 20 bed volumes depending on the type
of system and the endpoint requirements.
Rinse recycle instead of the final fast rinse is sometimes employed as a method of re-
ducing wastewater volume in demineralizers. When the effluent purity during the slow
rinse reaches about the same level as in the raw water, the rinse is recycled to the front
end of the system rather than discharged to waste. Since the effluent quality is changing
rapidly, there is very little load on the resins. Rinse recycle can usually be employed when
the regenerant concentration has dropped to less than about 0.1% in the slow rinse efflu-
ent. Rinse recycle cannot be employed with salt regenerated units as there is no mecha-
nism for the removal of the salt left in the rinse water; however, final rinse waters can be
saved for use in the next backwash or regenerant dilution. This is usually not practical
except on large systems.

Vessel Design

The ion exchange vessel must contain the ion exchange resin beads while allowing the
liquid to flow through them in such a fashion that the resin bed remains packed. The de-
sign must ensure that the various chemical solutions used during regeneration flow through
the media properly. Most ion exchange systems employ pressurized tanks, although there
are a few gravity flow ion exchange systems in use. Gravity flow type of systems are gen-
erally applicable only to very large systems and systems that have sufficient surface area
to allow the necessary flow rates without large pressure drops.
There are two basic choices of tank materials for pressure vessels: fiberglass and steel.
Fiberglass tanks are generally less expensive, but due to the way the fiberglass tank is
made, there are fewer options for accessories and connections that can be made part of
the system design. Fiberglass tanks are lightweight and have an advantage over steel tanks