Civil Engineering Reference
In-Depth Information
regenerant be used, which can prove to be very costly. As a result, only a portion of
the available exchange capacity is normally restored during the regeneration cycle. The
extent of regeneration is referred to as the
regeneration level
.
The regeneration efficiency is higher for weak ionic resins than for strong ionic
resins, because the weak ionic resins' affinity is higher for the H
and OH
-
ions. This
means that regeneration is more favorable for weak ionic resins, with the result that
less regenerant is required to achieve the same degreee of exchange. This can be
explained by considering that the value of the selectivity coefficient for the regenera-
tion reaction is the reciprocal of the selectivity coefficient for the initial exchange
reaction.
Rinsing
After the regeneration step, the ion-exchange resin must be rinsed free of
excess regenerant before being put back into operation. The rinsing procedure consists
of using a slow and fast rinse with product water. The slow rinse of one bed volume
displaces regenerant, and the waste from this rinse is combined with the regenerant
brine for disposal. The fast rinse washes away excess ions, and the waste from that
rinse is often collected and used for regenerant dilution water.
Brine Disposal
Potential techniques for brine disposal are discharge to sanitary sewers, evaporation
ponds (lined or unlined), the ocean or an estuary, or disposal wells. In all cases, the
disposal technique must be approved by regulatory agencies, and, in the case of sewer
disposal, by the sewering agency. Particular attention must be given to degradation of
groundwater and surface water quality. Chlorination may be necessary in some cases.
The cost of brine disposal is a key factor in the overall economic analysis that
should be made to select the treatment technique.
Pilot Testing
Use of scale-model ion-exchange reactors is recommended to establish design and
operational parameters for the particular water supply being evaluated. Pilot testing
will help in optimization of design parameters, and in large systems may avoid costly
errors in treatment process design.
In smaller systems where the cost of pilot testing may not be economically justified,
the knowledge and experience of ion-exchange equipment manufacturers and resin
suppliers is a valuable resource. Many manufacturers will analyze representative water
samples and will recommend design criteria and resin type. The cost for such services
is usually minimal. A prudent measure is to obtain recommendations from at least two
manufacturers.
Softening Applications
Softening is the most commonly used ion-exchange application in water treatment.
The resins of choice are strong-acid, cation-exchange resins in the sodium form. Typ-
ical resins are Amberlite IR-120 Plus, Duolite C-20, Dowex HCR, and Permutit
Q-100.
The resin can be contained in a pressure steel vessel or an open concrete basin.
Pressure vessels are most commonly used in small plants, whereas open concrete
basins are usually in larger facilities.
