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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12,45

an operator manually fill the tank with dilution water and concentrated regenerant. This
can expose the operator to dangerous chemicals. The larger, in-line dilution stations do
not routinely expose the worker to the chemical. However, the chemicals are pumped un-
der pressure, and there is the potential danger to workers should the chemical line rup-
ture. Salt exchanges are not considered hazardous, but acid and base exchanges are.
The use of bulk chemical storage generally requires a containment structure in the area
surrounding the dilution station, where the pressurized chemicals are used, or some type
of enclosure to prevent a worker from being accidentally sprayed, should a leak or rup-
ture in the pipe line occur.
For small systems the eductor type of pump is ideal because it can be used for pump-
ing and diluting the regenerant in one step. Larger systems generally employ positive dis-
placement pumps such as diaphragm pumps and gear pumps, although sometimes cen-
trifugal pumps are used. All the various types of chemical pumps have advantages and
drawbacks. Metering pumps have problems with pulsation and are not particularly reli-
able; gear pumps are far better in this respect, but are not free of maintenance problems.
During regeneration the resin bed changes size, and the pressure drop across the resin bed
will vary. It is harder to keep a constant flow rate with a centrifugal pump than with the
other types of pumps used. The chemical dilution station is one of the highest mainte-
nance areas of an ion exchange system.

Waste Collection and Disposal
In the past it was not uncommon for the wastewater leaving an ion exchange system to
go to the sewer with no treatment or monitoring. On the whole, this practice killed innu-
merable fish and helped to significantly pollute waterways. Today we are still paying for
these tragedies. The unwanted ions that are regenerated off the resin are present in the
wastewater in concentrated form. In addition, the wastewater contains either concentrated
salt solution or the concentrated acid and base solutions from the regenerant chemicals
used. The average ionic strength of the wastewater often exceeds 10,000 ppm, and it can
be as high as 60,000 ppm or more. The concentration factor is usually in the range of 10
to over 100. Most ion exchange systems today employ a wastewater tank where the re-
generant waste flow is placed prior to treatment and disposal. As a minimum, the waste-
water is neutralized and monitored to ensure that it is neutral prior to being discharged.
In some areas of the country, and for some types of wastewater, it is necessary to im-
pound the spent regenerant portion of the waste and to send it off-site to a place where it
can be further treated. Depending on the nature of the undesirable constituent and the lo-
cation within the country, this cost of waste collection and disposal can exceed the oper-
ating cost of the ion exchange equipment itself and is an increasingly important concern
in the design of ion exchange systems.

Disposing of Regenerant Wastes. State and municipal codes should be examined to see
what limitations are for discharging regenerant wastes and for specific objectionable sub-
stances such as barium, radium, and suspended solids. Community or municipal size sys-
tems should include procedures for environmentally safe waste disposal. Regenerant treat-
ment can include brine reclamation in softening installations, precipitation and removal
of a particular contaminant, or collecting the contaminated spent brine and hauling it to
a central treatment facility that is equipped to safely dispose of the waste. In cases where
municipal, district, or sanitation district codes exist, they should be followed, providing
they do not exceed regulations with higher authority.
The expected level of a specific contaminant in the waste can easily be estimated at
the design stage. It takes about 80 gal/ft 3 to regenerate an ion exchange resin, including