Operating Characteristics of Strong-Base
Anion Exchange Reactors
WILLIAM S. MIDKIFF, Assistant Professor
New Mexico State University
Las Cruces, New Mexico
WALTER J. WEBER, JR., Professor
The University of Michigan
Ann Arbor, Michigan
INTRODUCTION
As water quality and waste discharge requirements continue to rise, so do
needs for development and refinement of techniques for removing mineral
constituents from wastewaters. Certain inorganic anions are of principal interest
because of their significance as algal nutrients; namely, nitrate and phosphate.
Because of its simplicity and completeness, ion exchange presents an attractive
alternative to reverse osmosis and electrodialysis for treating wastewater for removal
of dissolved minerals. Whereas cost per equivalent of salt removed increases with
decreasing salt concentration for electrodialysis and reverse osmosis operations, unit
removal costs are relatively constant for ion exchange. Economically, then, ion
exchange is particularly attractive at lower salt concentrations, and as higher water
quality levels are approached.
Several aspects of the ion exchange process have been examined in the present
to work to determine operating characteristics, design parameters, and the feasibility
of ion exchange for removal of nitrates and phosphates from waters and wastewaters
containing sulfates, chlorides, and bicarbonates. Factors studied include selectivity,
hydraulic loading, and depth of resin bed.
Process design for treatment of multisolute systems should allow for the effects
of differences in selectivities of an exchange resin for various ions. All ions have
selectivities relative to one another which determine the tendency for them to be
preferentially retained by a resin. Knowing how effectively a specific ion will
compete for exchange sites is important in establishing the efficiency of the resin for
that ion in a particular application.
The current trend toward ion exchange systems with high hydraulic loading
rates (1,2,3) is based in part on the development of resins with high rates of exchange
reaction. If flow velocity can be increased without substantially decreasing efficiency
of exchange, a smaller cross-sectional area of resin bed can be employed.
The depth of a resin bed also will affect efficiency of operation. The
exchanging ions form a concentration profile in the beds as shown in Figure 1. This
profile has a finite length and occupies a definite zone within the bed. The relative
length of the profile, and thus the percentage of the bed occupied by the profile
zone, varies with bed depth. A proper understanding of the effect of bed depth on
the breakthrough profile is useful in reactor design.
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