Environmental Engineering Reference
In-Depth Information
39.4 Electrodialysis Reversal and Electrodeionization
An alternative to RO, the capabilities of electrodialysis reversal (EDR)—a process com-
mercialized by GE in the 1950s—are helping many municipal and industrial users in
desalination applications where high water recovery is important for resource con-
servation and cost control. Both EDR and RO use semipermeable membranes to ilter
out dissolved ions from water. However, whereas RO uses the application of pressure
to overtake osmotic pressure and force water through the membrane, EDR uses volt-
age potential and polarity reversal to remove unwanted constituents by pulling them
through the membranes.
During the last 10-15 years, numerous advances in membrane and system technol-
ogy have made EDR an especially attractive technology. Improved membrane tech-
nology now allows for one-step machine manufacture of ion-exchange membranes,
reducing costs and lowering membrane resistivity. Moreover, new high-performance
spacers (placed between the membranes) allow >90%-95% transport of contaminants
like nitrates, arsenic, and perchlorate, speeding the process, reducing the number of
membrane stacks required, and shrinking costs. Major improvements to the EDR sys-
tem design have taken shape in the form of the next-generation GE EDR 2020 product
line. This new design streamlines the process low with simpler hydraulics and stan-
dardized components, substantially lowering the capital and operating costs of EDR
demineralization.
In recent years, completely new EDR installations have taken place worldwide for a vari-
ety of applications. At the Ruth Fisher School in Arizona, an EDR system was installed
with the objective of removing inorganic components from groundwater and reducing
nitrates to meet US Environmental Protection Agency drinking water standards. The
nitrate concentration in the feed is >130 mg/L; however, the EDR system produces water
with extremely low total dissolved solids and nitrate concentrations. At the Bermuda
Water Works, EDR is used to reduce hardness in the island's existing water supply. The
brackish water lens under the island is contaminated from septic tank leach ields, making
nitrate removal essential. The 600,000 gpd plant removes 86% of the nitrates while achiev-
ing 90% water recovery (Figure 39.5).
Electrodeionization (EDI) is a modiication of conventional electrodialysis systems with
ion-exchange resins installed in the electrodialysis stacks. The technology—similar to RO
and EDR in that it uses semipermeable membranes with ion exchange media to provide
a high eficiency demineralization process—is a continuous electrochemical process of
water puriication where ion-speciic membranes, mixed bed resin and a DC voltage across
them, replace the standard acid-caustic chemical regeneration process. An EDI cell, like
GE's E-Cell*, consists of a series of thin chambers that alternately contain mixed bed resin
for water puriication and a concentrate water low to carry away impurities.
While the fundamental concept is somewhat simple with the basic desalting unit being an
electrodialysis dilute cell illed with mixed-bed ion-exchange resin, some complex chemi-
cal reactions take place within the resin-illed cell, helping to produce very high purity
water. When low enters the resin-illed diluting compartment of an EDI stack, several pro-
cesses are set in motion. Strong ions are scavenged out of the feed stream by the mixed-bed
resin. Under the inluence of the strong DC ield applied across the stack of components,
charged ions are pulled off the resin and drawn toward the respective, oppositely charged
electrodes, cathode or anode. As these strongly charged species, such as sodium and chlo-
ride, migrate toward the ion-exchange membrane, they are continuously removed and
