SNOWPURE, LLC, 2005-2018
VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 8
The electrodeionization process uses a combination of ion-selective membranes and
ion-exchange resins sandwiched between two electrodes (anode (+) and cathode (-))
under a DC voltage potential to remove ions from RO-pretreated water.
Ion-selective membranes operate using the same principle and materials as ion-
exchange resins, and they are used to transport specific ions away from their
counterions. Anion-selective membranes are permeable to anions but not to cations;
cation-selective membranes are permeable to cations but not to anions. The
membranes are not water-permeable.
By spacing alternating layers of anion- and cation-selective membranes within a plate-
and-frame mo
dule, a “stack” of parallel purifying and concentrating compartments are
created. The ion-selective membranes are fixed to an inert polymer frame, which is filled
with mixed ion-exchange resins to form the purifying chambers. The screens between
the purifying chambers form the concentrating chambers.
This basic repeating element of the EDI, called a “cell-pair,” is illustrated in Figure 1. The
“stack” of cell-pairs is positioned between the two electrodes, which supply the DC
potential to the module. Under the influence of the applied DC voltage potential, ions
are transported across the membranes from the purifying chambers into the
concentrating chambers. Thus, as water moves through the purifying chambers, it
becomes free of ions. This is the pure water product stream.
The RO feed to the Electropure™ EDI module is split into three separate streams:
1. Product stream (up to 99% water recovery)
2. Concentrate stream (typically 10%, may be recovered as RO feed*)
3. Electrode stream (10 l/h, 0.05 gpm, always to drain)
* Note: for recovery of the concentrate stream, we recommend
use of a break tank and pump, and we recommend against a
direct connection.
The electrode stream flows past the anode and cathode sequentially. The anolyte-
bathing stream first flows past the anode (+) through a compartment, formed by a
gasketed monofilament screen, which is located between the anode and an adjacent
anion-selective membrane. In this compartment the pH becomes acidic, and O
2
(gas)
and a small amount of Cl
2
(dissolved) are generated. This acidic stream then flows into
the cathode compartment, formed between the cathode (-) and its adjacent cation-
selective membrane. In this compartment the pH becomes neutral, and H
2
(gas) is
generated. Thus, the waste stream expels the unwanted chlorine, oxygen, and
hydrogen gas from the electrodes. The unique Electropure™ electrode system is
designed to be non-scaling since neither stream becomes high in pH. The
Electropure™ anode is further engineered to minimize the amount of chlorine (a strong
oxidizer) formed.
Details of the Electropure™ EDI Process
Water from all sources contains impurities including dissolved salts, which are
composed of negatively charged ions (anions) and positively charged ions (cations).