SNOWPURE, LLC, 2005-2018
VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 9
Typical ions include sodium, calcium, magnesium, chloride, sulfate, nitrate, carbonate,
bicarbonate, etc. Over 98% of these ions can be removed by appropriate reverse
osmosis (RO) treatment. Water sources also contain organics, dissolved gases (e.g.,
O2, CO2), trace metals, and weakly-ionized inorganic compounds (e.g., boron and
silica), which must be removed for use in most industrial processes. The RO system
(and its pretreatment) also removes many of these impurities.
RO permeate (the EDI feedwater) should “ideally” range from 1-6 μS/cm (conductivity),
or a FCE = 1-9
μS/cm. Ultrapure (deionized) water ranges from 2.0-18.2 MΩ.cm
depending on the application. Typically, fewer ions in the EDI feed leads to the highest
quality EDI product water.
Electropure’s EDI process removes the unwanted ions from the water by adsorbing them
on the resins in the purifying chambers, and then transports them into the concentrate
stream. The exchange reaction takes place in the purifying compartments of the module
where the anion-exchange resins trade their hydroxyl ions (OH
-
) for the anion of the
dissolved salt (e.g., chloride, Cl
-
). The cation-exchange resins trade their hydrogen ions
(H
+
) for the cation of the dissolved salt (e.g., sodium Na
+
).
The adsorption step removes the ions from the influence of the water, whose residence
time in the module is limited (approximately 10-15 seconds). When adsorbed, the ions
are only influenced by force of the external DC potential.
The DC electrical field is applied via the anode (+) and cathode (-) arranged at either end
of the stack. The DC potential attracts or repels the adsorbed ions, forcing movement
along the surface of the resin beads, through the membranes into the concentrating
compartments. The DC potential also “splits” water molecules to form hydroxyl ions and
hydrogen ions:
H
2
O = OH
-
+ H
+
In Figure 1, the ion-exchange membranes are represented by the vertical lines labeled in
terms of their ionic permeability. Since these ion-selective membranes do not allow
water to permeate through them, they are barriers to water flow.
The negatively-charged anions (e.g., OH
-
, Cl
-
) are attracted to the anode (+) and
repelled by the cathode (-). The anions pass through the anion-selective membrane and
into the adjacent concentrate stream. They are blocked by the cation-selective
membrane on the far side of the chamber, and are thus trapped and carried away by the
water in the concentrate stream. The positively-charged cations (e.g., H
+
, Na
+
) in the
purifying stream are attracted to the cathode (-) and repelled by the anode (+). The
cations pass through the cation-selective membrane and into the adjacent concentrate
stream, where they are blocked by the anion-selective membrane, and are carried away.
In the concentrate stream, electrical neutrality is maintained. Transported ions from the
two directions neutralize one another’s charge. The current draw from the power supply
is proportional to the number of ions moved. Both the “split” water (H
+
and OH
-
) and the
intended ions are transported, and therefore add to the current demand.