WHAT IS ELECTRODEIONISATION or EDI ?
Electrodeionization uses an electric field to remove ions and polar
species from an aqueous stream. EDI is used with reverse osmosis to replace ion
exchange resin-mixed beds, which require onsite or offsite chemical
By eliminating resin regenerating chemicals, EDI delivers significant
economic and environmental benefits. In addition, EDI’s continuous process
improves water quality by reducing spikes and upsets.
The Electropure XL™ EDI product line is offered for use in
ultrapure water systems for pharmaceutical, electronics, power generation, and
process systems replace conventional DI mixed resin beds to produce
deionized water. Unlike DI resin, EDI does not require shutdowns for
replacing resin beds or for resin regeneration using chemicals. Because
of this, EDI:
- Enables a simpler system (no concentrate recirculation)
- Produces ultrapure water (up to 18 megohm.cm)
- Eliminates regeneration chemicals
- Single unit capacities from 1/4 gpm to 10 gpm (50 l/hr to 2.3 m3/hr)
- Multiple unit arrays up to 1,000 gpm (200 m3/hr)
- Compact, lightweight, patented design
• minimizes water quality upsets and
• minimizes operating costs.
removes ions from aqueous streams, typically in conjunction with
reverse osmosis (RO) and other purification devices. Our high-quality
modules continually produce ultrapure water up to 18.2 MΩ.cm. EDI may
be run continuously or intermittently.
Advantages of EDI over Conventional DI
• EDI is Continuous, does not require shutdowns or changeovers
• Provides water of consistent quality
• EDI does not require chemicals (as does DI resin regeneration)
• Electropure™ EDI modules are the smallest and lightest per unit flow on the market; EDI skids are therefore compact
• Requires little energy
• Economic use of capital—saves operating expense
Process of Electrodeionization
The Electropure™ EDI design combines two well-established water purification technologies—electrodialysis and ion-exchange resin deionization. Through this revolutionary technique, dissolved salts can be removed with low energy cost and without the need for chemical regeneration; the result is high-quality pure water of multi-MΩ.cm resistivity which can be produced continuously at substantial flow rates.
Electropure’s EDI removes ions from water by forcing them out of the feed stream into adjacent streams via an electric potential. EDI is different from ED by using resins in the diluting chambers—the resins allow for more efficient migration of ions in very low conductivity water. The resins operate in steady state; they act not as an ion reservoir but as an ion conduit.
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 ionexchange 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 module, “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 stream 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. Electrolyte 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
electrolyte stream flows past the anode and cathode sequentially.
The anolytebathing 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 O2 (gas) and a small amount of Cl2 (dissolved) are
generated. This acidic stream
then flows into the cathode compartment, formed between the cathode (-)
and its adjacent cationselective membrane. In this compartment the pH
becomes neutral, and H2 (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 designed to minimize the
amount of chlorine (a strong oxidizer) formed.