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WHAT IS ELECTRODEIONISATION or EDI ?
Electro Deionisation EDI schematic diagram

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 regeneration.

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 laboratory applications.

  • 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
EDI 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:
• minimizes water quality upsets and
• minimizes operating costs.
EDI 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.

 

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