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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 module, 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 the figure below. 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 EDI module is split into three separate streams.
- Product stream
- Concentrate stream (typically 10%, may be recycled to RO feed)
- Electrolyte stream (1%, anolyte + catholyte to drain)
The third (electrolyte) 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 drops and Cl2 (dissolved) and O2 (gas) are generated. This stream then flows into the cathode compartment, formed between the cathode (-) and its adjacent cation selective membrane. In this compartment H2 (gas) is generated. Thus, the electrolyte waste stream expels the unwanted chlorine, oxygen, and hydrogen gas from the electrodes.
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