Electrodeionization combines two well-established water treatment methods: electrodialysis and ion exchange using ion exchange resin. This technique makes it possible to remove dissolved salts without chemical regeneration, at a very low cost/energy ratio.
Electrodeionization process description
Before being fed into the electrodeionization module, the permeate produced by the reverse osmosis system is split into three streams. The feed water passes through the dilute chambers, which are filled with ion exchange resin. Under the effect of the electric field, the anions migrate through the resin bed towards the anode. They pass through the anion-permeable membrane into the adjacent concentrate stream. The cations migrate through the resin bed towards the cathode, pass through the cation-permeable membrane and likewise reach the adjacent concentrate stream. The ions are removed from the module with the concentrate stream. The electrolyte stream carries away the gases produced at the electrodes, along with ions.
As a result of the electrical voltage, water splitting takes place in the resin bed of the dilute chamber. H+ and OH- ions are produced, which are needed to regenerate the ion exchange resin. They regenerate the resin bed continuously, so operation continues without having to stop the system for regeneration.
Unit description
Stainless steel frame with a plastic front panel containing the instruments and controls.
Electrodeionization module for continuous desalination of the RO permeate, consisting of a robust housing that contains a series of ion-selective membranes forming desalination chambers filled with ion exchange resin. Two special electrodes create an electric field across the chambers.
Diaphragm valve for concentrate flow control, sampling valves for EDI feed and EDI product, EDI feed solenoid valve and 5 µm filter.
Pressure monitoring using vibration-resistant pressure gauges for EDI feed water, EDI concentrate and EDI product pressure.
Flow meters for EDI feed water, EDI concentrate and electrolyte, and minimum flow control of the EDI concentrate. On-line resistivity measurement with temperature compensation as defined by ASTM D 1125-95, measuring range 0 - 20 MΩx cm.
Control panel with lockable main switch. Integrated power supply to maintain the EDI module voltage. Digital display of current and voltage for the EDI module.
Precision potentiometer for adjusting the EDI current. Residual current circuit breaker with a 30mA trip threshold. Collective fault signals in situations such as: EDI concentrate flow below the minimum limit, power supply problems, low permeate resistivity.
Common fault signal via a potential-free contact.
The unit is designed for a permeate conductivity of 3-20 µS/cm, a CO2 content of 10 mg/l, a maximum SiO2 content of 0.5 mg/l, a colloid index of max 0.25 and free product discharge. The feed water must be softened (<0.1°GH) and pre-treated by means specific to the reverse osmosis process (no intermediate permeate tank). The quality of the EDI product depends on the quality of the permeate.
