High-Purity Electrodeionization Systems

ELECTRODEIONIZATION (EDI)

Is a continuous, chemical-free process of removing ionized and ionizable species from feedwater using DC power. EDI is typically used to polish reverse osmosis (RO) permeate, and is an alternative to, and effective replacement of conventional mixed bed ion exchange (IX). Using EDI eliminates the need to store and handle hazardous chemicals used for resin regeneration in mixed beds. Since electricity is the only consumable, permeate polishing with EDI does not produce a hazardous waste stream.

HOW DOES ELECTRODEIONIZATION WORK?

Electrolysis involves the passage of an electrical current through an electrolyte solution, with the subsequent movement of positively and negatively charged to negative and positive electrodes. This process splits water molecules, and is the driving force of electrodeionization. Electrodialysis then separates the hydroxal (OH-) and hydrogen (H+) ions from the electrolyte solution, while EDI overcomes the limits of electrodialysis, allowing for ion separation without increasingly higher voltage.

The fundamental process is: Imagine a simple model of a battery connected to two electrodes immersed in a saltwater bath. When charge is introduced on the electrodes, a reduction reaction involving the water molecules occurs at the cathode: hydrogen gas is released and OH- ions are left behind. At the anode, oxygen gas is released and H+ ions are left behind in an oxidation reaction. The presence of salt in the solution facilitates continuous reactions at the electrodes, drawing hydroxyl ions from the cathode and hydrogen ions from the anode.

In electrodialysis, electrical current drives ions across a semipermeable membrane. In an EDI system, a membrane that allows for the passage of cations (OH- ions) only is positioned next to the cathode, and a membrane permeate to anions (H+ ions) only is positioned next to the anode. A central chamber now contains the saline solution. When the electrical charge is applied to the system and the chemical reactions occur, the ions will move across the membranes out of the central chamber to their respective electrodes, leaving behind the constituents of the salt molecules and any other impurities.

Electrodialysis is limited, however. As the water becomes purer, the system’s voltage requirement increases, even exceeding 600 volts, which can cause arching. EDI resolves this challenge by introducing IX resins, or ionically conductive media, into the central chamber. This allows the ions to easily migrate out of the central, dilute chamber without high voltage.

ELECTRODEIONIZATION (EDI)
SEPARATION PROCESS

EDI APPLICATIONS

EDI is useful for any application that requires constant and economic removal of water impurities without using dangerous chemicals. Some examples include: