Dow Water & Process Solutions

How does spiral wound EDI work?

The DOW™ EDI modules use electrical current to force a continuous migration of contaminant ions out of the feed water and into the reject stream while continuously regenerating the resin bed with H⁺ (hydrogen) and OH^- (hydroxyl) ions that are derived from water splitting. The patented flow process of the dilute and concentrate streams make the DOW EDI module completely unique.

Feed water (dilute stream) enters from the bottom of the DOW™ EDI module and is diverted into vertically spiraled cells known as the 'D' (dilute) chambers. The dilute stream flows vertically through ion exchange resins located between two membranes (an anion membrane specifically designed to only allow migration of anions, and a cation membrane specifically designed to only allow migration of cations).

Concentrate enters the bottom of the module through the center pipe and is diverted into cells known as the ‘C’ (concentrate) chambers.

DC current is applied across the cells. The DC electrical field splits a small percentage of water molecules (H₂O) into hydrogen (H⁺) and hydroxyl (OH^-) ions. The H⁺ and OH^- ions attach themselves to the cation and anion resin sites, continuously regenerating the resins. Hydrogen ions have a positive charge and Hydroxyl ions have a negative charge. Each will migrate through its respective resin, then through its respective permeable membrane and into the concentrate chamber due to their ionic attraction to the cathode or anode. Cation membranes are permeable only to cations and will not allow anions or water to pass, and anion membranes are permeable only to anions and will not allow cations or water to pass. The H⁺ and OH^- ions collect in the concentrate chamber to yield water.

Contaminant ions, dissolved in the feed water, attach to their respective ion exchange resin, displacing H⁺ and OH^- ions. Once within the resin bed, the ions join in the migration of other ions and permeate the membrane into the ‘C’ chambers. The contaminant ions are trapped in the ‘C’ chamber and are recirculated and bled out of the system.

The feed water continues to pass through the dilute chamber and is purified and collected on the outlet of the “D” chambers and exits the DOW EDI module. All DOW EDI modules product flows are collected and exit the system.