Electro deionization

Electro Deionization is a process that evolved from conventional ion exchange technology. EDI provides continuous demineralization at recovery rates of 90% or more. In EDI, just as in conventional ion exchange, cations and anions in the feed water are exchanged for hydrogen and hydroxyl ions in the ion exchange resins, producing demineralized water. The key operational difference is that with EDI, the ion exchange resin is regenerated continuously, while with conventional ion exchange, chemical regeneration is performed intermittently. Continuous regeneration in EDI is achieved electrochemically, by means of ion conducting membranes and an imposed electric current. The hydrogen and hydroxyl ions necessary for regeneration are formed in-situ, without addition of chemical reagents, by means of the familiar water dissociation reaction, sometimes called water splitting. is an illustration representing a water molecule splitting into hydrogen and hydroxyl ions.

To understand how Electro-Deionization (EDI) removes ions from water, we need to understand the components of an EDI stack and the function of each. An EDI stack consists of multiple beds of ion-exchange material sandwiched with membrane walls and open channels between two electrodes.

The two electrodes are located at opposite ends of the stack. These electrodes supply an electric current to the water flowing inside the cells. One of these electrodes is the cathode. It is negatively charged and is a source of electrons. The cathode attracts cations (positively charged ions).

The second electrode is the anode. This electrode is positively charged and attracts anions (negatively charged ions). This attraction occurs because opposite charges attract and like charges repel. Therefore, a negatively charged cathode attracts positively charged ions, and a positively charged anode attracts negatively charged ions.

There are two types of membranes: anion membranes and cation membranes. Using the same reasoning used for the electrodes, the anion membranes allow only negatively charged ions (anions) to permeate, and the cation membranes allow only the positively charged ions (cations) to permeate. Water does not permeate these membranes.

The membranes are situated so that they become the walls that separate chambers. There are three types of chambers; the Dilute or "D" chambers, the Concentrate or "C" chambers and the Electrolyte or "E" chambers.

Water to be treated is fed into the Dilute or "D" chamber. This chamber contains a resin bed that consists of cation and anion beads compressed between an anion membrane and a cation membrane. These resin beads allow the ions to adsorb into the respective beads and travel through the beads toward the membranes. Once contaminant ions pass through the membranes, they are in the Concentrate or "C" chamber. Ions are swept away by a recirculating flow (the concentrate loop) which has a bleed to prevent an excessive buildup of ions, and a corresponding makeup flow to introduce more fresh water.

One D chamber, one cation membrane, one C chamber and one anion membrane together are called one cell (or one cell pair). An EDI stack is made up of multiple cells operating in parallel.

The final chambers are the Electrolyte or "E" chambers. These two chambers contain the electrodes and are fed water from the concentrate loop. Like the Concentrate chambers, the E chambers receive contaminant ions from the nearest membrane.

The cathode E chamber also receives small amounts of hydrogen gas generated by the reduction of H+ ions, which have migrated to the cathode: 2H+ + 2e- = H2

The anode E chamber similarly receives small amounts of oxygen and chlorine gas generated by the oxidation of OH- and Cl- ions: 4OH- 4e- + 2H20 + O2 2Cl- 2e- + Cl2

The flow from the E chambers is generally sent to waste to prevent membrane attack by any chlorine gas generated.