Electrodialysis

These cations also stay in the C stream, prevented from further migration toward the cathode by the positively charged anion-exchange membrane.

[5] The E stream is usually employed to prevent the reduction and/or oxidation of salt ions from the feed into the electrode plates.

These gases are typically subsequently dissipated as the E stream effluent from each electrode compartment is combined to maintain a neutral pH and discharged or re-circulated to a separate E tank.

[citation needed] However, for water streams with lower salt concentration electrodialysis may be the most energy efficient process.

In a continuous process, feed is passed through a sufficient number of stacks placed in series to produce the final desired product quality.

[4] In normal potable water production without the requirement of high recoveries, reverse osmosis is generally believed to be more cost-effective when total dissolved solids (TDS) are 3,000 parts per million (ppm) or greater, while electrodialysis is more cost-effective for TDS feed concentrations less than 3,000 ppm or when high recoveries of the feed are required.

The main usage of EDI systems are in electronics, pharmaceutical, power generation, and cooling tower applications.

Electrodialysis can adapt to intermittent energy input and voltage variations, so it can be easily coupled to renewable electricity sources[14] Selective electrodialysis uses ion selective exchange membranes to concentrate only some ions, whereas other species remain in the diluted channel.

Therefore, monovalent selective electrodialysis may provide water with an ideal composition for agriculture, reducing the necessity for minerals fertilization.

[16] Electrodialysis has inherent limitations, working best at removing low molecular weight ionic components from a feed stream.

[17] As with RO, electrodialysis systems may require feed pretreatment to remove species that coat, precipitate onto, or otherwise "foul" the surface of the ion-exchange membranes.

Species of concern include calcium and magnesium hardness, suspended solids, silica, and organic compounds.