Waterguide - Electrodeionisation

What is electrodeionisation?

Electrodeionisation (abbreviated to “EDI”) or electrodialysis is a process for the continuous desalination of permeate from a reverse osmosis unit.

An EDI module consists of several chambers that are separated from one another by ion-selective membranes. The chambers are filled with ion exchange resin and arranged between two DC electrodes.

 

The figure shows the schematic design of an EDI

How is an electrodeionisation module constructed?

An EDI module consists of an electrodialysis stack that contains several cells. Each of these cells forms a chamber that is separated from the next cell by a cation and anion exchange membrane. This stack of cells is inserted into a frame, at the end of which there is an anode or cathode.

Spacers are used to separate the cells and ensure a constant cell thickness, prevent contact with the membrane and keep the electrical resistance in the cell low.

 

How are the ions selected in the electrode ionisation?

If a permeate is conveyed from an RO unit to an EDI module, the ions and water molecules flow though the individual cells with their chambers. DC voltage induced at the two electrodes causes the anions to wander in the direction of the anode and pass through the anion exchange membrane, which is how they get into the next cell. In the reverse process, the cations pass through the cation exchange membrane and in this way enter the neighbouring cell.

There is no risk of the ions continuing to wander into further cells, because once they have entered one neighbouring cell, their charge prevents them from penetrating the next membrane. The accumulation of the ions (formation of retentate) is thus carried out in the cells with an uneven number, and the dilute is formed in cells with an even number. The following explains this process schematically.

 

What are the advantages of using EDI modules?

The main advantages of this method include the ability to run your operations without interruption and without the need for chemicals. The units also do not take up much space. The EDI concentrate, which is concentrated RO permeate, does not have to be neutralised but can be recirculated within the unit or used for other purposes.

What is unique about this process?

The diluate produced with the EDI module has a very low conductivity: < 0.2 µS/cm or considerably lower than this (ultrapure water). The physical limit of EDI technology is approx. 0.056 µS/cm. It is not quite as useful to measure conductivity in the ultrapure range due to the low values. For this reason, the reciprocal value – i.e. the resistance – is used instead of the conductivity.

The following values correspond:

0.2 µS/cm = 5 MOhm*cm

0.1 µS/cm = 10 MOhm*cm

0.056 µm/cm = 17.8 MOhm*cm

What does a customer have to take into account when selecting an EDI module?

In order to achieve the projected water quality, an EDI module should ideally be in constant operation over longer periods of time. This is because each time the module is started up, it takes five to fifteen minutes before there is a balance in the stack and the conductivity values of the diluate reach the projected quality. If the EDI module is continually switched on and off within short periods of time, the average quality of the ultrapure water is not as good as would otherwise be technically possible. It is therefore essential that the dimensioning and operating mode of an EDI are correct. Another good option is to install a polisher downstream of the ultrapure water tank in order to balance out fluctuations in the conductivity. Our Herco DESTILLO mixed bed full demineralisation unit can be used for this purpose, as it guarantees a stable conductivity measurement below < 0.1 µS/cm.

The engineers at Herco would be happy to advise you in this respect.

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