Potable water treatment
Potable water treatment

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Potable water treatment

5.14.3 Electrodialysis

Electrodialysis is an electrochemical process in which ion transfer separates salt from water. It is effective only for substances that can be ionized: for example, salt (NaCl) becomes, in solution, a mixture of Na+ and Cl ions. (Silica, on the other hand, does not ionize and hence is not removed by electrodialysis. It could, however, be removed by reverse osmosis.) When electrodes, connected to a suitable direct current supply, are immersed in a salt solution, current will flow, carried by the ions. The ions with a positive charge are attracted towards the negative cathode and are called cations. Negatively charged anions flow towards the positive anode. In electrodialysis, filters or membranes selectively impervious to cations or anions are placed alternately between the electrodes (Figure 35). Cation filters permit the flow of anions but act as a barrier to positively charged cations. Conversely, anions are held back by the anion filter while cations pass through. In certain compartments of the tank, ions will collect as their flow is checked by an appropriate filter. Cells of increasing salt concentration thus alternate with cells of salt depletion. Water that is sufficiently desalinated is extracted from the appropriate compartments. Electrodialysis is only generally used with brackish waters as it is uneconomic for sea water desalination. It is used together with ion exchange and activated carbon to produce ultra-pure water for the electronics and pharmaceutical industries.

Figure 35
Figure 35 The principle of electrodialysis
T210_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus