MEMBRANE TECHNIQUES-Electrodialysis

Electrodialysis:The ions in a water solution can be made to migrate by applying an electric field to the solution. By arranging various barriers to the flow of ions, it is possible to directly desalinate water with electricity. Such barriers are called ion-exchange membranes.

Membranes that allow a reasonable flow of cations, but block or reduce the flow of anions, are called cationic-exchange membranes.

Membranes that allow a reasonable flow of anions, but block or reduce the flow of cations, are called anion-exchange membranes.

Membranes that pass both anions and cations are called neutral membranes.

A.Theory. In solutions containing dissolved ions, electric currents are carried by movement of the ions.Positive ions migrate in the direction of the current flow, and negative ions migrate against the current direction.When the anions are blocked by a cationic-exchange membrane, they stop and form a localized charge at the membrane face. This accumulated negative charge is neutralized by the flow of cations across the cationic membrane.

This generates a concentrated solution on the side of a cationic-exchange membrane that faces the negative electrode. It also generates a dilute solution on the side of the cationic membrane that faces the positive electrode as shown in figure.

B.Electrodialysis stack. If both a cationic and anionic membrane are placed across a current flow in an electrolyte solution, the side of the cationic membrane facing the positive electrode and the side of the anionic membrane facing the negative electrode will become less saline. If the cationic membrane is closer to the negative electrode and the anionic membrane is closer to the positive electrode, the solution between the membranes will become less saline as the ions migrate in their respective directions.

Any number of pairs of cationic and anionic membranes can be placed across a current-carrying solution, such that the cationic membrane is closest to the negative electrode, and the solution between will be diluted.

A battery of several such membrane pairs is called an electrodialysis stack. Several variations of the standard electrodialysis stack have been developed, but none have been proven superior to this standard stack of alternating cationic and anionic-exchange membranes to desalinate natural brackish water

Electrodialysis C.Electrodialysis reversal. One important improvement now used in electrodialysis installations is to reverse the polarity periodically and move the ions in the opposite direction.

This returns anions across the anionic membranes and helps break up scale formed on the concentrating face of the membranes. Water will flow osmotically across both membranes from the dilute product stream to the concentrated brine stream in an electrodialysis-reversal stack.

This osmotic product water loss concentrates uncharged material, such as turbidity and bacteria. This concentration effect must be considered during the design to ensure meeting water turbidity and product water bacterial count requirements.

Most electrodialysis membranes are not tolerant of chlorine. When possible, water desalinated by electrodialysis reversal should be disinfected after desalination is completed. The membranes should be protected by a 10-micron cartridge filter.

Article by Sivanandan

T.P.Sivanandan

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