Electrodialysis Desalination of Brackish Water Technology

Demineralization of Brackish Water by Electrodialysis

This detailed explanation covers the process of demineralization of brackish water using Electrodialysis (ED), based on standard water chemistry concepts.

Brackish Water Characteristics

Brackish water contains dissolved salts, with salinity levels higher than freshwater but lower than seawater. It tastes salty and is generally unsuitable for drinking or industrial use. To make it potable, salts must be removed (desalination).

Electrodialysis Principle

Electrodialysis is an electrochemical desalination process. When an electric field is applied, ions (salts) in the water migrate through specialized ion-selective membranes:

• Cation-exchange membranes: Allow only cations (e.g., Na⁺, Ca²⁺, Mg²⁺) to pass.
• Anion-exchange membranes: Allow only anions (e.g., Cl⁻, SO₄²⁻, HCO₃⁻) to pass.

As salts are transferred away, the water becomes deionized (demineralized).

Construction and Working Process

1. The Electrodialysis chamber contains many compartments separated by alternating cation and anion exchange membranes.
2. Electrodes are fitted at the ends:
   • Anode (+): Attracts anions (Cl⁻, SO₄²⁻).
   • Cathode (–): Attracts cations (Na⁺, Mg²⁺, Ca²⁺).
3. When electricity is applied:
   • Salts migrate through the membranes into side compartments, forming the concentrate stream.
   • The middle compartments retain desalinated (fresh) water.
4. Result:
   • The central stream is low-salt potable water (the diluate).
   • The side streams are highly concentrated brine (waste).

Example: NaCl Removal in Brackish Water

In brackish water containing Sodium Chloride (NaCl):

• The cation Na⁺ moves through the cation-exchange membrane toward the cathode.
• The anion Cl⁻ moves through the anion-exchange membrane toward the anode.

Thus, the salt concentration significantly decreases in the central compartment.

Advantages of Electrodialysis

• Compact unit and easy to operate.
• Economical, especially where electricity costs are low.
• Removes salts effectively without adding chemicals.
• Suitable for brackish water desalination (but generally not for seawater with very high salinity).

Limitations of Electrodialysis

• Not effective for water with very high salinity (such as seawater).
• Requires pretreatment if the source water is turbid (to avoid clogging membranes).
• Membranes are expensive and require periodic replacement.