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How It Works
Electrodialysis (ED) uses an electric field to pull dissolved ions through ion-selective membranes, separating salts from water. The current equation I = F × N × Q × E₁ / (n × E₂) ties Faraday's constant (96,487 C/eq) to the solution strength, flow rate, and membrane efficiency so you can size the electrical supply for a given stack.
Example Problem
A brackish-water ED stack has 200 cell pairs. The feed is 0.05 eq/L at 0.5 L/s. Removal efficiency is 0.70 and current efficiency is 0.90. What current is needed?
- Numerator: 96,487 × 0.05 × 0.5 × 0.70 = 1,688.5 C/s
- Denominator: 200 × 0.90 = 180
- I = 1,688.5 / 180 = 9.38 A
Frequently Asked Questions
What is electrodialysis used for?
ED is primarily used for brackish-water desalination, food-industry demineralization (whey, juice, wine), wastewater ion recovery, and chemical solution concentration. It is most cost-effective when total dissolved solids are below about 5,000 mg/L.
How does electrodialysis differ from reverse osmosis?
Reverse osmosis pushes water through a membrane, leaving salts behind. Electrodialysis pulls ions out of the water through selective membranes using electricity. ED can be more energy-efficient for low-to-moderate salinity feeds and allows easier recovery of concentrated brine.
What is Faraday's constant in the ED equation?
Faraday's constant (F = 96,487 C/eq) is the electric charge carried by one mole of electrons. In the ED equation it converts ion equivalents per second into the amperes of current that the membrane stack must carry.
What is a typical current efficiency for an ED stack?
Current efficiency (E₂) typically ranges from 0.80 to 0.95. Losses come from back-diffusion of ions, water transport through the membranes, and electrical leakage between cells. Well-maintained stacks with tight seals usually exceed 0.90.
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