Electrodialysis and EDR: Where Ion-Exchange Membranes Still Beat Reverse Osmosis on Cost
Electrodialysis sits at the intersection of an electrochemical process and a dialysis-diffusion process. Driven by an applied DC electric field and using the selective permeability of ion-exchange membranes, cations migrate toward the cathode through cation-exchange membranes and anions toward the anode through anion-exchange membranes. Where the fixed charge of a membrane is opposite to an ion's charge, the ion passes; where the charges match, the ion is repelled. The result is desalination, concentration, refining or purification of a solution. Compared with reverse osmosis, the other membrane technology that entered wide use in the same period, electrodialysis is cheaper but achieves a lower desalination rate — and that trade defines where each belongs.
Selective permeability, and why membranes need no regeneration
The semi-permeable membrane used in electrodialysis is in fact an ion-exchange membrane, supplied in two types. In an electrolyte solution the cationic membrane allows cations through and blocks anions; the anionic membrane does the reverse. That is selective permeability. Importantly, the membrane does not exchange ions the way an ion-exchange resin does — it merely permits ions of one charge sign to pass. There is therefore no exhaustion and no regeneration cycle, no acid and caustic storage, and no regeneration waste stream. For plants without a chemical handling infrastructure, that alone can decide the question.
The compartment formed by the electrodes and membranes is the electrode compartment, and the reactions there are ordinary electrode reactions. Oxidation occurs at the anode, so anode water turns acidic and the anode itself corrodes readily. Reduction occurs at the cathode, so cathode water turns alkaline and scale forms easily. Both ends need materials and flushing provisions designed for those conditions.
What actually happens in a stack
Insert one anion-exchange membrane and one cation-exchange membrane into an electrodialyser and the compartment between them loses salt through directional ion migration, while the compartments nearest the electrodes become concentration chambers. That middle compartment is the diluting cell. In practice a stack is never built from a single membrane pair — efficiency would be useless — but from one hundred pairs or even several hundred, which is what turns the principle into a practical production rate.
EDR: reversing polarity to stay online
Electrodialysis with automatic frequent electrode reversal, or EDR, is what made the process operationally realistic. By periodically reversing polarity, the scale and fouling that would otherwise build at the cathode are dissolved before they mature, and operation and management become markedly simpler. Raw-water utilisation can reach 80%, with the general raw-water recovery rate somewhere between 45% and 70%. Desalination rate typically lands between 45% and 90%, which places EDR as a primary desalination step rather than a polishing one — brackish water and seawater desalination, primary desalination ahead of pure-water preparation, and desalting and softening of boiler and power-plant feedwater.
Operating envelope and running cost
Typical operating conditions are modest by membrane standards: pressure around 0.5-3.0 kg/cm2, voltage and current in the range of 100-250 V and 1-3 A, and energy consumption of roughly 0.2-2.0 kWh per ton of fresh water produced. Because the driving force is electrical rather than hydraulic, energy scales with the salt load removed rather than with the osmotic pressure of the feed — which is exactly why electrodialysis stays competitive on brackish water and loses ground as salinity rises.
The secondary processes that erode performance
Six side effects work against ideal separation. Co-ion migration occurs because selectivity is never 100%, so a small amount of counter-ions always crosses. Ionic concentration diffusion pushes a little salt back from the concentrating to the diluting cell. Water osmosis moves solvent toward the concentrate despite the membrane's nominal rejection. Water electro-osmosis carries hydration water along with migrating ions. Water polarization ionisation occurs when poor operating conditions force water to split into hydrogen and hydroxide ions that then cross into the concentration chamber. Water pressure permeation pushes molecules from the high-pressure side to the low-pressure side. All six are unfavourable, and all six can be avoided or controlled through operating conditions rather than hardware changes.
Beyond desalination
The process reaches well past water. Newly developed charged membranes offer higher selectivity, lower membrane resistance, better thermal and chemical stability and greater mechanical strength, so electrodialysis now appears in food, pharmaceutical and chemical production. Industrial applications include recovering acid and metals from metal-surface pickling liquors, recovering heavy-metal ions from electroplating wastewater, recovering sulfate from synthetic-fibre wastewater and sulfite from pulp waste liquor. In food it desalinates milk for infant formula; in chemicals it separates ionic from non-ionic substances; clinically it can serve as an artificial kidney. It also concentrates seawater for salt making, deacidifies and purifies juice, and pretreats high-purity water for electronics and pharmaceutical use. Because an electrodialyser is in principle an electrolytic cell with membranes, it can also put the electrode redox reactions to productive use.