Electrodialysis for Brackish Water and Industrial Recovery: Where It Beats Reverse Osmosis
The electrodialysis process combines an electrochemical process with a dialysis-diffusion process. Driven by an applied direct-current electric field, it exploits the selective permeability of ion-exchange membranes - cations pass cation-exchange membranes, anions pass anion-exchange membranes - so anions and cations migrate toward the anode and cathode respectively. During migration, an ion whose charge is opposite the membrane's fixed charge passes through; an ion of the same charge is repelled. The outcome is desalination, concentration, refining or purification of the solution.
Against reverse osmosis, the other membrane separation technology that arrived in the same period, electrodialysis is cheaper but achieves a lower desalination rate. Domestically produced ion-exchange membranes are now stable in quality and operation and management are convenient, which has kept the process competitive.
Membranes and Electrode Reactions
The semi-permeable membrane in electrodialysis is an ion-exchange membrane, divided by the charge of the ions it handles into cation-exchange and anion-exchange types. In an electrolyte solution, cation membranes pass cations and block anions; anion membranes do the reverse. That is selective permeability.
Importantly, unlike ion-exchange resin the membrane does not exchange with any ion in solution - it merely permits ions of one polarity through, so it never needs regeneration. The compartment formed by the electrodes and membranes is the electrode compartment, where the reactions are ordinary electrode reactions. Oxidation occurs in the anode compartment, anode water is acidic and the anode corrodes easily. Reduction occurs in the cathode compartment, cathode water is alkaline and scale forms readily on the cathode.
Where It Is Applied
Electrodialysis is among the more mature membrane separation processes and is widely used in brackish-water desalination; in some regions it is the main method of producing fresh water. Newer charged membranes have higher selectivity, lower membrane resistance, better thermal and chemical stability and higher mechanical strength, so the process has moved well beyond desalination.
In food, pharmaceutical and chemical industries it recovers acid and metals from spent liquor generated when acid cleans metal surfaces, recovers heavy metal ions from electroplating wastewater, recovers sulfate from synthetic fiber wastewater, and recovers sulfite from pulp waste liquor. In food it desalts milk for infant formula; in chemicals it separates ionic from non-ionic substances; in clinical treatment it can serve as an artificial kidney. It also recovers metals from waste liquid, such as nickel from electroplating waste.
EDR and the Performance Envelope
Automatically controlled electrodialysis reversal, EDR, with frequent polarity reversal makes operation and management easier. Raw water utilisation can reach 80 percent, with the general raw water recovery rate between 45 and 70 percent. Electrodialysis is mainly used for primary desalination, with a desalination rate between 45 and 90 percent. It serves seawater and brackish water desalination, primary desalination in pure water preparation, and desalination and softening of feed water for boilers and power equipment.
In practice an electrodialyzer is not built from a single pair of membranes, which would be hopelessly inefficient, but from one hundred pairs or even several hundred, which is where the efficiency comes from. Typical operating conditions are an operating pressure of about 0.5-3.0 kg/cm2, operating voltage and current of 100-250 V and 1-3 A, and power consumption of the unit itself of about 0.2-2.0 kWh per ton of fresh water.
What the Process Can Do at Once
It desalinates, concentrates, separates and purifies an electrolyte solution simultaneously. It purifies non-electrolytes such as sucrose by removing the electrolytes present. And because an electrodialyzer is in principle an electrolytic cell with a diaphragm, it can exploit the high efficiency of oxidation-reduction at the electrodes.
Six Secondary Processes That Cost You
Alongside the intended migration, several secondary processes run:
- Co-ion migration: selective permeability is never 100 percent, so a small number of counter-ions cross the membrane.
- Concentration-difference diffusion: the concentration difference between the concentrate and diluting compartments drives a few ions back from concentrate to diluate, reducing dialysis efficiency.
- Osmosis of water: although the membrane blocks solvent molecules, the concentration difference pushes some water into the concentrate compartment.
- Electro-osmosis of water: ion hydration and the electric double layer move water from the diluting to the concentrate compartment under the DC field.
- Polarization ionization of water: under poor working conditions water is forced to ionize into hydrogen and hydroxide ions, which then pass through the membrane into the concentrate compartment.
- Pressure osmosis of water: a fluid pressure difference between compartments forces water from the higher-pressure side to the lower.
All six are unfavourable, and all six can be avoided or controlled by changing operating conditions. That is the practical message: electrodialysis rewards attention to operating parameters more than most membrane processes do.
The wider point is that electrodialysis is rarely the right answer where a single pass to potable quality is required - that is a reverse osmosis duty. It earns its place where the stream is brackish rather than saline, where recovery of a specific ion has value, or where a plant needs a robust primary desalination step ahead of polishing. Judged on those terms rather than on desalination rate alone, the economics are usually straightforward.