Electrodialysis for Desalting: Membrane Selectivity, Energy Use and the Six Loss Mechanisms
The electrodialysis process is a combination of an electrochemical process and a dialysis diffusion process; driven by an applied direct-current electric field and utilizing the selective permeability of ion-exchange membranes, cations and anions migrate toward the anode and cathode, respectively. During ion migration, if the fixed charge of the membrane is opposite to the charge of the ion, the ion can pass; if their charges are the same, the ion is repelled, thus achieving the purposes of solution desalination, concentration, refining, or purification.
Selectivity, and why the membrane never regenerates
The semi-permeable membrane used in electrodialysis is actually an ion-exchange membrane, divided into cation-exchange membranes and anion-exchange membranes. In an electrolyte aqueous solution, the cationic membrane allows cations to pass through while repelling and blocking anions, and the anionic membrane allows anions to pass through while repelling and blocking cations. That is the selective permeability.
One distinction is worth holding on to: the ion-exchange membrane does not exchange with a certain ion in the aqueous solution like an ion-exchange resin does, but only plays a selective-permeation role for ions of different electrical natures, i.e. the ion-exchange membrane does not need regeneration. There is no regeneration chemical to buy, no regeneration waste to dispose of.
The compartment composed of the electrodes and membranes is called the electrode compartment, where the electrochemical reaction is the same as an ordinary electrode reaction. In the anode compartment an oxidation reaction occurs, the anode water becomes acidic, and the anode itself is easily corroded. In the cathode compartment a reduction reaction occurs, the cathode water becomes alkaline, and scale easily forms on the cathode. Those two ends are where most maintenance hours go.
What EDR changed
Automatically controlled electrodialysis with frequent polarity reversal (EDR) makes operation and management more convenient. The raw-water utilization rate can reach 80%, and the general raw-water recovery rate is between 45-70%. Electrodialysis is mainly used for primary desalination of water, with a desalination rate between 45-90%. It is widely used for desalination of seawater and brackish water; primary desalination when producing pure water; and desalination and softening of boiler and power-equipment feed water.
Essentially, electrodialysis can be said to be a desalting technique, because all kinds of water contain a certain amount of salts, and the cations and anions that make up these salts migrate toward electrodes of opposite directions under the action of a DC electric field. If one anion-exchange membrane and one cation-exchange membrane are inserted into an electrodialyzer, the salt concentration in the compartment between the two membranes will decrease due to directional migration, while the two compartments near the electrodes become concentration compartments. In practice an electrodialyzer is not composed of just one pair of membranes, because that is very inefficient, but uses one hundred pairs or even several hundred pairs.
Position against reverse osmosis
Compared with reverse osmosis, another membrane-separation technology introduced in recent years, electrodialysis is cheaper but has a lower desalination rate. When the quality of domestically produced ion-exchange membranes is also very stable, and operation and management are very convenient, that cost gap tends to dominate the selection for brackish water rather than seawater.
The operating window is narrow and well defined. Operating pressure is about 0.5-3.0 kg/cm2. Operating voltage and current are 100-250 V and 1-3 A. Body power consumption is about 0.2-2.0 kWh per ton of fresh water. Those three numbers are the whole utility budget.
Industrial uses beyond drinking water
Electrodialysis is a relatively mature technique among membrane-separation processes and has been widely used in brackish-water desalination, being a major method of producing fresh water in some regions of the world. Newly developed charged membranes have higher selectivity, lower membrane resistance, better thermal and chemical stability and higher mechanical strength, so the process is no longer limited to desalination.
Industrial applications include the recovery of acids and metals from waste liquids formed by acid cleaning of metal surfaces; the recovery of heavy-metal ions from electroplating wastewater; the recovery of sulfates from synthetic-fiber wastewater; and the recovery of sulfites from pulp waste liquids. In the food industry it is used for milk desalination to produce infant milk powder; in the chemical industry to separate ionic from non-ionic substances; and in clinical treatment electrodialysis can be used as an artificial kidney.
Other duties: seawater concentration for salt production, dairy-product refining, fruit-juice deacidification, refinement and purification, production of chemical products, pure water for the food and light industries, pre-treatment for high-purity water in electronics and pharmaceuticals, primary softening and desalting of boiler feed water, and desalinating brackish water into drinking water. It also handles the treatment of wastewater and waste liquids and the recovery of precious metals, such as the recovery of nickel from electroplating waste liquids. Broadly, it can simultaneously perform desalting, concentration, separation and purification of electrolyte aqueous solutions, and it can be used for the purification of non-electrolytes such as sucrose, to remove the electrolytes therein.
The six losses you are really fighting
During the electrodialysis process, secondary processes also take place, and they are where current efficiency disappears. Migration of ions of the same name: the selective permeability of ion-exchange membranes can never be 100%, so a small amount of counter-ions always pass through the exchange membrane. Concentration-diffusion of ions: because there is a concentration difference between the concentration compartment and the desalting compartment, a small amount of ions always diffuse from the concentration compartment to the desalting compartment, reducing dialysis efficiency. Water permeation: part of the solvent molecules will permeate toward the concentration compartment. Electro-osmosis of water: due to the hydration of ions and the formation of an electric double layer, water molecules can also migrate from the desalting compartment to the concentration compartment under a DC electric field. Polarization ionization of water: sometimes, due to poor working conditions, water is forced to ionize into hydrogen ions and hydroxide ions, which can pass through the exchange membrane into the concentration compartment. Pressure permeation of water: because there is a fluid-pressure difference between the compartments, water molecules are forced to permeate from the higher-pressure side to the lower-pressure side. Obviously these are unfavorable factors, but they can all be avoided or controlled by changing the operating conditions.
One upside sits on the other side of the ledger: in principle, an electrodialyzer is an electrolytic cell with a diaphragm, and the high efficiency of the redox reaction at the electrodes can be exploited.