Wastewater Treatment Terminology: Electrodialysis
Electrodialysis is a combination of an electrochemical process and a diffusion dialysis process. Driven by an external DC electric field and utilizing the selective permeability of ion-exchange membranes (cations pass through cation-exchange membranes, anions through anion-exchange membranes), cations and anions migrate toward the anode and cathode respectively. During ion migration, if the membrane's fixed charge is opposite to the ion's charge, the ion passes; if the charges are the same, the ion is repelled — thereby achieving desalination, concentration, refining, or purification of a solution.
Compared with reverse osmosis, another membrane-separation technology introduced in recent years, electrodialysis is cheaper but has a lower desalination rate. Domestic ion-exchange membranes are now of stable quality and convenient to operate and manage.
The semi-permeable membrane used in electrodialysis is in fact an ion-exchange membrane, divided by ion charge into cation-exchange membranes (cationic membranes) and anion-exchange membranes (anionic membranes). In an electrolyte aqueous solution, a cationic membrane allows cations to pass while blocking anions, and an anionic membrane allows anions to pass while blocking cations — this is the selective permeability of ion-exchange membranes. During electrodialysis, the ion-exchange membrane does not exchange with a certain ion in the solution as an ion-exchange resin does; it only selectively passes ions of different charge signs, so the ion-exchange membrane needs no regeneration. The compartment formed by the electrodes and membranes is called the electrode compartment; the electrochemical reaction there is the same as an ordinary electrode reaction. Oxidation occurs in the anode compartment, making the anode water acidic and the anode itself prone to corrosion; reduction occurs in the cathode compartment, making the cathode water alkaline and the cathode prone to scaling.
Electrodialysis is a relatively mature membrane-separation technology, widely used for brackish-water desalination and a major method of producing fresh water in some regions of the world. Newly developed charged membranes with higher selectivity, lower membrane resistance, better thermal and chemical stability, and higher mechanical strength have extended electrodialysis beyond desalination to many other industrial applications in food, pharmaceutical, and chemical industries — such as industrial wastewater treatment: recovering acid and metal from metal-surface pickling waste liquid; recovering heavy-metal ions from electroplating wastewater; recovering sulfate from synthetic-fiber wastewater; recovering sulfite from pulp waste liquid. In the food industry it is used for milk desalination to make infant formula; in the chemical industry for separating ionic from non-ionic substances; and clinically electrodialysis can serve as an artificial kidney.
Automatic frequent-electrode-reversal electrodialysis (EDR) makes operation and management more convenient. Raw-water utilization can reach 80%, with general recovery rates of 45%–70%. Electrodialysis is mainly used for primary desalination of water, with a desalination rate of 45%–90%. It is widely used for seawater and brackish-water desalination, primary desalination when producing pure water, and desalination and softening of boiler and power-equipment feed water.
In essence, electrodialysis is a desalination technology, because various waters (including natural water, tap water, and industrial wastewater) all contain a certain amount of salt, and the anions and cations that compose the salt migrate toward opposite electrodes under a DC electric field. If one cation-exchange membrane and one anion-exchange membrane are inserted into an electrodialyzer, the selective permeability means the salt concentration in the compartment between the two membranes drops due to directional ion migration, while the two compartments near the electrodes become concentration compartments for anions and cations respectively, ultimately achieving desalination in the middle compartment.
In practical applications, an electrodialyzer is not composed of just one pair of membranes (which would be inefficient) but of one hundred or even several hundred pairs, greatly improving efficiency.
Electrodialyzers are now widely used: in water desalination and de-salting, seawater concentration for salt production, dairy-product refining, juice deacidification and purification, and chemical-product production; and as pretreatment for high-purity water in electronics, pharmaceuticals, and other industries. They are also used for primary softening and desalination of boiler feed water and for converting brackish water into drinking water.
Electrodialyzers suit feed-water treatment in electronics, pharmaceutical, chemical, thermal-power, food, beer, beverage, printing-dyeing, and coating industries, and can also be used for concentration, purification, and separation in physical-chemical processes.
Electrodialysis can also be used for wastewater and waste-liquid treatment and precious-metal recovery, such as recovering nickel from electroplating waste liquid.
Typical operating parameters:
Operating pressure: about 0.5–3.0 kg/cm²
Operating voltage/current: 100–250 V, 1–3 A
Energy consumption: about 0.2–2.0 kWh per ton of fresh water
Main features:
Can simultaneously desalinate, concentrate, separate, and purify electrolyte aqueous solutions.
Can be used to purify non-electrolytes such as sucrose by removing electrolytes.
In principle, an electrodialyzer is an electrolytic cell with a diaphragm, able to exploit the high redox efficiency at the electrodes.
Secondary processes that also occur during electrodialysis:
Co-ion migration: membrane selectivity is never 100%, so a small amount of counter-ions always passes through.
Concentration diffusion: the concentration difference between concentration and dilution compartments causes a small amount of ions to diffuse from concentration to dilution, lowering efficiency.
Water osmosis: despite membranes not allowing solvent molecules through, the concentration difference drives some water molecules toward the concentration compartment.
Electro-osmosis of water: due to ion hydration and the electric double layer, water molecules can also migrate from dilution to concentration under the DC field.
Water pressure osmosis: the fluid-pressure difference between compartments forces water from the high-pressure to the low-pressure side.
Clearly, these secondary processes are unfavorable to electrodialysis, but they can all be avoided or controlled by changing operating conditions.
