Basic Wastewater Treatment Terminology: What Is Electrodialysis?

Electrodialysis 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 can pass through cation exchange membranes and anions through anion exchange membranes), so that anions and cations migrate toward the anode and the cathode respectively. During ion migration, if the fixed charge of the membrane is opposite to that of the ion, the ion can pass through; if the charges are the same, the ion is repelled. In this way, desalination, concentration, refining or purification of a solution is achieved [1].

Compared with reverse osmosis, another membrane separation technology introduced in recent years, electrodialysis is cheaper but has a lower desalination rate. When

③ Water permeation: although the exchange membrane does not allow solvent molecules to pass, the concentration difference between the dilute compartment and the concentrate compartment causes some solvent molecules (water) to permeate into the concentrate compartment;

The semi-permeable membrane used in electrodialysis is in fact an ion exchange membrane. According to the charge of the ions involved, such membranes are divided into cation exchange membranes (cation membranes) and anion exchange membranes (anion membranes). In an aqueous electrolyte solution, cation membranes allow cations to pass while repelling and blocking anions, and anion membranes allow anions to pass while repelling and blocking cations — this is the selective permeability of ion exchange membranes. In electrodialysis, the ion exchange membrane does not exchange with a particular ion in the aqueous solution as an ion exchange resin does; it only selectively allows ions of different charge to pass, which means the membrane does not need regeneration. The compartment formed by the electrodes and membranes in the electrodialysis process is called the electrode compartment, and the electrochemical reactions occurring there are the same as ordinary electrode reactions. Oxidation occurs in the anode compartment, the anode water is acidic and the anode itself is prone to corrosion. Reduction occurs in the cathode compartment, the cathode water is alkaline and the cathode is prone to scaling [2].

Electrodialysis is a relatively mature membrane separation technology that has been widely used for brackish water desalination and is the main method of producing fresh water in some parts of the world. Because newly developed charged membranes offer higher selectivity, lower membrane resistance, better thermal and chemical stability and greater mechanical strength, electrodialysis is no longer limited to desalination; it has many other industrial applications in the food, pharmaceutical and chemical industries. These include industrial wastewater treatment — mainly recovering acid and metals from spent acid used to clean metal surfaces, recovering heavy metal ions from electroplating wastewater, recovering sulfates from synthetic fibre wastewater and recovering sulfites from pulp waste liquor. In the food industry it is used, for example, to desalinate milk for infant formula; in the chemical industry to separate ionic from non-ionic substances; and in clinical treatment electrodialysis can serve as an artificial kidney [3].

Automatically controlled electrodialysis reversal (EDR) makes operation and management more convenient. Raw water utilization can reach 80%, with typical raw water recovery between 45% and 70%. Electrodialysis is mainly used for primary desalination of water, with desalination rates between 45% and 90%. It is widely used for desalination of seawater and brackish water, for primary desalination in the production of pure water, and for desalination and softening of feed water for boilers and power equipment.

In essence, electrodialysis can be described as a desalination technology, because all kinds of water (including natural water, tap water and industrial wastewater) contain a certain amount of salt, and the anions and cations making up these salts move toward electrodes of opposite polarity under a direct-current electric field. If one cation exchange membrane and one anion exchange membrane are inserted into an electrodialyzer, then because ion exchange membranes are selectively permeable — a cation exchange membrane allows only cations to pass freely and an anion exchange membrane only anions — the salt concentration in the compartment between the two membranes falls as ions migrate directionally, while the two compartments nearer the electrodes become the concentrate compartments for anions and cations respectively, finally achieving desalination in the central dilute compartment.

In practice, an electrodialyzer is not made up of a single pair of anion and cation exchange membranes (which would be very inefficient) but of a hundred or even several hundred pairs of membranes, greatly improving efficiency.

Electrodialyzers are currently used across a wide range of applications: water desalination, concentration of seawater for salt production, refining of dairy products, deacidification and purification of fruit juice, and production of chemical products. They can also be used in the food and light industries to produce pure water, and as pretreatment for producing high-purity water in the electronics and pharmaceutical industries; for primary softening and desalination of boiler feed water; and for desalinating brackish water into drinking water.

Electrodialyzers are suitable for water treatment in the electronics, pharmaceutical, chemical, thermal power, food, brewing, beverage, printing and dyeing, and coating industries. They can also be used for physicochemical processes such as concentration, purification and separation of materials.

Electrodialysis can also be used to treat wastewater and waste liquor and to recover precious metals, such as recovering nickel from electroplating waste liquor.

(1) Operating pressure: approximately 0.5–3.0 kg/cm²

(2) Operating voltage and current: 100–250 V, 1–3 A

(3) Power consumption of the unit itself: about 0.2–2.0 kWh per tonne of fresh water

① It can simultaneously desalinate, concentrate, separate and purify aqueous electrolyte solutions;

② It can be used to purify non-electrolytes such as sucrose by removing the electrolytes they contain;

③ In principle, an electrodialyzer is an electrolytic cell with a diaphragm, and the oxidation-reduction reactions at the electrodes can be utilized with high efficiency.

4. The following secondary processes also occur during electrodialysis

① Migration of co-ions: the selective permeability of ion exchange membranes can never be 100%, so a small number of oppositely charged ions always pass through the membranes;

② Concentration-difference diffusion of ions: because a concentration difference exists between the solutions in the concentrate and dilute compartments, a small number of ions always diffuse from the concentrate compartment into the dilute compartment, reducing dialysis efficiency;

domestically produced ion exchange membranes are now also very stable in quality, and operation and management are very convenient.

④ Electro-osmosis of water: because of ion hydration and the formation of electric double layers, water molecules can also migrate from the dilute compartment to the concentrate compartment under the direct-current field;

⑤ Polarization-induced water dissociation: sometimes, under poor operating conditions, water is forced to dissociate into hydrogen ions and hydroxide ions, which can pass through the exchange membranes into the concentrate compartment;

⑥ Pressure-driven water permeation: because a fluid pressure difference exists between the concentrate and dilute compartments, water molecules are forced to permeate from the higher-pressure side to the lower-pressure side. Clearly these secondary processes are unfavourable to electrodialysis, but they can all be avoided or controlled by changing the operating conditions.