WHAT IS ELECTRODIALYSIS?
Electrodialysis (ED) is a separation process in which charged membranes and electrical potential difference are used to separate ionic species from an aqueous solution and other uncharged components.
HOW DOES ELECTRODIALYSIS WORK?
The Electrodialysis process utilizes an electrodialysis stack built on the filter press principle. The stack consists of 200–400 alternate cationic and anionic membranes between two electrodes; the aqueous feed solution flows through the cells between each pair of membranes as shown in the picture below..
When an electrical potential difference is applied between the two electrodes, positively charged cations in the feed solution move toward the cathode. These ions easily pass through the negatively charged cation exchange membranes, but are retained by the positively charged anion exchange membranes. Similarly, negatively charged anions migrate towards the anode, pass through the anion exchange membrane and are retained by the cation exchange membrane.
Because of the arrangement of ion-selective membranes, the migrating ions become concentrated in each alternate cell in the stack. Thus, ions removed from the aqueous feed solution are concentrated into two separate streams.
In application, electrodialysis systems can be operated as continuous production or batch production processes. In a continuous process, feed is passed through a sufficient number of stacks placed in series to produce the final desired product quality. In batch processes, the diluate and/or concentrate streams are re-circulated through the electrodialysis systems until the final product or concentrate quality is achieved.
Electrodialysis is usually applied to deionization of aqueous solutions. However, desalting of sparingly conductive aqueous organic and organic solutions is also possible.
Some applications of electrodialysis include:
Large scale brackish and seawater desalination and salt production
Small and medium scale drinking water production (e.g., towns & villages, construction & military camps, nitrate reduction, hotels & hospitals)
Water reuse (e.g., desalination brine treatment, industrial laundry wastewater, produced water from oil/gas production, cooling tower makeup & blowdown, metals industry fluids, wash-rack water)
Pre-demineralization (e.g., boiler makeup & pretreatment, ultrapure water pretreatment, process water desalination, power generation, semiconductor, chemical manufacturing, food and beverage)
Agricultural water (e.g., water for greenhouses, hydroponics, irrigation, livestock)
Glycol desalting (e.g., antifreeze / engine-coolants, capacitor electrolyte fluids, oil and gas dehydration, conditioning and processing solutions, industrial heat transfer fluids, secondary coolants from heating, venting, and air conditioning (HVAC))
The major application of electrodialysis has historically been the desalination of brackish water or seawater as an alternative to RO for potable water production and seawater concentration for salt production.
In normal potable water production without the requirement of high recoveries, reverse osmosis is generally believed to be more cost-effective when total dissolved solids (TDS) are 3,000 parts per million (ppm) or greater, while electrodialysis is more cost-effective for TDS feed concentrations less than 3,000 ppm or when high recoveries of the feed are required.
Another important application for electrodialysis is the production of pure water and ultrapure water by electrodeionization (EDI). In EDI, the purifying compartments and sometimes the concentrating compartments of the electrodialysis stack are filled with ion-exchange resin. When fed with low TDS feed, the product can reach very high purity levels. The ion-exchange resins act to retain the ions, allowing these to be transported across the ion-exchange membranes.