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How Has EDI Technology Changed Since 1987?
By Fred Wiesler
Electrodeionization technology has evolved over the last 30 years to become a recognized process for final polishing of RO permeate water. In the first years of its development, pretreatment was not so well understood by the early adaptors of the technology, which meant challenging feedwater quality was sent to early electrodeionization units. Because of this, the technology was reported to be unstable and prone to scaling and fouling. This impacted the penetration of the technology and delayed its widespread entry into the water treatment market.
After some diligent promotion by the early developers, EDI technology was first accepted by the healthcare industry to produce high-purity water for manufacturing pharmaceutical products. The technology was well suited for low flowrates in these applications, and its hot water sanitization capability proved to be a strong driver in the industry
As the EDI installed base increased, the technology was reevaluated for use in other markets that required high-purity water. These markets included the power and semiconductor industries. Today, EDI is a well-accepted technology for polishing high-purity water for a variety of applications.
As the technology evolved, several major design improvements were made by the original developers as well as by new suppliers who had entered the market. These improvements have made the technology commercially attractive as a replacement to mixed-bed ion exchangers used for polishing water. This article reviews some of the design innovations that have occurred since the technology was first developed.
From ED to EDI
In order to fully understand the technology improvements, it is important to understand the electrodialysis (ED) process from which EDI evolved. In the ED process, ions are transported through ion-selective membranes. Water with dissolved ions is an electrolytic solution that can conduct electricity. Water flows through compartments that have alternating cationic and anionic membranes. The cationic membranes allow only cations to pass through, and the anionic membranes permit only anions to pass. When an electrical potential is applied across these compartments, the anions move toward the anode, and cations move towards the cathode. The water exiting the chambers can be separated, resulting in a concentrated stream and a diluted stream.
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