Electrodeionization (EDI) has recently gained widespread acceptance by water treatment companies and end users alike for almost all applications requiring high-purity water.
EDI has proven to be a reliable, cost-effective solution for both end users and water treatment companies. End users purchasing capital equipment can eliminate on-site chemical regeneration. Water treatment companies selling capital equipment can provide this ben-efit to their customers. Service exchange companies can eliminate chemical regeneration on their own premises and sell water by the gallon using a system that requires less maintenance than traditional service exchange.
Historically, ultrapure water systems were based completely on ion exchange. These systems consisted of cation units, followed by anion units, followed by mixed-bed units. While these systems provide high-quality water, they also require significant amounts of chemical regeneration.
Over the last 20 years., reverse osmosis (RO) has gained industry acceptance to replace the two-bed cation and anion vessels. Now EDI has replaced the mixed-bed, ion-exchange unit used for polishing. In combination with RO, EDI provides a continuous, chemical-free system.
EDI eliminates hazardous regeneration chemicals and the need to transport, store or handle acid and caustic. The continuous, simple operation eliminates the complicated mixed-bed regeneration process and requires less labor.
The process also eliminates the need for auxiliary equipment, such as regeneration skids, storage tanks, pH neutralization system and associated infrastructure. The process produces no h?iZ ardous waste streams, making special discharge permits unnecessary. In fact, most of the EDI waste can be recovered back into the water treatment system inlet.
In many cases, using EDI will result in lower operating and capital costs. The main consumables for mixed beus are chemicals, replacement resin, labor and wastewater. Hie main consumable in ED' is electricity. Replacement stacks may need to be refurbished or replaced from time to time. Labor and wastewater costs are significantly lower in EDI than for a mixed bed with a comparable flow.
The eiectrical cost of running an EDI system is usually between 0.5 to 3.0 kilowatts an hour for every 1,000 gallons of product water, depending on feedwater quality and product water specifications. In many cases, this results in lower operating costs compared with mixed-bed ion exchange.
Software is available from EDI manufacturers that compares the operating cost of EDI to mixed beds by incorporating application and site-specific requirements and costs. RO effluent projections serve as input to EDI process software.