Electrodeionization (EDI) uses electrical current, in lieu of acid and caustic chemicals, to regenerate cation and anion resin. Reverse osmosis/EDI systems are now widely used to replace conventional mixed-bed ion-exchange (IX) technology in a broad range of applications.
Proven effective at high flowrates, EDI has in recent years been incorporated in water treatment plants around the world, primarily in the power generation, chemical, electronics, and pharmaceutical industries. The benefits of EDI are well known. While conventional mixed-bed systems require batch regeneration of the resin beds, which leads to variations in water quality, EDI offers continuous operation. As a result, quality is consistent and predictable. Because no regeneration chemicals are required, compliance with stringent environmental and safety standards is simplified. The hazardous waste stream, common to all mixed-bed systems, is eliminated. The EDI reject water can be recovered by the system ahead of the RO, or sent directly to drain, with no neutralization infrastructure or permitting required. Electrodeionization systems also have a smaller footprint than mixed-bed IX systems of comparable capacity. The most widely used systems offer a modular design, with the flexibility to accommodate any height and space requirements.
As more companies make the move to EDI, the specific cost and performance benefits of this technology compared to mixed-bed systems are increasingly apparent. However, for the comparison to be accurate, it is important to look at the entire system. If the water treatment plant has already been specified, it will be necessary to take a step back and look at all the factors which affect system performance and payback. For many applications, EDI offers significant performance improvements while substantially reducing system infrastructure and operating costs. This article relates Fuji Photofilm's experience with the EDI technology.
When facility expansion at Fuji Photo-film in Greenwood, S.C., necessitated additional high-purity water production capacity, new water plant designs were evaluated. These design considerations included: initial capital cost, operating costs, physical space requirements, reliable performance, chemical usage, and waste disposal requirements.
The old plant consisted of an activated carbon filter pretreatment stage, an RO stage, and a mixed-bed deioniza-tion (Dl) stage. The capability of the new system was to be similar to the old one, consisting of two trains, each producing 250 gallons per minute (gpm) of product water. The decision to upgrade mixed-bed technology with more efficient EDI was made based on the results of a pilot test, as well as the reduced capital and operating costs that could result from its use.
A key aspect of the manufacture of photographic emulsion is the formation and growth of silver halide crystals. This critical reaction between silver nitrate and halide is process sensitive, and requires close quality control of the chemical constituents. Pure water, used as a solvent, is a primary ingredient in producing the silver chloride and halide solutions. Trace contaminants in the process water can have deleterious affects on the silver halide crystal formation and growth. Pure water is used further throughout the process in the making of numerous sensitizing chemicals, preparation of the gelatin used to suspend the halide crystals, for chemical 'washing' of the emulsion, and for post-batch clean in place.
In addition to the cost and performance considerations addressed below, Fuji had concerns about the reliability of mixed-bed technology. Upset conditions had previously been experienced causing process contamination and product loss. Electrodeionization offered fewer potential process problems with less possibility of contamination, and lower potential for operator exposure to hazardous chemicals. The new EDI system was installed in the spring of 2000.