Electrodeionization system

Electrodeionization systems were first suggested to remove small amounts of radioactive elements from contaminated waters [27], but the principal current application is the preparation of ultrapure water for the electronics and pharmaceutical industries [28], The process is sometimes used as a polishing step after the water has been pretreated with a reverse osmosis unit. 

The main difference between the electrodeionization system with the mixed-bed ion-exchange resins and the system with separate beds is that in mixed-bed electrodeionization systems anions and cations are simultaneously removed from the feed... 

Continuous electrodeionization systems can achieve 95% rejection of boron and silica, and 99+% rejection of sodium and chloride. This performance is possible due to voltage-induced dissociation of water that effectively regenerates a portion of the resin thereby allowing removal of weakly ionized species such as silica and boron.19 In fact, the boron in the effluent from a CEDI system can be lower than that in the effluent from a mixed-bed ion exchange system.13... 

S. Yoshida, T. Watanabe and H. Uchino, Electrodeionization system - novel technique of ultrapure water production, Kaisui Gahkaishi (Bull. Soc. Seawater Sci.), 2002, 56, 220-227. 

Reverse osmosis/electrodeionization (RO/EDI) plants are available in modular form to suit any desired input-output water quality and flow rate. A RO/EDI system should be capable of producing high-purity water of perhaps 5 to 20 xS/cm conductivity (0.2-0.05 MO/cm resistance). By providing a second EDI stack in series, it is possible to achieve even higher quality of up to 0.055 xS/cm conductivity (18.2 Mfl/cm resistance). 

In this chapter, the impact of other membrane technologies on the operation of RO systems is discussed. Technologies considered include microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) as pretreatment to RO, and continuous electrodeionization (CEDI) as post-treatment to RO. This chapter also describes the HERO (high efficiency RO—Debasish Mukhopadhyay patent holder, 1999) process used to generate high purity water from water that is difficult to treat, such as water containing high concentrations of silica. 

Continuous electrodeionization is primarily used as an alternative to ion exchange. Because of the extensive pretreatment required by CEDI systems, the technology has grown into a polisher for RO (see Figure 16.18). Continuous deionization can achieve mixed-bed water quality of RO permeate without the need to store and... 

Hybrid systems have attracted interest in recent years due to their potential for reducing costs, decreasing environmental impact from discharge streams, and expanding the envelope of source waters that can be treated [52], For example, electrodeionization (EDI) is a hybrid process involving ED and IX. In an EDI system, the space between the IX membranes is packed with IX resins. The addition of the resin improves the conductivity across the cell and allows the production of highly deionized water. EDI systems have attracted interest for boiler feedwater treatment applications [53]. Hybrid systems using combinations of RO and IX have also been piloted and commercialized [54]. 

A typical ultrapure water system for the semiconductor industry is illustrated in Eigure 13.1. The reverse osmosis (RO)/electrodeionization (EDI) system is gaining more and more in importance in a typical UPW system due to its contamination-free design. 

An integrated membrane approach in UPW systems consists of four major membrane-based water treatment components ultrafiltration (UF), reverse osmosis (RO), electrodeionization (EDI), and membrane degasification. Each process is unique and contributes particular advantages to the system design. As the need increases and the costs become more acceptable, these technologies will become lynchpins of UPW systems. 

Electrodeionization S em Material Balance The percentage of the feed water that becomes product is referred to as the recovery of the system. The recent EDI units are typically operated at very high system recovery. With the addition of a concentrate recycle system, the new generation EDI modules are routinely operated at 80-95% feed water recovery in post-RO application. Recovery rate is decided by EDI feed water hardness. Recovery for the EDI units can be expressed as follows ... 

Electrodeionization vendors recommend injecting brine in the concentrate loop of their systems to maintain the conductivity of the concentrate stream between 250 and 600 pS/cm. The use of Canners evaporated salt or equivalent is recommended for this purpose. At lower concentrate water conductivity, a rise in the electrical resistance will augment the power consumption. 

Dey, A. (2005a). Spiral wound electrodeionization An emerging energy-efficient deionization process in high purity water systems. Paper presented at the Semiconductor Pure Water Chemical Conference (SPWCC), Santa Clara, CA, Feb. 14-16. 

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