Experimental Study on Operation of Electrodeionization (EDI) System

Electrodeionization (EDI) technology effectively integrates electrodialysis and ion exchange technologies. It is a novel water treatment method that uses mixed-bed resin filled between ion exchange membranes to achieve continuous desalination under a direct current electric field. It combines the advantages of continuous desalination in electrodialysis and deep desalination in ion exchange, while avoiding the concentration polarization problems of electrodialysis and the acid-base regeneration issues of ion exchange. Originating in the 1950s, this technology achieved breakthroughs in the 1990s amidst rapid industrial development and is now widely used in electronics, pharmaceuticals, energy, and laboratories, showing promise as a mainstream water treatment technology in the future.

This paper primarily investigates the pre-desalinated water at the reverse osmosis outlet of Zhejiang Jiaxing Power Generation Co., Ltd. through operational testing of the EDI system. It examines whether the pre-decarbonized water at the power plant's reverse osmosis outlet can meet boiler water quality requirements after EDI treatment. This study also provides relevant data and experience for the operation and maintenance of EDI systems in power generation.

System Overview

EDI Principle

EDI is also known as packed bed electrodialysis in my country. The desalination chamber of the electrodialyzer is filled with a mixture of anion and cation exchange resins (particles, fibers or woven fabrics), and the two processes of electrodialysis and ion exchange are carried out in the same container, so that the two processes are intrinsically linked.

It is generally believed that the principle of EDI can be divided into three aspects in the horizontal direction: ion exchange, selective migration of ions under DC electric field and electroregeneration of resin [1]. In high-purity water, the conductivity of ion exchange resin is 2 to 3 orders of magnitude higher than that of the water in contact with it, so almost all ion migration from solution to lipid surface is accomplished by resin. Ions in water are first adsorbed on resin particles due to exchange, and then migrate to the membrane surface through the ion propagation channel formed by resin particles under the action of electric field and enter the concentrate chamber through the ion selective membrane. At the same time, at the interface where resin, membrane and water are in contact, the polarization in the interface diffusion causes water to dissociate into hydrogen ions and hydroxide ions. Besides participating in the load current, most of these components also play a role in regenerating the resin, thus enabling the three processes of ion exchange, ion migration, and electroregeneration to occur simultaneously and promote each other, achieving continuous deionization.

Vertically, the EDI process can be divided into three parts from the influent side to the product water side. The part closer to the influent side is called the saturation zone, where the filled resin has already undergone ion exchange with the ions in the influent. The part closer to the effluent side is called the regeneration zone, where most of the ions in the effluent have been removed, and a small number of weakly ionized ions are removed. Simultaneously, pure water is ionized in this zone, and the generated H+ and OH- regenerate the filled resin. The area between the saturation zone and the regeneration zone is called the working zone, where ion exchange and electroregeneration tend to reach equilibrium.

Characteristics of EDI Technology

In chemical desalination systems, ion exchange devices have evolved from single-stage mixed beds to two-stage mixed beds, and finally to mixed beds. Using ion exchange, high-purity water with a resistivity close to theoretical pure water (18.2 MΩ·cm) can be produced. However, the advantage of ion exchange resins being repeatedly regenerable brings the waste acid and alkali from resin regeneration, causing environmental pollution. To overcome this pollution, reverse osmosis technology was introduced into water desalination systems, namely reverse osmosis + mixed bed desalination systems. Compared to ion exchange desalination systems, this reduces waste acid and alkali discharge by 90%, essentially solving the problem. However, with increasing process requirements, this method has revealed two drawbacks: mixed bed regeneration requires the storage of acid and alkali, making operation cumbersome. With the development of EDI technology, replacing mixed beds with EDI equipment to form RO-EDI desalination systems can overcome pollution and enable automated pure water production.

Characteristics of RO-EDI desalination systems: no acid or alkali required, no environmental pollution; continuous production without backup equipment; unattended operation, stable water quality; small footprint, low operating costs; special requirements for the influent water of both RO and EDI equipment.

System Process Flow Selection

Currently, the chemical makeup water supply system of Jiaxing Power Generation Co., Ltd. consists of a single-stage RO system plus a two-stage ion exchange desalination system. Therefore, this operational test adopted a combination of a single-stage RO system and an EDI system. The specific process flow is as follows:

Single-stage RO → Pre-deionized water tank after reverse osmosis → Booster pump → Decarbonizer → EDI booster pump → EDI module → Effluent

Since the pre-deionized water after reverse osmosis contains free CO2, a CO2 decarbonizer was installed before the EDI module to reduce the load on the EDI module.

Data Statistics during the EDI System Start-up Phase

The data from the start-up phase shows that, with the voltage remaining constant, both the system current and the resistance of the effluent increase.