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Electrodeionization(EDI)desalination

I. System Introduction

EDI (Electrodeionization) is a pure water production technology that combines ion exchange technology, ion exchange membrane technology, and ion electromigration technology. It ingeniously integrates electrodialysis and ion exchange technology, utilizing high voltage applied to electrodes at both ends to move charged ions in water, combined with ion exchange resins and selective resin membranes to accelerate ion removal and achieve water purification. Therefore, the EDI system here is a pure water production system. In the EDI desalination process, ions are removed through ion exchange membranes under the action of an electric field. Simultaneously, water molecules produce hydrogen ions and hydroxyl ions under the action of an electric field, which continuously regenerate the ion exchange resins to maintain their optimal state.

EDI Ultrapure Water Equipment

Historical Development of Ultrapure Water Production

Phase 1: Pretreatment Filter→Cation Bed →Anion Bed→Mixed Bed

Phase 2: Pretreatment Filter→Reverse Osmosis→ Mixed Bed

Current Phase: Pretreatment Filter→Reverse Osmosis→EDI (No Acid/Base Required)

For decades, mixed bed ion exchange technology (D) has been the standard process for ultrapure water production. However, it requires periodic regeneration, which consumes large amounts of chemicals (acid/base) and industrial pure water and poses environmental issues. Therefore, there is a need to develop acid/base-free ultrapure water systems. As traditional ion exchange increasingly fails to meet the needs of modern industry and environmental protection, EDI technology, which combines membranes, resins, and electrochemical principles, has revolutionized water treatment technology. Its ion exchange resins are regenerated using electrical energy, eliminating the need for acid/base and thus meeting today's environmental requirements.

II. Working Principle

The Electrodeionization (EDI) system is mainly a scientific water treatment technology that purifies water quality. Under the action of a direct current electric field, the dielectric ions in the water separated by partitions move in a directional manner, and the selective permeability of ion exchange membranes to ions is utilized.Between a pair of electrodes in an electrodialyzer, multiple groups of anion exchange membranes, cation exchange membranes and partitions (A and B) are usually arranged alternately to form concentrated chambers and dilute chambers (that is, cations can pass through the cation exchange membranes, and anions can pass through the anion exchange membranes).In the dilute chamber, the cations in the water migrate towards the negative electrode and pass through the cation exchange membranes, and are retained by the anion exchange membranes in the concentrated chamber. The anions in the water migrate towards the positive electrode and pass through the anion exchange membranes, and are retained by the cation exchange membranes in the concentrated chamber. In this way, the number of ions in the water passing through the dilute chamber gradually decreases, turning into fresh water. In the water of the concentrated chamber, due to the continuous influx of anions and cations in the concentrated chamber, the concentration of dielectric ions keeps increasing, and the water becomes concentrated water. Thus, the purposes of desalination, purification, concentration or refinement are achieved.

III. System Characteristics

EDI systems have been vigorously developed in industries such as pharmaceuticals, semiconductors, power generation, and surface cleaning, and are also widely used in wastewater treatment, beverages, microbiology, and other fields. EDI equipment is used after reverse osmosis systems to produce stable ultrapure water, replacing traditional mixed bed ion exchange technology (MB-DI). Compared to mixed ion exchange technology, EDI technology has the following advantages:

Stable water quality

Easy to achieve fully automatic control

No downtime due to regeneration

No need for chemical regeneration

Low operating costs

Small plant area

No sewage discharge

EDI Working Principle：

The EDI module sandwiches ion exchange resins between anion/cation exchange membranes to form an EDI unit. The EDI working principle is shown in the diagram. A certain number of EDI units in the EDI module are separated by partitions to form concentrated water chambers and dilute water chambers. An anion/cation electrode is set at both ends of the unit group. Driven by direct current, the cations and anions in the water flow through the anion and cation exchange membranes into the concentrated water chamber, respectively, and are removed from the dilute water chamber. The water passing through the concentrated water chamber carries ions out of the system, becoming concentrated water. EDI equipment generally uses secondary reverse osmosis (RO) pure water as feed water for EDI. The resistivity of RO pure water is typically 40-2 μS/cm (25°C). The resistivity of EDI pure water can reach up to 18 MΩ·cm (25°C), but depending on the application of deionized water and system configuration, EDI ultrapure water is suitable for producing pure water with a resistivity requirement of 1-18.2 MΩ·cm (25°C).

IV. Factors Affecting System Operation

Factors Affecting Operation：

(1) Impact of EDI Feed Water Conductivity: At the same operating current, as the conductivity of raw water increases, the removal rate of weak electrolytes by EDI decreases, and the conductivity of the outlet water also increases. If the conductivity of raw water is low, the ion content is also low, and low-concentration ions result in a large electromotive force gradient formed on the surface of the resin and membrane in the dilute chamber, enhancing the degree of water dissociation, increasing the limiting current, and producing more H+ and OH- ions, leading to good regeneration of the anion and cation exchange resins filled in the dilute chamber.

(2) Impact of Operating Voltage-Current: As the operating current increases, the quality of the produced water continuously improves. However, if the current is increased beyond the maximum point, excessive amounts of H+ and OH- ions produced by water ionization, in addition to being used for resin regeneration, serve as carrier ions for electrical conduction. At the same time, due to the accumulation and blockage of a large number of carrier ions during movement, and even reverse diffusion, the quality of the produced water decreases.

(3) Impact of Turbidity and Silt Density Index (SDI): The water production channels of EDI modules are filled with ion exchange resins. Excessively high turbidity and SDI can cause channel blockage, resulting in an increase in system differential pressure and a decrease in water production.

(4) Impact of Hardness: If the residual hardness of the inlet water in EDI is too high, it can lead to scaling on the membrane surface of the concentrated water channel, decreasing the flow rate of concentrated water and the resistivity of produced water, affecting the quality of produced water, and in severe cases, blocking the concentrated water and electrode water flow channels of the module, causing damage to the module due to internal heating.

(5) Impact of TOC (Total Organic Carbon): Excessively high organic content in inlet water can cause organic contamination of resins and selective permeable membranes, leading to an increase in system operating voltage and a decrease in water quality. It can also easily form organic colloids in the concentrated water channel, blocking the channel.

(6) Impact of CO2 in Inlet Water: HCO3- generated by CO2 in inlet water is a weak electrolyte, which can easily penetrate the ion exchange resin layer and degrade the quality of produced water.

(7) Impact of Total Anion Content (TEA): High TEA will reduce the resistivity of EDI produced water or require an increase in EDI operating current, while excessively high operating current will lead to an increase in system current and an increase in residual chlorine concentration in electrode water, adversely affecting the lifetime of electrode membranes. In addition, inlet water temperature, pH, SiO2, and oxides also affect the operation of EDI systems.

Inlet Water Quality Control:

(1) Control of Inlet Water Conductivity: Strictly control the conductivity during pretreatment to ensure that the conductivity of EDI inlet water is less than 40 μS/cm, guaranteeing qualified outlet water conductivity and removal of weak electrolytes.

(2) Control of Operating Voltage-Current: Select an appropriate operating voltage-current when the system is in operation. At the same time, since there is a limiting voltage-current point on the voltage-current curve of EDI water purification equipment, related to factors such as inlet water quality, membrane and resin performance, and membrane structure, etc., to produce a sufficient amount of H+ and OH- ions through a certain amount of water ionization to regenerate a certain amount of ion exchange resins, the selected voltage-current operating point of the EDI water purification equipment must be greater than the limiting voltage-current point.

(3) Control of Inlet Water CO2：Adjust the pH before RO by adding alkali to maximize the removal of CO2, or use degassing towers and degassing membranes to remove CO2.

(4) Control of Inlet Water Hardness：Combine CO2 removal with softening and alkalization of RO inlet water; when the inlet water salinity is high, combine desalination with an additional stage of RO or nanofiltration.

(5) Control of TOC：Combine with other indicator requirements and add an additional stage of RO to meet requirements.

(6) Control of Turbidity and Silt Density Index: Turbidity and SDI are one of the main indicators for inlet water control in RO systems, and qualified RO outlet water generally meets the inlet water requirements of EDI.

(7) Control of Fe：Control the Fe content of EDI inlet water to be below 0.01 mg/L during operation. If the resin has already been "poisoned," acid solution can be used for resuscitation treatment with good results.

(8) EDI system inlet water quality requirements

Based on the analysis of the above aspects, the water quality requirements for EDI inlet water are shown in the table, which can ensure that the outlet water indicators meet the high-purity water requirements for semiconductor manufacturing in the electronics industry.

Ⅴ.Application areas

1.Semiconductor and electronics industry - ultrapure water

2.Biological and pharmaceutical industry - purified water

3.Power plant - boiler makeup water

4.Surface coating

5.Consumer goods and cosmetics industry

6.Replacement of various types of distilled water

7.Other industries with high purity requirements for water

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