01

What is EDI?

The full English name for EDI is electrode ionization, which translates into electrodesalting, also known as electrodeionization, or packed bed electrodialysis.

Electrodeionization technology combines ion exchange and electrodialysis. It is a desalination technology developed on the basis of electrodialysis, and is increasingly widely used and achieving good results in water treatment technology after ion exchange resins.

It not only utilizes the advantages of continuous desalting by electrodialysis technology, but also utilizes ion exchange technology to achieve deep desalting effect;

This not only improves the defect of decreased current efficiency when electrodialysis processes low concentration solutions, enhances ion transfer, but also enables the regeneration of ion exchange agents, avoids the use of regenerants, reduces secondary pollution generated during the use of acid and alkali regenerants, and realizes continuous deionization operations.

Schematic diagram of EDI principle

The basic principle of EDI deionization includes the following three processes:

1. Electrodialysis process

Under the action of an applied electric field, electrolytes in water undergo selective migration through ion exchange resins in the water and are discharged with concentrated water, thereby removing ions from the water.

2. Ion exchange process

Impurity ions in water are exchanged through ion exchange resins, which combine with impurities in water to effectively remove ions from water.

3. Electrochemical regeneration process

Electrochemical regeneration of the resin is carried out using H+and OH - generated by polarization of water at the interface of the ion exchange resin to achieve self regeneration of the resin.

02

What are the influencing factors and control measures for EDI?

1. Effect of influent conductivity

At the same operating current, with the increase in the conductivity of raw water, the removal rate of weak electrolytes by EDI decreases, and the conductivity of effluent also increases.

If the conductivity of raw water is low, the content of ions is also low, and the low concentration of ions causes a large electromotive force gradient to form on the surface of the resin and membrane in the fresh water chamber, resulting in an increase in the degree of dissociation of water, an increase in the limiting current, and a large amount of H+and OH - generated, resulting in a good regeneration effect of anion and cation exchange resins filled in the fresh water chamber.

Therefore, it is necessary to control the inlet conductivity to make the EDI inlet conductivity less than 40us/cm, which can ensure that the outlet conductivity is qualified and weak electrolyte is removed.

2. Influence of working voltage and current

The working current increases and the quality of produced water continuously improves.

However, if the current is increased after increasing to the highest point, due to the excessive amount of H+and OH - ions generated by water ionization, in addition to being used to regenerate resins, a large number of surplus ions act as current carrying ions to conduct electricity. At the same time, due to the accumulation and blockage of a large number of current carrying ions during their movement, and even reverse diffusion, the quality of the produced water decreases.

Therefore, it is necessary to select the appropriate working voltage and current.

3. Impact of turbidity and pollution index (SDI)

The water production channel of EDI module is filled with ion exchange resin, and excessive turbidity and pollution index can cause channel blockage, resulting in an increase in system pressure differential and a decrease in water production.

Therefore, appropriate pretreatment is required, and the RO effluent generally meets the EDI inflow requirements.

4. Effect of hardness

If the residual hardness of the incoming water in EDI is too high, it can cause scaling on the membrane surface of the concentrated water channel, resulting in a decrease in the concentrated water flow rate and the resistivity of the produced water, affecting the quality of the produced water. In severe cases, it can block the concentrated water and extreme water flow channels of the module, causing the module to be damaged due to internal heating.

It can be combined with CO2 removal to soften and add alkali to RO influent water; When the salt content of the influent water is high, the effect of hardness can be adjusted by adding a primary RO or nanofiltration in combination with desalination.

5. Effect of TOC (Total Organic Carbon)

If the content of organic matter in the influent water is too high, it will cause organic pollution of the resin and selective permeable membrane, leading to a rise in the system operating voltage and a decline in the quality of the produced water. At the same time, it is also easy to form organic colloids in the concentrated water channel, blocking the channel.

Therefore, during processing, a level R0 can be added in combination with other indicator requirements to meet the requirements.

6. Effects of metal ions such as Fe and Mn

Metal ions such as Fe and Mn can cause "poisoning" of the resin, while metal "poisoning" of the resin can cause rapid deterioration of the EDI effluent quality, especially the rapid decline in the silicon removal rate.

In addition, the oxidation catalysis of variable valence metals on ion exchange resins can cause permanent damage to the resins.

Generally speaking, during operation, the Fe content of EDI inlet water is controlled to be lower than 0.01 mg/L.

7. Effect of C02 in influent water

The HCO3 - generated by CO2 in the influent water is a weak electrolyte that easily penetrates the ion exchange resin layer, resulting in a decline in the quality of the produced water.

The water can be removed by a degassing tower before entering the water.

8. Effect of Total Anion Content (TEA)

High TEA will reduce the EDI water production resistivity or need to increase the EDI operating current, while high operating current will lead to an increase in the system current and increase the residual chlorine concentration in the electrode water, which is detrimental to the electrode film life.

In addition to the above eight influencing factors, inlet water temperature, pH value, SiO2, and oxides also have an impact on the operation of the EDI system.

03

Characteristics of EDI

In recent years, EDI technology has been widely used in industries that require high water quality, such as electric power, chemical engineering, and medicine.

The long-term application research in the field of water treatment shows that EDI treatment technology has the following six characteristics:

1. High water quality and stable effluent

EDI technology combines the advantages of continuous desalination by electrodialysis and deep desalination by ion exchange. Continuous scientific research and practice have shown that using EDI technology to conduct secondary desalination can effectively remove ions from water, and the purity of the effluent is high.

2. Low equipment installation conditions and small floor area

Compared with ion exchange beds, EDI devices are small in size, light in weight, and do not require acid and alkali storage tanks, which can effectively save space.

Moreover, the EDI device is of a packaged structural type, with a short construction period and a small on-site installation workload.

3. Simple design, easy operation and maintenance

EDI processing devices can be produced in a modular manner and can be automatically and continuously regenerated without the need for large and complex regeneration equipment. After being put into operation, operation and maintenance are simple and convenient.

4. Simple and convenient automatic control of water purification process

The EDI device can connect multiple modules in parallel to the system, with safe and stable module operation and reliable quality, making the operation and management of the system easy to achieve program control and easy to operate.

5. No waste acid, waste alkali discharge, beneficial to environmental protection

The EDI device does not require acid and alkali chemical regeneration, and there is basically no chemical waste discharge.

6. The water recovery rate is high, and the water utilization rate of EDI treatment technology is generally as high as 90% or more

In summary, EDI technology has great advantages in terms of water quality, operational stability, ease of operation and maintenance, safety and environmental protection.

However, it also has certain shortcomings. EDI devices have high requirements for influent water quality, and their one-time investment (infrastructure and equipment costs) is high.

It should be noted that although the infrastructure and equipment costs of EDI are slightly higher than those of the mixed bed process, after comprehensive consideration of the costs of device operation, EDI technology still has certain advantages.

For example, a pure water station has compared the investment and operating costs of the two processes, and the EDI device can offset the investment difference with the mixed bed process after one year of normal operation.

04

Reverse osmosis+EDI VS traditional ion exchange

1. Comparison of initial investment of the project

In terms of the initial investment of the project, in the water treatment system with small water production flow, due to the elimination of the large regeneration system required by the traditional ion exchange process by the reverse osmosis+EDI process, especially the elimination of two acid storage tanks and two alkali storage tanks, not only greatly reducing the cost of equipment procurement, but also saving about 10% to 20% of the floor area, thereby reducing the civil engineering costs and land acquisition costs of building a plant.

Due to the fact that the height of traditional ion exchange equipment is generally above 5m, while the height of reverse osmosis and EDI equipment is within 2.5m, the height of the water treatment workshop building can be reduced by 2 to 3m, thereby saving another 10% to 20% of the building civil engineering investment.

Considering the recovery rates of reverse osmosis and EDI, all the concentrated water from secondary reverse osmosis and EDI is recovered, but the concentrated water from primary reverse osmosis (about 25%) needs to be discharged, and the output of the pre-treatment system needs to be increased accordingly. When the pre-treatment system adopts the traditional coagulation, clarification, and filtration process, the initial investment needs to be increased by about 20% compared to the pre-treatment system of the ion exchange process.

Overall, the initial investment of the reverse osmosis+EDI process in small water treatment systems is substantially equivalent to that of traditional ion exchange processes.

2. Comparison of operating costs

It is well known that in terms of reagent consumption, the operating costs of reverse osmosis processes (including reverse osmosis dosing, chemical cleaning, wastewater treatment, etc.) are lower than those of traditional ion exchange processes (including ion exchange resin regeneration, wastewater treatment, etc.).

However, in terms of power consumption and spare parts replacement, the reverse osmosis plus EDI process is much higher than the traditional ion exchange process.

According to statistics, the operation cost of the reverse osmosis plus EDI process is slightly higher than that of the traditional ion exchange process.

Overall, the overall operation and maintenance cost of the reverse osmosis plus EDI process is 50% to 70% higher than that of the traditional ion exchange process.

3. Reverse osmosis+EDI has strong adaptability, high degree of automation, and little environmental pollution

The reverse osmosis+EDI process has a strong adaptability to the salt content of raw water. From seawater, brackish water, mine drainage water, groundwater, to river water, the reverse osmosis process can be used, while the ion exchange process is not economical when the dissolved solids content in the influent water is greater than 500 mg/L.

Reverse osmosis and EDI do not require acid and alkali regeneration, do not require a large amount of acid and alkali consumption, and do not generate a large amount of acid and alkali wastewater. Only a small amount of acid, alkali, scale inhibitor, and reducing agent dosing is required.

In terms of operation and maintenance, reverse osmosis and EDI also have the advantages of high automation and convenient program control.

4. Reverse osmosis+EDI equipment is expensive and difficult to repair, and it is difficult to treat concentrated brine

Although the reverse osmosis plus EDI process has many advantages, when equipment fails, especially when the reverse osmosis membrane and EDI membrane stack are damaged, they can only be stopped for replacement. In most cases, professional technicians are required to replace them, and the shutdown time may be longer.

Although reverse osmosis does not generate a large amount of acid and alkali wastewater, the recovery rate of primary reverse osmosis is generally only 75%, resulting in a large amount of concentrated water, which has a much higher salt content than raw water. Currently, there are no mature treatment measures for this part of concentrated water, and once discharged, it will pollute the environment.

Currently, in domestic power plants, the recovery and utilization of concentrated brine from reverse osmosis is mostly used for coal handling washing and ash humidification; Some universities are conducting research on concentrated brine evaporation and crystallization purification processes, but due to their high cost and difficulty, they have not yet been widely applied in industry.

The cost of reverse osmosis and EDI equipment is relatively high, but in some cases even lower than the initial engineering investment of traditional ion exchange processes.

In large water treatment systems (when the system produces a large amount of water), the initial investment in reverse osmosis and EDI systems is much higher than in traditional ion exchange processes.

In small-scale water treatment systems, the reverse osmosis plus EDI process is substantially equivalent to the traditional ion exchange process in terms of initial investment in small-scale water treatment systems.

In summary, when the output of the water treatment system is low, priority can be given to using the reverse osmosis plus EDI treatment process, which has low initial investment, high degree of automation, and low environmental pollution.