01
What is EDI?
The full English name of EDI is electrolysis, which translates to electric desalination, also known as electro deionization technology or packed bed electrodialysis.
Electrodeionization technology combines ion exchange and electrodialysis techniques. It is a desalination technology developed on the basis of electrodialysis, and is an increasingly widely used and effective water treatment technology after ion exchange resins.
It not only utilizes the advantages of continuous desalination through electrodialysis technology, but also achieves deep desalination through ion exchange technology;
This not only improves the defect of decreased current efficiency in the electrodialysis process for treating low concentration solutions, enhances ion transfer, but also enables the regeneration of ion exchangers, avoiding the use of regenerators and reducing secondary pollution generated during the use of acid-base regenerators, achieving continuous operation of deionization.
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EDI schematic diagram
The basic principles of EDI deionization include the following three processes:
1. Electrodialysis process
Under the action of an external electric field, electrolytes in water selectively migrate through ion exchange resins and are discharged with concentrated water, thereby removing ions from the water.
2. Ion exchange process
By using ion exchange resin to exchange impurity ions in water, combined with impurity ions in water, the effect of effectively removing ions from water can be achieved.
3. Electrochemical regeneration process
Utilizing the H+and OH - generated by the polarization of water at the interface of ion exchange resin for electrochemical regeneration of the resin, achieving self regeneration of the resin.
02
What are the influencing factors and control measures of EDI?
1. The influence of inlet conductivity
Under the same operating current, as the conductivity of the raw water increases, the removal rate of weak electrolytes by EDI decreases, and the conductivity of the effluent also increases.
If the conductivity of the raw water is low, the ion content 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 freshwater chamber, leading to an increase in the degree of water dissociation, an increase in the maximum current, and a larger amount of H+and OH - produced, resulting in a good regeneration effect of the anion and cation exchange resin filled in the freshwater chamber.
Therefore, it is necessary to control the conductivity of the incoming water to ensure that the EDI incoming water conductivity is less than 40us/cm, which can guarantee the qualified effluent conductivity and the removal of weak electrolytes.
2. The influence of working voltage and current
The working current increases, and the quality of the produced water continues to improve.
But if the current is increased after reaching the highest point, due to the excessive amount of H+and OH - ions generated by water ionization, a large number of surplus ions act as current carrying ions for conduction, in addition to being used for regenerating resins. At the same time, due to the accumulation and blockage of a large number of current carrying ions during their movement, even counter diffusion occurs, resulting in a decrease in the quality of the produced water.
Therefore, it is necessary to choose appropriate working voltage and current.
3. The impact of turbidity and pollution index (SDI)
The EDI component's water production channel is filled with ion exchange resin. Excessive turbidity and pollution index can cause channel blockage, resulting in an increase in system pressure difference and a decrease in water production.
Therefore, appropriate pretreatment is required, and RO effluent generally meets the EDI inlet requirements.
4. The influence of hardness
If the residual hardness of the incoming water in EDI is too high, it will cause scaling on the membrane surface of the concentrated water channel, reduce the flow rate of concentrated water, decrease the electrical resistivity of the produced water, affect the quality of the produced water, and in severe cases, block the concentrated water and extreme water channels of the components, resulting in damage to the components due to internal heating.
Can be combined with CO2 removal to soften and add alkali to RO influent; When the salt content in the influent is high, the effect of hardness can be adjusted by adding a first stage RO or nanofiltration in conjunction with desalination.
5. The impact of Total Organic Carbon (TOC)
If the organic content in the influent is too high, it will cause organic pollution of the resin and selective permeable membrane, leading to an increase in system operating voltage and a decrease in the quality of the produced water. At the same time, it is also easy to form organic colloids in the concentrated water channel, which can block the channel.
Therefore, in processing, an additional level R0 can be added in conjunction with other indicator requirements to meet the requirements.
6. The influence of metal ions such as Fe and Mn
Metal ions such as Fe and Mn can cause resin "poisoning", and the metal "poisoning" of resin can lead to rapid deterioration of EDI effluent quality, especially the rapid decrease in silicon removal rate.
In addition, the oxidation catalytic effect of variable valence metals on ion exchange resins can cause permanent damage to the resin.
Generally speaking, the Fe content in the EDI inlet during operation is controlled to be below 0.01mg/L.
7. The impact of CO2 in influent
The HCO3- generated by CO2 in the inflow is a weak electrolyte that can easily penetrate the ion exchange resin layer and cause a decrease in the quality of the produced water.
Before entering the water, a degassing tower can be used for removal.
8. The influence of total anion content (TEA)
A high TEA will reduce the resistivity of EDI water production or require an increase in EDI operating current, while excessively high operating current will lead to an increase in system current and residual chlorine concentration in the polar water, which is detrimental to the lifespan of the polar membrane.
In addition to the above 8 influencing factors, the 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 applied in industries such as power, chemical, and pharmaceutical industries that require high water quality.
Long term application research in the field of water treatment has shown that EDI processing 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 for further desalination can effectively remove ions from water and achieve high effluent purity.
2. Low equipment installation conditions and small footprint
Compared with ion exchange beds, EDI devices are smaller in size, lighter in weight, and do not require acid or alkali storage tanks, which can effectively save space.
Moreover, the EDI device is a fully assembled structure with a short construction period and minimal on-site installation workload.
3. Simple design, easy operation and maintenance
The EDI processing device can be modularized for production and can automatically regenerate continuously without the need for large and complex regeneration equipment. After being put into operation, it is easy to operate and maintain.
4. Automatic control of water purification process is simple and convenient
EDI devices can connect multiple modules in parallel to the system, ensuring safe and stable module operation, reliable quality, and easy program control for system operation and management.
5. No discharge of waste acid and alkali solution, beneficial for environmental protection
EDI devices do not require acid or alkali chemical regeneration, and there is basically no discharge of chemical waste.
6. The water recovery rate is high, and the water utilization rate of EDI treatment technology is generally over 90%
In summary, EDI technology has significant advantages in terms of water quality, operational stability, ease of operation and maintenance, safety and environmental protection.
But it also has certain shortcomings. EDI devices have high requirements for the quality of incoming water, and their one-time investment (infrastructure and equipment costs) is relatively high.
It should be noted that although the infrastructure and equipment costs of EDI are slightly higher than those of mixed bed processes, considering the overall operating costs of the equipment, EDI technology still has certain advantages.
For example, a pure water station compared the investment and operating costs of 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 for the project
In terms of initial investment for the project, in water treatment systems with low water production flow rates, the reverse osmosis+EDI process eliminates the large regeneration system required by traditional ion exchange processes, especially by eliminating two acid and alkali storage tanks each. This not only greatly reduces equipment procurement costs, but also saves about 10% to 20% of the land area, thereby reducing the civil engineering and land acquisition costs for building factories.
Due to the fact that the height of traditional ion exchange equipment is generally over 5m, while the height of reverse osmosis and EDI equipment is within 2.5m, the height of the water treatment workshop can be reduced by 2-3m, thereby saving 10% to 20% of the construction investment in the workshop.
Considering the recovery rates of reverse osmosis and EDI, all 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 pretreatment system needs to be correspondingly increased. When using traditional coagulation clarification and filtration processes in the pretreatment system, the initial investment needs to be increased by about 20% compared to the pretreatment system using ion exchange processes.
Taking all factors into consideration, the reverse osmosis+EDI process is roughly equivalent in initial investment to traditional ion exchange processes in small-scale water treatment systems.
2. Comparison of operating costs
As is well known, in terms of drug consumption, the operating cost of reverse osmosis technology (including reverse osmosis dosing, chemical cleaning, wastewater treatment, etc.) is lower than that of traditional ion exchange technology (including ion exchange resin regeneration, wastewater treatment, etc.).
However, in terms of power consumption and replacement of spare parts, the reverse osmosis and EDI process is much higher than the traditional ion exchange process.
According to statistics, the operating cost of reverse osmosis combined with EDI process is slightly higher than that of traditional ion exchange process.
Taking all factors into consideration, the overall operation and maintenance cost of reverse osmosis combined with EDI process is 50% to 70% higher than that of traditional ion exchange process.
3. Reverse osmosis+EDI has strong adaptability, high degree of automation, and minimal environmental pollution
The reverse osmosis+EDI process has strong adaptability to the salt content of raw water, and can be used for seawater, brackish water, mine dewatering water, groundwater, and river water. However, the ion exchange process is not economical when the dissolved solid content in the influent is greater than 500 mg/L.
Reverse osmosis and EDI do not require acid-base regeneration, consume a large amount of acid-base, or generate a large amount of acid-base wastewater. Only a small amount of acid, alkali, scale inhibitor, and reducing agent need to be added.
In terms of operation and maintenance, reverse osmosis and EDI also have the advantages of high automation and easy program control.
4. Reverse osmosis+EDI equipment is expensive and difficult to repair, and the treatment of concentrated saltwater is challenging
Although the reverse osmosis plus EDI process has many advantages, in the event of equipment failure, especially when the reverse osmosis membrane and EDI membrane stack are damaged, it can only be shut down for replacement. In most cases, professional technicians are needed for replacement, and the shutdown time may be longer.
Although reverse osmosis does not produce a large amount of acidic and alkaline wastewater, the recovery rate of primary reverse osmosis is generally only 75%, which produces a large amount of concentrated water. The salt content of concentrated water is much higher than that of raw water. There is currently no mature treatment measure for this concentrated water, and once discharged, it will pollute the environment.
At present, in domestic power plants, the recovery and utilization of concentrated salt water from reverse osmosis is mostly used for coal washing and ash humidification; Some universities are conducting research on the evaporation and crystallization purification process of concentrated salt water, but the cost is high and the difficulty is high, so it has not been widely applied in industry yet.
The cost of reverse osmosis and EDI equipment is relatively high, but in some cases, the initial investment is even lower than that of traditional ion exchange processes.
In large-scale 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 that of traditional ion exchange processes.
In small-scale water treatment systems, the reverse osmosis plus EDI process has a similar initial investment compared to traditional ion exchange processes.
In summary, when the output of the water treatment system is low, the reverse osmosis and EDI treatment process can be prioritized. This process has low initial investment, high degree of automation, and minimal environmental pollution.
