09 Aug 2024
Comparison of Reverse Osmosis + EDI and Traditional Ion Exchange Process Technology
1.What is EDI？
The full name of EDI is electrode ionization, which translates to electric desalination, also known as electrodeionization technology, or packed bed electrodialysis.
Electrodeionization technology combines ion exchange and electrodialysis. It is a desalination technology developed on the basis of electrodialysis. It is a water treatment technology that has been widely used and achieved good results after ion exchange resins.
It not only utilizes the advantages of continuous desalination of electrodialysis technology, but also utilizes ion exchange technology to achieve deep desalination;
It not only improves the defect of decreased current efficiency when treating low-concentration solutions in the electrodialysis process, enhances ion transfer, but also enables ion exchangers to be regenerated, avoids the use of regeneration agents, reduces secondary pollution generated during the use of acid-base regeneration agents, and realizes continuous deionization operation.
The basic principle of EDI deionization includes the following three processes:
1. Electrodialysis process
Under the action of an external electric field, the electrolyte in the water selectively migrates through the ion exchange resin in the water and is discharged with the concentrated water, thereby removing the ions in the water.
2. Ion exchange process
The impurity ions in the water are exchanged and combined with the impurity ions in the water through the ion exchange resin, thereby achieving the effect of effectively removing the ions in the water.
3. Electrochemical regeneration process
The H+ and OH- generated by the polarization of water at the ion exchange resin interface are used to electrochemically regenerate the resin to achieve self-regeneration of the resin.
02 What are the factors affecting EDI and what are the control measures?
1. Influence of inlet water conductivity
Under the same operating current, as the raw water conductivity increases, the EDI removal rate of weak electrolytes decreases, and the effluent conductivity also increases.
If the raw water conductivity is low, the ion content is also low, and the low concentration of ions makes the electromotive force gradient formed on the surface of the resin and membrane in the fresh water chamber also large, resulting in an enhanced degree of water dissociation, an increase in the limiting current, and a large number of H+ and OH-, so that the regeneration effect of the anion and cation exchange resins filled in the fresh water chamber is good.
Therefore, it is necessary to control the inlet water conductivity so that the EDI inlet water conductivity is less than 40us/cm, which can ensure the qualified effluent conductivity and the removal of weak electrolytes.
2. Influence of working voltage and current
As the working current increases, the water quality of the produced water continues to improve.
However, if the current is increased after reaching the highest point, due to the excessive amount of H+ and OH- ions produced by water ionization, in addition to being used for regeneration of resin, a large number of surplus ions act as carrier ions for conduction. At the same time, due to the accumulation and blockage of a large number of carrier ions during movement, even reverse diffusion occurs, resulting in a decrease in the quality of produced water.
Therefore, it is necessary to select appropriate working voltage and current.
3. Influence of turbidity and pollution index (SDI)
The water production channel of the EDI component is filled with ion exchange resin. Excessive turbidity and pollution index will block the channel, causing the system pressure difference to rise and the water production to decrease.
Therefore, appropriate pretreatment is required, and RO effluent generally meets the EDI inlet requirements.
4. Influence of hardness
If the residual hardness of the inlet water in the EDI is too high, it will cause scaling on the membrane surface of the concentrated water channel, reduce the concentrated water flow rate, reduce the resistivity of the produced water, affect the water quality of the produced water, and in severe cases, block the concentrated water and polar water flow channels of the component, causing the component to be destroyed due to internal heating.
The RO inlet water can be softened and alkali can be added in combination with CO2 removal; when the inlet water has a high salt content, a first-level RO or nanofiltration can be added in combination with desalination to adjust the impact of hardness.
5. Impact of TOC (Total Organic Carbon)
If the organic content in the influent is too high, it will cause organic pollution of the resin and the selective permeable membrane, resulting in an increase in the 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 and block the channel.
Therefore, when treating, you can combine other index requirements to increase the level of R0 to meet the requirements.
6. Impact of metal ions such as Fe and Mn
Metal ions such as Fe and Mn will cause "poisoning" of the resin, and the metal "poisoning" of the resin will cause the rapid deterioration of the EDI effluent quality, especially the rapid decrease in the removal rate of silicon.
In addition, the oxidative catalytic effect of variable valence metals on ion exchange resins will cause permanent damage to the resin.Generally speaking, the Fe of EDI influent is controlled to be less than 0.01 mg/L during operation.
7. Impact of CO2 in influent
HCO3- generated by CO2 in the influent is a weak electrolyte, which can easily penetrate the ion exchange resin layer and cause the quality of the produced water to decrease.A degassing tower can be used to remove it before influent.
8. Influence of total anion content (TEA)
High TEA will reduce the resistivity of EDI produced water, or require an increase in EDI operating current. Excessive operating current will increase system current and increase residual chlorine concentration in the electrode water, which is not good for the life of the electrode membrane.
In addition to the above 8 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
EDI technology has been widely used in industries with high water quality requirements such as electricity, chemical industry, and medicine.
Long-term application research in the field of water treatment shows that EDI treatment technology has the following 6 characteristics:
1. High water quality and stable water output
EDI technology combines the advantages of continuous desalination by electrodialysis and deep desalination by ion exchange. Continuous scientific research practice shows that the use of EDI technology for desalination can effectively remove ions in water and produce high purity water output.
2. Low equipment installation conditions and small footprint
Compared with ion exchange beds, EDI devices are small in size and light in weight, and do not require acid or alkali storage tanks, which can effectively save space.
Not only that, the EDI device is a prefabricated structure with a short construction period and small on-site installation workload.
3. Simple design, easy operation and maintenance
EDI treatment devices can be produced in modularized form, can be automatically and continuously regenerated, do not require large and complex regeneration equipment, and are easy to operate and maintain after being put into operation.
4. Simple automatic control of water purification process
EDI device can connect multiple modules to the system in parallel. The modules are safe and stable, with reliable quality, making the operation and management of the system easy to implement program control and convenient operation.
5. No waste acid and waste alkali liquid discharge, which is beneficial to environmental protection
EDI device does not require acid and alkali chemical regeneration, and basically no chemical waste discharge
.
6. High water recovery rate. 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 higher requirements for influent water quality, and their one-time investment (infrastructure and equipment costs) is relatively high.
It should be noted that although the cost of EDI infrastructure and equipment is slightly higher than that of mixed bed technology, after comprehensively considering the cost of device operation, EDI technology still has certain advantages.
For example, a pure water station compared the investment and operating costs of the two processes. After one year of normal operation, the EDI device can offset the investment difference with the mixed bed process.
04 Reverse Osmosis + EDI VS Traditional Ion Exchange
1. Comparison of initial investment of the project
In terms of initial investment of the project, in the water treatment system with a small water flow rate, the reverse osmosis + EDI process eliminates the huge regeneration system required by the traditional ion exchange process, especially the elimination of two acid storage tanks and two alkali storage tanks, which not only greatly reduces the equipment procurement cost, but also saves about 10% to 20% of the floor area, thereby reducing the civil engineering cost and land acquisition cost of building the plant.
Since 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 can be reduced by 2 to 3m, thereby saving another 10% to 20% of the civil engineering investment of the plant.
Considering the recovery rate of reverse osmosis and EDI, the concentrated water of the secondary reverse osmosis and EDI is fully recovered, but the concentrated water of the primary reverse osmosis (about 25%) needs to be discharged, and the output of the pretreatment system needs to be increased accordingly. When the pretreatment system adopts the traditional coagulation, clarification and filtration process, the initial investment needs to be increased by about 20% compared with the pretreatment system of the ion exchange process.
Taking all factors into consideration, the initial investment of reverse osmosis + EDI process in small water treatment system is roughly equivalent to that of traditional ion exchange process.
2. Comparison of operating costs
As we all know, in terms of reagent consumption, the operating cost of reverse osmosis process (including reverse osmosis dosing, chemical cleaning, wastewater treatment, etc.) is lower than that of traditional ion exchange process (including ion exchange resin regeneration, wastewater treatment, etc.).
However, in terms of power consumption, replacement of spare parts, etc., reverse osmosis plus EDI process is much higher than traditional ion exchange process.
According to statistics, the operating cost of reverse osmosis plus 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 plus 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 low environmental pollution
The reverse osmosis + EDI process has strong adaptability to the salt content of raw water. The reverse osmosis process can be used for seawater, brackish water, mine drainage water, groundwater and river water, while the ion exchange process is not economical when the dissolved solid content of the influent water is greater than 500 mg/L.
Reverse osmosis and EDI do not require acid and alkali regeneration, do not consume a large amount of acid and alkali, and do not produce a large amount of acid and alkali wastewater. Only a small amount of acid, alkali, scale inhibitor and reducing agent are required.
In terms of operation and maintenance, reverse osmosis and EDI also have the advantages of high degree of automation and easy program control.
4. Reverse osmosis + EDI equipment is expensive, difficult to repair, and difficult to treat brine
Although the reverse osmosis plus EDI process has many advantages, when the equipment fails, 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 required to replace it, and the shutdown time may be long.
Although reverse osmosis does not produce a large amount of acid and alkaline wastewater, the recovery rate of the first-level reverse osmosis is generally only 75%, which will produce a large amount of concentrated water. The salt content of the concentrated water will be much higher than that of the raw water. There is currently no mature treatment measure for this part of concentrated water, and once discharged, it will pollute the environment.
At present, the recovery and utilization of reverse osmosis brine in domestic power plants is mostly used for coal washing and ash humidification; some universities are conducting research on brine evaporation and crystallization purification processes, but the cost is high and the difficulty is great, and it has not yet been widely used in industry.
The cost of reverse osmosis and EDI equipment is relatively high, but in some cases it is even lower than the initial investment of the traditional ion exchange process.
In large-scale water treatment systems (when the system produces a large amount of water), the initial investment of reverse osmosis and EDI systems is much higher than that of traditional ion exchange processes.
In small water treatment systems, the reverse osmosis plus EDI process is roughly equivalent to the traditional ion exchange process in terms of initial investment.
In summary, when the output of the water treatment system is small, the reverse osmosis plus EDI treatment process can be given priority. This process has low initial investment, high degree of automation, and low environmental pollution.
For specific prices, please contact us！
The full name of EDI is electrode ionization, which translates to electric desalination, also known as electrodeionization technology, or packed bed electrodialysis.
Electrodeionization technology combines ion exchange and electrodialysis. It is a desalination technology developed on the basis of electrodialysis. It is a water treatment technology that has been widely used and achieved good results after ion exchange resins.
It not only utilizes the advantages of continuous desalination of electrodialysis technology, but also utilizes ion exchange technology to achieve deep desalination;
It not only improves the defect of decreased current efficiency when treating low-concentration solutions in the electrodialysis process, enhances ion transfer, but also enables ion exchangers to be regenerated, avoids the use of regeneration agents, reduces secondary pollution generated during the use of acid-base regeneration agents, and realizes continuous deionization operation.
The basic principle of EDI deionization includes the following three processes:
1. Electrodialysis process
Under the action of an external electric field, the electrolyte in the water selectively migrates through the ion exchange resin in the water and is discharged with the concentrated water, thereby removing the ions in the water.
2. Ion exchange process
The impurity ions in the water are exchanged and combined with the impurity ions in the water through the ion exchange resin, thereby achieving the effect of effectively removing the ions in the water.
3. Electrochemical regeneration process
The H+ and OH- generated by the polarization of water at the ion exchange resin interface are used to electrochemically regenerate the resin to achieve self-regeneration of the resin.
02 What are the factors affecting EDI and what are the control measures?
1. Influence of inlet water conductivity
Under the same operating current, as the raw water conductivity increases, the EDI removal rate of weak electrolytes decreases, and the effluent conductivity also increases.
If the raw water conductivity is low, the ion content is also low, and the low concentration of ions makes the electromotive force gradient formed on the surface of the resin and membrane in the fresh water chamber also large, resulting in an enhanced degree of water dissociation, an increase in the limiting current, and a large number of H+ and OH-, so that the regeneration effect of the anion and cation exchange resins filled in the fresh water chamber is good.
Therefore, it is necessary to control the inlet water conductivity so that the EDI inlet water conductivity is less than 40us/cm, which can ensure the qualified effluent conductivity and the removal of weak electrolytes.
2. Influence of working voltage and current
As the working current increases, the water quality of the produced water continues to improve.
However, if the current is increased after reaching the highest point, due to the excessive amount of H+ and OH- ions produced by water ionization, in addition to being used for regeneration of resin, a large number of surplus ions act as carrier ions for conduction. At the same time, due to the accumulation and blockage of a large number of carrier ions during movement, even reverse diffusion occurs, resulting in a decrease in the quality of produced water.
Therefore, it is necessary to select appropriate working voltage and current.
3. Influence of turbidity and pollution index (SDI)
The water production channel of the EDI component is filled with ion exchange resin. Excessive turbidity and pollution index will block the channel, causing the system pressure difference to rise and the water production to decrease.
Therefore, appropriate pretreatment is required, and RO effluent generally meets the EDI inlet requirements.
4. Influence of hardness
If the residual hardness of the inlet water in the EDI is too high, it will cause scaling on the membrane surface of the concentrated water channel, reduce the concentrated water flow rate, reduce the resistivity of the produced water, affect the water quality of the produced water, and in severe cases, block the concentrated water and polar water flow channels of the component, causing the component to be destroyed due to internal heating.
The RO inlet water can be softened and alkali can be added in combination with CO2 removal; when the inlet water has a high salt content, a first-level RO or nanofiltration can be added in combination with desalination to adjust the impact of hardness.
5. Impact of TOC (Total Organic Carbon)
If the organic content in the influent is too high, it will cause organic pollution of the resin and the selective permeable membrane, resulting in an increase in the 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 and block the channel.
Therefore, when treating, you can combine other index requirements to increase the level of R0 to meet the requirements.
6. Impact of metal ions such as Fe and Mn
Metal ions such as Fe and Mn will cause "poisoning" of the resin, and the metal "poisoning" of the resin will cause the rapid deterioration of the EDI effluent quality, especially the rapid decrease in the removal rate of silicon.
In addition, the oxidative catalytic effect of variable valence metals on ion exchange resins will cause permanent damage to the resin.Generally speaking, the Fe of EDI influent is controlled to be less than 0.01 mg/L during operation.
7. Impact of CO2 in influent
HCO3- generated by CO2 in the influent is a weak electrolyte, which can easily penetrate the ion exchange resin layer and cause the quality of the produced water to decrease.A degassing tower can be used to remove it before influent.
8. Influence of total anion content (TEA)
High TEA will reduce the resistivity of EDI produced water, or require an increase in EDI operating current. Excessive operating current will increase system current and increase residual chlorine concentration in the electrode water, which is not good for the life of the electrode membrane.
In addition to the above 8 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
EDI technology has been widely used in industries with high water quality requirements such as electricity, chemical industry, and medicine.
Long-term application research in the field of water treatment shows that EDI treatment technology has the following 6 characteristics:
1. High water quality and stable water output
EDI technology combines the advantages of continuous desalination by electrodialysis and deep desalination by ion exchange. Continuous scientific research practice shows that the use of EDI technology for desalination can effectively remove ions in water and produce high purity water output.
2. Low equipment installation conditions and small footprint
Compared with ion exchange beds, EDI devices are small in size and light in weight, and do not require acid or alkali storage tanks, which can effectively save space.
Not only that, the EDI device is a prefabricated structure with a short construction period and small on-site installation workload.
3. Simple design, easy operation and maintenance
EDI treatment devices can be produced in modularized form, can be automatically and continuously regenerated, do not require large and complex regeneration equipment, and are easy to operate and maintain after being put into operation.
4. Simple automatic control of water purification process
EDI device can connect multiple modules to the system in parallel. The modules are safe and stable, with reliable quality, making the operation and management of the system easy to implement program control and convenient operation.
5. No waste acid and waste alkali liquid discharge, which is beneficial to environmental protection
EDI device does not require acid and alkali chemical regeneration, and basically no chemical waste discharge
.
6. High water recovery rate. 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 higher requirements for influent water quality, and their one-time investment (infrastructure and equipment costs) is relatively high.
It should be noted that although the cost of EDI infrastructure and equipment is slightly higher than that of mixed bed technology, after comprehensively considering the cost of device operation, EDI technology still has certain advantages.
For example, a pure water station compared the investment and operating costs of the two processes. After one year of normal operation, the EDI device can offset the investment difference with the mixed bed process.
04 Reverse Osmosis + EDI VS Traditional Ion Exchange
1. Comparison of initial investment of the project
In terms of initial investment of the project, in the water treatment system with a small water flow rate, the reverse osmosis + EDI process eliminates the huge regeneration system required by the traditional ion exchange process, especially the elimination of two acid storage tanks and two alkali storage tanks, which not only greatly reduces the equipment procurement cost, but also saves about 10% to 20% of the floor area, thereby reducing the civil engineering cost and land acquisition cost of building the plant.
Since 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 can be reduced by 2 to 3m, thereby saving another 10% to 20% of the civil engineering investment of the plant.
Considering the recovery rate of reverse osmosis and EDI, the concentrated water of the secondary reverse osmosis and EDI is fully recovered, but the concentrated water of the primary reverse osmosis (about 25%) needs to be discharged, and the output of the pretreatment system needs to be increased accordingly. When the pretreatment system adopts the traditional coagulation, clarification and filtration process, the initial investment needs to be increased by about 20% compared with the pretreatment system of the ion exchange process.
Taking all factors into consideration, the initial investment of reverse osmosis + EDI process in small water treatment system is roughly equivalent to that of traditional ion exchange process.
2. Comparison of operating costs
As we all know, in terms of reagent consumption, the operating cost of reverse osmosis process (including reverse osmosis dosing, chemical cleaning, wastewater treatment, etc.) is lower than that of traditional ion exchange process (including ion exchange resin regeneration, wastewater treatment, etc.).
However, in terms of power consumption, replacement of spare parts, etc., reverse osmosis plus EDI process is much higher than traditional ion exchange process.
According to statistics, the operating cost of reverse osmosis plus 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 plus 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 low environmental pollution
The reverse osmosis + EDI process has strong adaptability to the salt content of raw water. The reverse osmosis process can be used for seawater, brackish water, mine drainage water, groundwater and river water, while the ion exchange process is not economical when the dissolved solid content of the influent water is greater than 500 mg/L.
Reverse osmosis and EDI do not require acid and alkali regeneration, do not consume a large amount of acid and alkali, and do not produce a large amount of acid and alkali wastewater. Only a small amount of acid, alkali, scale inhibitor and reducing agent are required.
In terms of operation and maintenance, reverse osmosis and EDI also have the advantages of high degree of automation and easy program control.
4. Reverse osmosis + EDI equipment is expensive, difficult to repair, and difficult to treat brine
Although the reverse osmosis plus EDI process has many advantages, when the equipment fails, 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 required to replace it, and the shutdown time may be long.
Although reverse osmosis does not produce a large amount of acid and alkaline wastewater, the recovery rate of the first-level reverse osmosis is generally only 75%, which will produce a large amount of concentrated water. The salt content of the concentrated water will be much higher than that of the raw water. There is currently no mature treatment measure for this part of concentrated water, and once discharged, it will pollute the environment.
At present, the recovery and utilization of reverse osmosis brine in domestic power plants is mostly used for coal washing and ash humidification; some universities are conducting research on brine evaporation and crystallization purification processes, but the cost is high and the difficulty is great, and it has not yet been widely used in industry.
The cost of reverse osmosis and EDI equipment is relatively high, but in some cases it is even lower than the initial investment of the traditional ion exchange process.
In large-scale water treatment systems (when the system produces a large amount of water), the initial investment of reverse osmosis and EDI systems is much higher than that of traditional ion exchange processes.
In small water treatment systems, the reverse osmosis plus EDI process is roughly equivalent to the traditional ion exchange process in terms of initial investment.
In summary, when the output of the water treatment system is small, the reverse osmosis plus EDI treatment process can be given priority. This process has low initial investment, high degree of automation, and low environmental pollution.
For specific prices, please contact us！