16 Feb 2023
Which is better, reverse osmosis + EDI or traditional ion exchange?
The full English name of EDI is electrode ionization, also known as electrodeionization technology, or packed bed electrodialysis
Electrodeionization technology combines the two technologies of ion exchange and electrodialysis. It is a desalination technology developed on the basis of electrodialysis, and it is a water treatment technology that has been widely used and achieved better results after ion exchange resins.
It not only takes advantage of the advantages of continuous desalination by electrodialysis technology, but also uses ion exchange technology to achieve the effect of deep desalination;
It not only improves the defect that the current efficiency drops when the electrodialysis process is used to treat low-concentration solutions, enhances ion transfer, but also enables the ion exchanger to be regenerated, avoiding the use of regenerants, and reducing the secondary generated during the use of acid-base regenerants. Secondary pollution, realize the continuous operation of deionization.
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 will selectively migrate through the ion exchange resin in the water and be discharged with the concentrated water, thereby removing the ions in the water.
2. Ion exchange process
The impurity ions in the water are exchanged by the ion exchange resin, and the impurity ions in the water are combined to achieve the effect of effectively removing the ions in the water.
3. Electrochemical regeneration process
The resin is electrochemically regenerated by using the H+ and OH- generated by the polarization of the interfacial water of the ion exchange resin to realize the self-regeneration of the resin.
02 Influencing factors and control means of EDI?
1. Influence of influent 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 content of ions 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 enhanced water dissociation, an increase in the limit current, and the generated H+ And the quantity of OH- is more, so that the regeneration effect of the anion and cation exchange resin filled in the fresh water chamber is good.
Therefore, it is necessary to control the conductivity of the influent water so that the conductivity of the EDI influent water is less than 40us/cm, which can ensure the qualified conductivity of the effluent water and the removal of weak electrolytes.
2. The influence of working voltage and current
As the working current increases, the 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 generated by water ionization, in addition to being used to regenerate the resin, a large number of surplus ions act as carrier ions for conduction, and at the same time due to the large amount of carrier ion movement process Accumulation and clogging occur in the medium, and even back diffusion occurs, resulting in a decline in the quality of the produced water.
Therefore, the appropriate working voltage and current must be selected.
3. The influence of turbidity and pollution index (SDI)
The water production channel of the EDI module is filled with ion exchange resin. Excessive turbidity and pollution index will block the channel, resulting in an increase in system pressure difference and a decrease in water production.
Therefore, proper pretreatment is required, and RO effluent generally meets the requirements of EDI influent.
4. The influence of hardness
If the residual hardness of the feed water in EDI is too high, it will cause fouling on the membrane surface of the concentrated water channel, the flow rate of the concentrated water will decrease, the resistivity of the produced water will decrease, and the water quality will be affected. In severe cases, the concentrated water and polar water channels of the module will be blocked. Resulting in component destruction 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 influent water is high, it can be combined with desalination to increase the level of RO or nanofiltration to adjust the impact of hardness.
5. The impact 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 the selectively permeable membrane, which will lead to an increase in the operating voltage of the system and a decrease in the quality of the produced water. At the same time, it is also easy to form organic colloid in the concentrated water channel and block the channel.
Therefore, when dealing with it, one level of R0 can be added in combination with other index requirements to meet the requirements.
6. The influence 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 decline 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 resins.
Generally speaking, the Fe in the EDI influent is controlled to be lower than 0.01mg/L during operation.
7. The influence of C02 in the influent
The HCO3- generated by CO2 in the influent water is a weak electrolyte, which can easily penetrate the ion exchange resin layer and cause the quality of the produced water to decline.
It can be removed by degassing tower before entering water.
8. Effect of total anion content (TEA)
A high TEA will reduce the resistivity of the EDI produced water, or increase the EDI operating current, while an excessively high operating current will increase the system current, increase the concentration of residual chlorine in the electrode water, and be detrimental to the life of the electrode membrane.
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 Features of EDI
In recent years, EDI technology has been widely used in industries with high water quality requirements such as electric power, chemical industry, and medicine.
Long-term application research in the field of water treatment shows that EDI treatment technology has the following six characteristics:
1. The water quality is high and the water output is stable
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 desalination again can effectively remove ions in water, and the purity of effluent water is high.
2. Low equipment installation conditions and small footprint
Compared with the ion exchange bed, the EDI device is small in size and light in weight, and does not need to be equipped with acid and alkali storage tanks, which can effectively save space.
Not only that, the EDI device is a self-contained structure, the construction period is short, and the on-site installation workload is small.
3. Simple design, convenient operation and maintenance
The EDI processing device can be produced in a modular manner, and can be automatically and continuously regenerated without large and complicated regeneration equipment. After being put into operation, it is easy to operate and maintain.
4. The automatic control of the water purification process is simple and convenient
The EDI device can be connected to the system in parallel with multiple modules. The modules are safe and stable in operation and reliable in quality, making the operation and management of the system easy to realize program control and easy to operate.
5. No discharge of waste acid and waste lye, which is conducive to environmental protection
The EDI device does not need 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
To sum up, EDI technology has great advantages in terms of water quality, operation stability, ease of operation and maintenance, safety and environmental protection.
But it also has certain shortcomings. The EDI device has higher requirements on the quality of the influent water, and its one-time investment (infrastructure and equipment costs) is relatively high.
It should be noted that although the cost of infrastructure and equipment for EDI is slightly higher than that of the mixed bed process, EDI technology still has certain advantages after considering the cost of device operation.
For example, a pure water station 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 project investment
In terms of the initial investment of the project, in the water treatment system with a small water flow rate, because the reverse osmosis + EDI process cancels the huge regeneration system required by the traditional ion exchange process, especially cancels two acid storage tanks and two alkali storage tanks. Taiwan, not only greatly reduces the cost of equipment procurement, but also saves about 10% to 20% of the land area, thereby reducing the cost of civil engineering and land acquisition for the construction of factories.
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 2-3m, thereby saving another 10%-20% of the civil construction 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 system adopts the traditional coagulation, clarification and filtration process, the initial investment needs to increase by about 20% compared with the pretreatment system of the ion exchange process.
Comprehensive consideration, the reverse osmosis + EDI process is roughly equivalent to the traditional ion exchange process in terms of initial investment in small water treatment systems.
2. Comparison of operating costs
As we all know, in terms of reagent consumption, the operating cost of the reverse osmosis process (including reverse osmosis dosing, chemical cleaning, wastewater treatment, etc.) is lower than that of the traditional ion exchange process (including ion exchange resin regeneration, wastewater treatment, etc.).
However, in terms of power consumption, replacement of spare parts, etc., the reverse osmosis plus EDI process will be much higher than the traditional ion exchange process.
According to statistics, the operating cost of the reverse osmosis plus EDI process is slightly higher than that of the traditional ion exchange process.
Comprehensive consideration, 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 is highly adaptable to the salinity of the raw water. The reverse osmosis process can be used from seawater, brackish water, mine drainage water, groundwater to river water, while the ion exchange process has a dissolved solid content of more than 500 mg in the incoming water /L is uneconomical.
Reverse osmosis and EDI do not require acid-base regeneration, consume a large amount of acid-base, and do not generate a large amount of acid-base wastewater. They only need to add a small amount of acid, alkali, antiscalant and reducing agent.
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 it is difficult to treat concentrated 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 replaced by shutdown. In most cases, professional and technical personnel are required to replace it, and the shutdown time may longer.
Although reverse osmosis does not produce a large amount of acid-base wastewater, the recovery rate of primary reverse osmosis is generally only 75%, and a large amount of concentrated water will be produced. The salt content of the concentrated water will be much higher than that of the raw water. Treatment measures, once discharged will pollute the environment.
At present, in domestic power plants, most of the concentrated brine from reverse osmosis is recycled and used for coal washing and ash humidification; some universities are conducting research on the evaporation and crystallization of concentrated brine, but the cost is high and difficult, and there is no major issue yet. range of industrial applications.
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 small water treatment systems.
To sum up, 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.
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