Reverse osmosis +EDI and traditional ion exchange, which is better?
01
What is EDI?
EDI is electrode ionization, which translates to electrical deionization, also known as electroionization technique, or packed bed electrodialysis.
Electrodeionization technology combines ion exchange and electrodialysis. It is a desalting technology developed on the basis of electrodialysis. It is a water treatment technology which has been widely used and achieved good results after ion exchange resin etc.
It not only utilizes the advantage of continuous desalting by electrodialysis technology, but also utilizes ion exchange technology to achieve deep desalting effect.
It not only improves the defect of decreasing current efficiency when treating low concentration solution in electrodialysis process, enhances ion transfer, but also makes ion exchanger can be regenerated, avoids the use of regenerant, reduces the secondary pollution produced in the use of acid and base regenerant, and realizes the continuous operation of deionization.
The basic principle of EDI deionization includes the following three processes:
1. Electrodialysis process
Under the action of applied electric field, electrolytes in water carry out selective migration through ion exchange resin in water and discharge with concentrated water, thus removing ions in 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 electrochemical regeneration of resin was carried out by H+ and OH- generated by the polarization of water at the interface of ion exchange resin, and the self-regeneration of resin was realized.
02.The influencing Factors and control Means of EDI?
1. Influence of water inlet conductivity
Under the same operating current, with the increase of raw water conductivity, the removal rate of weak electrolyte by EDI decreases, and the conductivity of effluent also increases.
If the raw water conductivity is low, the content of ions is also low, and the low concentration of ions makes the electrodynamic force gradient formed on the surface of the resin and film in the fresh water chamber is also large, resulting in the enhancement of the degree of water dissociation, the increase of the limit current, the amount of H+ and OH- generated is large, so that the Yin and cation exchange resin filled in the fresh water chamber has a good regeneration effect.
Therefore, it is necessary to control the influent conductivity so that EDI influent conductivity is less than 40us/cm, which can ensure the qualified effluent conductivity and the removal of weak electrolytes.
2. The influence of working voltage and current
As the working current increases, the water quality becomes better.
However, if the current is increased after increasing to the highest point, due to the excessive amount of H+ and OH- ions generated by hydro-ionization, a large number of surplus ions will act as current-carrying ions to conduct electricity except for regenerated resins. Meanwhile, due to the accumulation and blockage of a large number of current-carrying ions in the process of movement, and even reverse diffusion, the quality of water produced will decline.
Therefore, the appropriate working voltage and current must be selecte.
3. Influence of turbidity and pollution index (SDI)
The water production channel of EDI module is filled with ion exchange resin. High turbidity and pollution index will block the channel, resulting in the increase of system pressure difference and the decrease of water production.
Therefore, proper pretreatment is required, and RO effluent generally meets EDI inlet requirements.
4. The influence of hardness
If the residual hardness of the inlet water in EDI is too high, it will cause scaling on the membrane surface of the concentrated water channel, decrease the flow rate of concentrated water, decrease the resistivity of water production, affect the quality of water production, and in serious cases, block the concentrated water and polar flow channel of the component, resulting in the destruction of the component due to internal heating.
It can be combined with CO2 removal to soften and add alkali to RO influent. When the influent salt content is high, the influence of hardness can be adjusted by adding a level of RO or nanofiltration combined with salt removal.
5. Influence of TOC (Total organic carbon)
If the content of organic matter in the water is too high, it will cause organic pollution of resin and selective permeable film, leading to the rise of the system operating voltage and the decline of water quality. At the same time, it is also easy to form organic colloids in the concentrated water channel, blocking the channel.
Therefore, in processing, a level of R0 can be added to meet the requirements combined with other indicators.
6. Influence of metal ions such as Fe and Mn
Fe, Mn and other metal ions will cause resin "poisoning", and the metal "poisoning" of resin will cause the rapid deterioration of EDI effluent water quality, especially the rapid decline in the removal rate of silicon.
In addition, the oxidation catalysis of variable metal to ion exchange resin will cause permanent damage to the resin.
Generally speaking, the Fe controlling EDI influent in operation is less than 0.01mg/L.
7. Influence of CO2 in water
HCO3- generated by CO2 in the influent is a weak electrolyte, which is easy to penetrate the ion-exchange resin layer and cause the quality of the produced water to decline.
Before water can be removed by degassing tower.
8. Influence of total anion content (TEA)
High TEA will reduce the resistivity of EDI water production, or the EDI running current needs to be increased, while high running current will lead to the increase of system current and the increase of residual chlorine concentration in polar water, which is unfavorable to the life of polar film.
In addition to the above 8 influencing factors, influent temperature, pH value, SiO2 and oxide also affect the operation of EDI system.
03.Characteristics of EDI
In recent years, EDI technology has been widely used in power, chemical, medicine and other industries with high water quality requirements.
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 effluent
EDI technology synthesizes the advantages of electrodialysis continuous desalting and ion exchange deep desalting. Continuous scientific research and practice show that using EDI technology to desalting again can effectively remove ions in water, and the effluent purity is high.
2. Low installation conditions and small footprint
Compared with ion exchange bed, EDI device is small in size and light in weight, and does not need to set up acid and alkali storage tank, which can effectively save space.
Moreover, EDI device is an integrated structure with short construction period and small installation workload.
3. Simple design, convenient operation and maintenance
EDI processing device can be modular production, automatic continuous regeneration, do not need large, complex regeneration equipment, put into operation, easy to operate and maintain.
4, water purification process automatic control is simple
EDI device can connect multiple modules to the system in parallel, the module operation is safe and stable, the quality is reliable, so that the operation and management of the system is easy to realize program control, easy to operate.
5, no waste acid waste lye discharge, conducive to environmental protection
EDI device does not need acid, alkali chemical regeneration, basically no chemical waste discharge.
6. The water recovery rate is high, and the water utilization rate of EDI treatment technology is generally above 90%
To sum up, EDI technology has great advantages in water quality, operation stability, operation and maintenance difficulty, safety and environmental protection, etc.
However, it also has some shortcomings, EDI device has higher requirements on inlet water quality, and its one-time investment (capital construction and equipment costs) is high.
It should be noted in particular that, although the cost of construction and equipment of EDI is slightly higher than that of mixed-bed technology, EDI technology still has certain advantages after considering the cost of device operation comprehensively.
For example, a pure water station compared the investment and operation cost of the two processes, and EDI device could 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 low water production flow, the huge regeneration system required by the traditional ion exchange process is cancelled due to the reverse osmosis +EDI process, especially the two acid storage tanks and alkali storage tanks respectively, which not only greatly reduces the equipment procurement cost, but also saves about 10% ~ 20% of the land area. Thus reducing the construction of plant construction costs and land acquisition costs.
Since the height of traditional ion exchange generation equipment is generally above 5m, and the height of reverse osmosis and EDI equipment is less than 2.5m, the height of water treatment workshop can be reduced by 2 ~ 3m, thus saving 10% ~ 20% of the investment in plant construction.
Considering the recovery rate of reverse osmosis and EDI, all the concentrated water of secondary reverse osmosis and EDI is recovered, but the concentrated water of primary reverse osmosis (about 25%) needs to be discharged, and the output of the pretreatment system needs to be increased correspondingly. When the pretreatment system adopts the traditional coagulation and filtration process, the initial investment needs to be increased by about 20% compared with the pretreatment system of ion exchange process.
Overall, the initial investment of reverse osmosis +EDI process in small water treatment system is similar to that of traditional ion exchange process.
2. Comparison of operating costs
It is well known that the operating cost of reverse osmosis processes (including reverse osmosis dosing, chemical cleaning, wastewater treatment, etc.) is lower than that of traditional ion exchange processes (including ion exchange resin regeneration, wastewater treatment, etc.) in terms of pharmaceutical consumption.
However, in terms of power consumption and replacement of spare parts, the reverse osmosis plus EDI process is much higher than the traditional ion exchange process.
According to statistics, the operation cost of reverse osmosis plus EDI process is slightly higher than that of traditional ion exchange process.
Overall, the overall operation and maintenance cost of reverse osmosis plus EDI process is 50% ~ 70% higher than that of traditional ion exchange process.
3. Reverse osmosis +EDI has strong adaptability, high degree of automation and little environmental pollution
Reverse osmosis + EDI process has strong adaptability to the salt content of raw water. The reverse osmosis process can be used in seawater, brackish water, mine drainage, groundwater and river water. However, the ion exchange process is uneconomical when the content of dissolved solids in the water is greater than 500 mg/L.
Reverse osmosis and EDI do not require acid and base regeneration, no large amount of acid and base consumption, nor produce a large amount of acid and base wastewater, only a small amount of acid, base, scale inhibitor and reducing agent dosing can be done.
In operation and maintenance, reverse osmosis and EDI also have the advantages of high degree of automation, easy to program control.
4. Reverse osmosis +EDI equipment is costly and difficult to repair, and it is difficult to deal with concentrated brine
Although reverse osmosis plus EDI process has many advantages, when the equipment fails, especially when the reverse osmosis membrane and EDI membrane reactor are damaged, it can only be replaced out of service. In most cases, professional and technical personnel are needed to replace it, and the outage time may be long.
Although reverse osmosis does not produce a large amount of acid and alkali wastewater, the recovery rate of primary reverse osmosis is generally only 75%, which will produce a large amount of concentrated water. The salt content of concentrated water is much higher than that of raw water. At present, there are no mature treatment measures for this part of concentrated water, and once discharged, it will pollute the environment.
At present, in domestic power plants for reverse osmosis of concentrated brine recovery and utilization of most used for coal transport washing, ash humidification; Some universities are conducting research on the process of evaporation, crystallization and purification of concentrated brine, but it is costly and difficult, and has not been widely used 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.
The initial investment of reverse osmosis and EDI systems is much higher than that of traditional ion exchange processes in large water treatment systems.
In small scale water treatment system, reverse osmosis plus EDI process in small scale water treatment system and traditional ion exchange process in terms of initial investment is about the same.
In summary, when the output of the water treatment system is small, the reverse osmosis plus EDI treatment process can be given priority, which has low initial investment, high degree of automation and little pollution to the environment.
