Publish Time: 2026-09-02 Origin: Site
Reverse osmosis can remove most dissolved salts and ions from water, but some high-purity water systems require additional deionization after RO.
For many years, mixed bed ion exchange was commonly used for this polishing step. Today, EDI is increasingly used in new high-purity water systems, especially where continuous operation and reduced chemical regeneration are important.
So, what is the difference between EDI and mixed bed? Does EDI actually provide better water quality, and is mixed bed still worth considering? The answer depends not only on ion removal, but also on how each technology operates, regenerates, and is maintained.
Reverse osmosis is an effective primary purification process for reducing dissolved salts, ions, and other impurities in water. However, RO does not remove every dissolved ion from the feed water, so the RO permeate may still contain a small amount of residual ionic content.
RO uses a semipermeable membrane to separate water from many dissolved substances. It can significantly reduce dissolved salts and ions, making it a common purification step before further treatment.
However, the quality of RO permeate depends on factors such as feed water quality, membrane performance, operating conditions, and system design. Some residual ions can remain in the treated water.
For applications requiring higher water purity, the remaining ionic content after RO may need to be reduced further. This is why an additional deionization or polishing step is used after reverse osmosis.
This is where EDI and mixed bed ion exchange can be used. Both technologies can further remove remaining ions from RO permeate, but they use different methods and have different operating and maintenance requirements.
Mixed bed ion exchange is a polishing process used to further remove remaining ions from RO permeate. It contains both cation exchange resin and anion exchange resin mixed together in the same vessel.
Cation exchange resin removes positively charged ions, while anion exchange resin removes negatively charged ions from the water. Through the ion exchange process, these remaining dissolved ions are exchanged with hydrogen and hydroxide ions, which combine to form water.
By removing both cations and anions in the same treatment vessel, mixed bed ion exchange can produce high-purity water after reverse osmosis.
As the resin exchanges ions, its capacity gradually becomes exhausted. Once the resin reaches its operating capacity, it must be regenerated before returning to service.
The common process is:
Resin exhaustion → chemical regeneration → rinsing → return to service
Regeneration normally involves acid for the cation resin and alkali for the anion resin. The process also generates regeneration wastewater and requires additional operating and maintenance procedures. Depending on the system design and regeneration method, the treatment process may also require downtime during regeneration.
Electrodeionization (EDI) is a continuous deionization technology used to further purify RO permeate. Unlike conventional mixed bed ion exchange, EDI combines ion exchange resin, ion-selective membranes, and an electrical field to remove remaining ions from the water.
Inside an EDI module, ion exchange resin helps capture dissolved ions from the water, while ion-selective membranes control the movement of cations and anions. An electrical field drives the captured ions through the membranes toward the appropriate electrode compartments.
This continuous ion migration allows EDI to further reduce the ionic content of RO permeate and produce high-purity water.
The electrical field also allows the ion exchange media inside the EDI module to be continuously regenerated during operation. This is the key difference from mixed bed ion exchange, where resin must be periodically removed from service for chemical regeneration.
EDI continuously regenerates the ion exchange media during operation instead of relying on periodic chemical regeneration.
This does not mean that EDI is maintenance-free. The system still requires proper operation, monitoring, and routine maintenance, but it does not normally require conventional acid and alkali regeneration of the ion exchange media.
Although both EDI and mixed bed ion exchange can be used to polish RO permeate, their operating principles and maintenance requirements are different. The main differences can be summarized as follows:
Factor | EDI | Mixed Bed |
Ion exchange | Continuous | Batch / periodic |
Regeneration | Electrical | Chemical |
Acid and alkali | Normally not required | Required for regeneration |
Operation | Continuous operation | Requires regeneration when resin is exhausted |
Chemical handling | Reduced | Required |
Routine maintenance | Generally lower | Generally higher |
Initial investment | Generally higher | Generally lower |
Continuous production | Well suited | Less convenient |
Automation | Well suited | Depends on system design |
Mixed bed can still produce high-purity water, but its reliance on periodic chemical regeneration makes it less suitable for many modern continuous-production systems. EDI avoids routine acid and alkali regeneration, making it increasingly preferred for new high-purity water systems.
The growing use of EDI is mainly related to how modern water treatment systems are operated. Compared with mixed bed systems, EDI reduces the need for periodic chemical regeneration and fits better with continuous and automated water production.
EDI is designed for continuous operation, allowing high-purity water to be produced without regularly stopping the polishing process for resin regeneration. This is particularly suitable for production facilities that require a stable and continuous supply of purified water.
One of the main reasons for choosing EDI is that it does not require routine acid and alkali regeneration of the ion exchange media. This simplifies the operation of the polishing stage compared with conventional mixed bed systems.
Without routine chemical regeneration, the system has less need for chemical storage, chemical preparation, regeneration operations, and handling of regeneration wastewater. This can reduce the routine workload associated with the water treatment system.
EDI can operate as a continuous polishing unit after RO and can be integrated into automated water treatment systems. Its continuous operation and reduced chemical regeneration requirements make it well suited to modern RO-based high-purity water systems.
Not necessarily.
Both EDI and mixed bed ion exchange can further remove ions from RO permeate and produce high-purity water. The achievable water quality depends on the specific system and operating conditions, rather than simply on whether EDI or mixed bed is used.
The final water quality is affected by the performance of the entire treatment system, including:
Pretreatment
RO performance
Feed water quality
Feed conductivity
EDI operating conditions
Overall system design
Therefore, the main difference between EDI and mixed bed is not simply the achievable water purity, but how the polishing process is operated and maintained.
Although EDI is increasingly preferred for new high-purity water systems, mixed bed ion exchange remains a practical option in certain operating conditions.
Mixed bed may be suitable when the water demand is relatively small or production is intermittent. It can also be considered when lower initial investment is important or when a facility already has mixed bed equipment and supporting infrastructure.
EDI is generally more suitable for continuous production, higher water demand, and new ro water purification system installations where reduced chemical regeneration is an important consideration. Its continuous operation also makes it easier to integrate into modern automated RO water treatment systems.
Mixed bed is becoming less attractive for many new continuous-production systems, but it remains a practical technology for certain applications.
EDI is normally installed after the 2-stage RO because the EDI unit works more effectively when the incoming water has already been highly purified. The 2-stage RO further reduces the ionic load and conductivity of the water before it enters the EDI unit.
The 2-stage RO further removes dissolved salts and ions from the permeate produced by the 1-stage RO. This reduces the ionic load and conductivity of the water, providing suitable feed water for the EDI polishing stage.
A common 2-stage RO water treatment system with EDI follows this process:
Raw Water → Pretreatment → 1-Stage RO → 2-Stage RO → EDI → High-Purity Water
Raw Water: The source water entering the treatment system.
Pretreatment: Reduces suspended solids, chlorine, hardness, and other substances that can affect downstream equipment.
1-Stage RO: Removes most dissolved salts and other impurities from the pretreated water.
2-Stage RO: Further reduces the remaining ionic load and conductivity.
EDI: Further removes ions from the 2-stage RO permeate to produce high-purity water.
This configuration is commonly described as a 2 Stage Reverse Osmosis with EDI system, with EDI serving as the final deionization or polishing stage.
EDI is increasingly preferred in modern ultrapure water systems not because mixed bed has stopped working, but because EDI is better suited to continuous operation with less reliance on chemical regeneration.
For a new RO water treatment system, the choice between EDI and mixed bed should be based on the required water quality, water demand, operating pattern, and long-term maintenance requirements.