Reverse osmosis (RO) systems effectively remove contaminants from water, but the resulting RO permeate often requires further treatment to achieve desired purity levels. This is where electrodeionization (EDI) enters the picture. But what exactly is an EDI unit and how does it work to polish and purify RO permeate?
In this comprehensive guide, as a professional EDI module supplier, we’ll cover everything you need to know about EDI technology, including:
- What is an EDI Unit for water treatment
- How EDI units work to remove impurities
- The components that make up an EDI system
- Key benefits EDI offers over other methods
- Limitations to consider
- Typical EDI applications for producing high purity water
After reading this guide, you’ll have a solid understanding of EDI systems and whether integrating one can help you meet your water treatment goals.

What is an EDI Unit for Water Treatment?
An EDI unit is an advanced, electricity-powered ion removal system capable of purifying water to precise quality standards for industrial applications. By pairing membrane deionization with automated, chemical-free electrical regeneration, EDI tech provides a green and cost-effective water treatment solution.
How Do EDI Units Work?
An EDI unit combines ion exchange resins, semi-permeable ion exchange membranes, and an electrical current to continuously remove ionized contaminants from water.
Here’s a high-level overview of the EDI process:
- Feedwater enters stacked EDI cells filled with cation and anion exchange resins
- An electrical field drives positively charged cations towards the cathode and negatively charged anions towards the anode
- Ions pass through their respective semi-permeable membranes into concentrate channels
- Trapped ions exit the EDI system through the concentrate stream
- Purified, deionized water exits through the dilute stream
So in a nutshell, applying an electrical current enables charged particles to move laterally through ion-specific membranes, separating contaminants from pure water.
But how exactly does the EDI unit facilitate this ion separation on a deeper level?
Electrodialysis Overview
To understand EDI, it helps to first look at a closely related process called electrodialysis (ED).
ED uses a stack of cation and anion exchange membranes to split salts into acid and base components under an electric field.
Positively charged cations migrate through cation membranes towards the negative cathode. Negative anions move through anion membranes in the opposite direction towards the positive anode.
This separates the dilute stream (purified water) from the concentrate stream containing higher salt concentrations.
However, as water purity increases, so does electrical resistance. This requires exponentially higher voltage to sustain ion transfer, making it hard to achieve consistent deionization.
Introducing Ion Exchange Resins
And that’s where EDI comes in!
Like ED, EDI utilizes stacked membrane cells and electrodes to facilitate ion separation.
But the key difference lies in the added ion exchange resins contained in each cell:
These mixed bed resins provide a conductive path between the membranes for ions to flow through.
So even as water purity increases, ions can still migrate easily to their respective membranes under low voltage conditions.
This innovation bypasses the limitations of conventional ED, enabling thorough separation of ions down to parts per billion impurity levels.
Components of an EDI Unit
Now that you understand the basics of how EDI works, let’s take a closer look at the key components that make up an EDI system:
Feedwater Pretreatment
For optimal EDI performance, feedwater requires extensive pretreatment, usually with a reverse osmosis system.
RO effectively reduces hardness, organics, and ionic impurities from the feed stream entering the EDI unit. This protects the ion exchange resins and membranes from fouling or scale buildup.
Some applications may also use additional pretreatment steps like micron filtration, activated carbon filters, and degassing.
Ion Exchange Resins
As outlined earlier, the ion exchange resins inside EDI cells provide a conductive media for ions to easily migrate through under an electric field.
They also facilitate the splitting of water molecules into hydrogen (H+) and hydroxyl (OH-) ions. The continuous regeneration of resins with these ions is what enables reliable, long-term EDI operation.
Ion Exchange Membranes
The anion and cation membranes act as selective barriers between the dilute and concentrate channels.
This forces the separation of ions based on charge across the membrane stack into their respective concentrate streams.
Different membrane materials can be combined to target specific ion separations. For example, monovalent selective membranes allow passage of either anions or cations while rejecting divalent ions.
Electrode Channels
Electrodes establish the electric field across the EDI membrane stack to induce the flow of ions.
The electrodes themselves are situated in electrode rinse channels isolated from the feedwater to prevent electrochemical byproducts from contaminating the dilute stream.
End Blocks
The end blocks house and secure the electrodes, facilitating electrical connections to the EDI stack.
Careful compression of the membrane stack between end blocks is required to prevent any leakage between cells while still allowing sufficient feedwater flow.
Key Benefits of Using EDI Technology
Now that you have a solid grasp of what an EDI system is made up of, let’s explore some of the major advantages EDI offers:
No Chemical Regeneration
Unlike conventional ion exchange deionization, EDI units utilize electrochemical regeneration of resins instead of harsh chemicals.
This makes EDI a very environmentally friendly process with no acid/caustic wastewater requiring treatment. Operational costs associated with buying, storing, and handling dangerous chemicals are also eliminated.
Continuous Operation
The electrical regeneration enables continuous purification without any interruptions or downtime. Traditional ion exchange systems require taking resin beds offline periodically for time-consuming chemical regeneration.
So EDI can reliably produce a constant flow of high purity water to meet process demands.
Removes Weakly Ionized Compounds
In addition to removing free ionic salts, EDI can also effectively eliminate weakly ionized contaminants like silica, carbon dioxide, boron and ammonia.
Conversion to ionic forms by hydroxyl ions allows subsequent separation across EDI membranes. This capability exceeds what single-pass reverse osmosis or ion exchange alone can achieve.
Lower Operating Costs
Despite higher capital equipment expenses, EDI operating costs are significantly lower over time compared to conventional ion exchange or distillation processes.
The only reoccurring costs are periodic membrane replacements and electricity to power the unit. EDI’s electrochemical regeneration also extends resin life indefinitely.
Compact, Modular Design
EDI’s stacked membrane configuration enables compact, space-saving systems relative to traditional ion exchange columns. And each membrane is a self-contained module for straightforward maintenance or replacements.
Limitations of EDI Technology
While EDI purification offers important advantages, the technology does come with inherent limitations to factor in as well:
Requires Extensive Pretreatment
High quality feedwater is critical to prevent fouling or scaling risk to EDI internals. This necessitates intensive pretreatment (typically RO).
Higher Capital Costs
Although long term OPEX is lower, EDI systems require a greater upfront investment over alternative technologies.
Limited Chemical Tolerance
Introducing oxidizing agents or free chlorine can rapidly degrade ion exchange resins or membranes, taking EDI systems offline. Careful feedwater monitoring is essential.
Slower Removal of Organics
EDI mainly targets ionic contaminants – while some organics are removed by ionization, the bulk of larger non-ionic organics may pass through unchanged.
Typical EDI Applications
The unique capabilities of EDI technology make it well suited for producing ultrapure water across multiple applications:
Power Industry – Treating boiler feedwater to minimize corrosion and scaling risk to steam systems
Semiconductor Manufacturing – Providing ultraclean rinse water to meet exacting purity demands
Pharmaceutical – Purifying water used in ingredients, processes, or final drug formulations
Electronics – Polishing rinsewater for washing and cleaning metal surfaces or circuits
Laboratories – Supplying consistent quality water for running sensitive analyses or experiments
In many cases, EDI systems provide the last stage of purification following extensive pretreatment – whether it’s polishing reverse osmosis permeate or taking deionized water to unprecedented levels of cleanliness.
This makes it an indispensable final step to remove trace ionic contaminants that could otherwise undermine end use process integrity and quality.
So in summary – EDI’s unmatched effectiveness at eliminating the “last traces” of dissolved salts or inorganic compounds make it ideal for fulfilling stringent water purity requirements across multiple industries.
Conclusion
Electrodeionization combines the chemical free process of electrodialysis with the continuous regeneration benefits of ion exchange technology.
This enables uninterrupted production of consistent purity water otherwise unattainable by conventional treatment methods.
By facilitating the thorough removal of all ionic contaminants down parts per billion levels, EDI systems ensure integrity of high purity water to safeguard vital processes and products.
Although intensive pretreatment is essential and capital costs run higher – the operational savings and process reliability justify EDI implementation for many applications demanding exceptional water quality.
So if your operations require pushing water cleanliness to the limits of current technology – integrating an electrodeionization unit may provide the competitive quality edge vital to your business.