Electrodialysis Explained: How Ion-Selective Membranes Purify Water

Electrodialysis is a water purification technology that removes dissolved salts using an electric field and special ion-selective membranes. Unlike conventional filters that trap mechanical impurities, electrodialysis targets charged particles-ions-formed when salts dissolve in water.

What Is Electrodialysis and Why Is It Needed?

To understand the principle of electrodialysis, it's important to know what happens to salt after it dissolves. For example, ordinary sodium chloride in water splits into positively charged sodium ions and negatively charged chloride ions. Although the solution appears homogeneous, it's filled with countless mobile charged particles at the molecular level.

Typical mechanical filters are nearly powerless against such ions. Their size is much smaller than the pores of most filter materials, so dissolved salts pass through cartridges along with water. That's why filters for sand, rust, or other suspended solids don't make salty water fresh.

Electrodialysis tackles the problem differently. Rather than physically trapping salt, the technology leverages the electric charge of the ions themselves. If a constant electric field is created between two electrodes, positive ions move toward the negative electrode (cathode), and negative ions move toward the positive electrode (anode).

However, an electric field alone isn't enough. If electrodes are simply placed in a saline solution, ions will move within the liquid, but this won't result in a stable separation of water into purified and concentrated streams. That's why special membranes, which selectively allow only certain types of ions to pass, are installed between the electrodes.

As a result, an electrodialysis setup creates two types of channels: in some, the amount of dissolved salts gradually decreases-producing purified water (dilute), while in adjacent channels, salts accumulate, forming a concentrate that's removed from the system separately.

The key feature of electrodialysis is that it removes only charged dissolved substances from water. It's excellent for reducing mineralization, removing some dissolved salts, and treating brackish water, but it's not a universal solution for all contaminants.

Organic compounds, large particles, microorganisms, and many uncharged substances may require other treatment steps. In practice, electrodialysis often works as part of a comprehensive system, combined with mechanical filtration, sorption, disinfection, and other methods.

How Electrodialysis Works

The foundation of electrodialysis is a simple physical property: dissolved salts break down into charged particles, and an electric field causes these particles to move. Positive ions (cations) move toward the cathode, while negative ions (anions) move toward the anode.

If only saline water were placed between the electrodes, ions would simply move throughout the solution. To separate them, alternating ion-selective membranes are arranged in the system. These membranes turn ordinary ion movement into a controlled purification process.

Why Do Ions Move in an Electric Field?

Molecules of many salts dissociate into ions in water. For example, sodium chloride forms sodium cations (Na⁺) and chloride anions (Cl⁻). When a constant voltage is applied to the electrodes, these particles move in opposite directions.

The more dissolved ions in the water, the higher its conductivity. Salty water conducts electricity well, while nearly fully desalinated water is much less conductive. This directly affects electrodialysis operation: as salts are removed, the electrical resistance of the treated solution increases.

The electric field doesn't destroy the salt or turn it into something else. It merely causes the ions in the solution to move from one channel of the unit to another. As a result, salts concentrate in a separate stream, which is then removed from the system.

How Do Ion-Selective Membranes Separate Salts?

Two main types of membranes are used in electrodialysis units: cation-exchange and anion-exchange membranes. They are alternately arranged between the anode and cathode.

The cation-exchange membrane mainly allows positively charged ions to pass while restricting negative ones. The anion-exchange membrane works the opposite way: it lets anions through more easily while blocking cations.

Imagine a channel filled with salty water. Under the electric field, cations move toward the cathode and pass through the nearest cation-exchange membrane. But then they encounter an anion-exchange membrane, which they can't pass.

Anions in the same channel move in the opposite direction. They pass through the anion-exchange membrane but are stopped by the cation-exchange membrane. Thus, both types of ions gradually leave the original channel and end up in adjacent ones.

Because the membranes alternate, one channel loses salts while its neighbors accumulate them. The channel with reduced mineralization is called the dilute chamber, while the channel with higher salt concentration is the concentrate chamber.

In real installations, there are not just two or three chambers, but an entire stack. Dozens or hundreds of membranes form a repeating structure of dilute and concentrate channels. This allows a large surface area of membranes to simultaneously participate in ion transfer, making it possible to treat significant volumes of water.

The degree of purification depends on several factors: the salt concentration in the source water, the membrane area, the strength of the electric field, the water flow rate, and the time it spends in the unit. If more salt needs to be removed, water can pass through several electrodialysis stages or circulate multiple times.

Importantly, electrodialysis removes not the water itself, but the charged particles dissolved in it. This is a key difference from many membrane technologies, where mainly water moves through the membrane under pressure.

Electrodialysis Unit Design

An electrodialysis unit consists of more than just membranes and two electrodes. For stable operation, it uses a whole set of elements that drive the water, maintain the electric field, and separate the purified and concentrate flows.

The core of the system is the membrane stack, where cation- and anion-exchange membranes alternate, separated by thin water channels. This structure creates many dilute and concentrate chambers simultaneously.

At the edges of the membrane stack are the anode and cathode. Constant voltage is applied to them, generating the electric field across the system. This field is what drives the dissolved ions through the membranes.

Water is supplied to the unit by pumps. The flows are distributed so that some channels produce water with reduced salt concentration, while adjacent channels collect the removed ions. At the outlet, these streams are separated and can be directed for further treatment, recirculation, or concentrate disposal.

Inside the unit, spacers and flow distributors are also used. They maintain the necessary distance between membranes and help water flow evenly across the surfaces. Uneven flow can decrease membrane efficiency and increase the risk of deposits in certain areas.

A power supply allows adjustment of voltage and current. The stronger the electric field, the faster ions can be transferred, but current cannot be increased indefinitely. Excessive load reduces process efficiency and can cause unwanted electrochemical effects at the membrane surfaces.

Pre-treatment of the water is crucial. Suspended particles, organic contaminants, and poorly soluble compounds can deposit on membrane surfaces and reduce their performance. Therefore, water is often filtered and its chemical composition adjusted before electrodialysis.

Electrodialysis Reversal (EDR)

One variant of the technology is Electrodialysis Reversal (EDR). The principle remains the same, but the direction of the electric field is periodically reversed.

The unit switches electrode polarity: the former cathode becomes the anode and vice versa. At the same time, the system redirects flows so that the dilute and concentrate chambers swap roles.

Reversing the direction of ion movement helps reduce the buildup of scale and fouling on membrane surfaces. Particles that start to concentrate in certain areas are sent the opposite way when the field is reversed.

This allows EDR to operate more stably with water prone to mineral deposits. However, reversal does not eliminate the need for pre-treatment and membrane maintenance-it only slows down fouling and helps extend reliable operation.

Industrial electrodialysis systems may include multiple membrane stages, conductivity, pressure, and flow sensors, automatic power control, and cleaning systems. This allows for continuous desalination monitoring and flexible adjustment based on source water quality.

Electrodialysis vs Reverse Osmosis: What's the Difference?

Both electrodialysis and reverse osmosis reduce dissolved salts in water, but they do so in fundamentally different ways. In electrodialysis, ions themselves move under the influence of an electric field, while in reverse osmosis, water passes through a membrane under high pressure.

In a reverse osmosis unit, pressure forces water molecules through a semi-permeable membrane that blocks most dissolved salts and many other impurities. This also creates two streams: purified water and a concentrate with high contaminant levels.

For electrodialysis, the main pressure is only needed to pump water through the channels; salt removal is achieved by electrical current. Cations and anions leave the treated stream through their respective ion-selective membranes, while most of the water remains in its channel.

As a result, the energy consumption of electrodialysis depends heavily on the mineralization of the feed solution. The more ions that need to be moved, the more electrical energy is required. This makes the technology especially attractive for brackish water and other solutions with moderate salt content.

Reverse osmosis, on the other hand, is widely used for water with very high mineralization, including seawater. In such systems, a significant portion of the energy is used to create the high pressure needed to overcome the osmotic pressure of the solution.

For more details about other methods of obtaining fresh water and how desalination systems work, you can read the article How Desalination Plants Turn Seawater into Fresh Water.

The nature of purification also differs. Electrodialysis primarily targets charged dissolved particles. Uncharged organic substances and some other contaminants may barely be removed unless additional treatment steps are used.

Reverse osmosis membranes provide broader separation, retaining not only salts but also many organic compounds, microorganisms, and other impurities. Therefore, reverse osmosis is often chosen when extremely low mineral content is required.

Electrodialysis offers advantages when complete desalination isn't necessary. The process intensity can be adjusted to remove only the desired portion of salts. For example, water with excess mineralization can be brought down to a target level instead of removing almost all dissolved substances.

Both technologies require pre-treatment. Reverse osmosis membranes are especially vulnerable to fouling and damage from suspended solids, organics, and sediments. In electrodialysis, similar issues can hinder ion movement and increase membrane pack resistance.

Thus, it's impossible to say that electrodialysis is always better than reverse osmosis or vice versa. The choice depends on the mineralization and composition of the source water, the required output quality, acceptable energy consumption, and which impurities need to be removed.

Where Is Electrodialysis Used and What Are Its Limitations?

Electrodialysis is used where reducing dissolved salt concentration or producing a more concentrated salt solution is needed. The technology is especially well-suited for water with moderate mineralization, where deep desalination to nearly zero salt content isn't required.

One major application is desalination of brackish water, which contains more salts than fresh water but far less than seawater. In such cases, electrodialysis may be more energy-efficient than technologies designed for very high mineralization.

Another area is industrial water treatment. Facilities use electrodialysis to lower certain ion concentrations before further processing, such as preparing water for equipment, reusing process wastewater, or adjusting solution compositions.

The technology is also used in the food industry to remove or redistribute mineral components in various liquid products and process solutions. Electrodialysis is convenient because separation occurs without significant heating, which is important for temperature-sensitive substances.

In some processes, the concentrate-not the purified stream-becomes the valuable product. Since ions are purposefully transferred between chambers, electrodialysis can be used to concentrate specific salts and recover components from industrial solutions.

Key advantages include the ability to precisely control the degree of desalination. The system doesn't have to remove nearly all dissolved substances: the mode can be set to reduce mineralization to the desired level.

Another benefit is efficient water use. In electrodialysis, mainly ions-not the bulk of the liquid-move through membranes. This enables systems with a high yield of purified water, although the exact result depends on the solution composition and unit configuration.

However, there are limitations. The higher the water's mineralization, the more charged particles must be transferred, which can increase energy consumption. Therefore, electrodialysis isn't always the most economical option for highly saline seawater.

Membrane fouling is another problem. Mineral deposits, organic compounds, and suspended particles can accumulate on membranes, increasing resistance, hindering ion transfer, and reducing system performance.

Electrodialysis is not a universal filter. It's most effective against dissolved charged components. If water contains microorganisms, organic pollutants, mechanical particles, or uncharged substances, additional treatment methods are needed.

As a result, modern electrodialysis units are usually part of larger water treatment systems. Combining pre-filtration, electrodialysis, and subsequent processing steps allows the strengths of the technology to be used where controlled removal of dissolved salts is most effective.

Conclusion

Electrodialysis purifies water not by mechanically trapping salts, but by controlled movement of charged particles. The electric field drives cations and anions in opposite directions, while alternating ion-selective membranes direct them into separate chambers. One stream loses salts, while the other becomes more concentrated.

The main advantage is the ability to control the degree of desalination and specifically remove dissolved ions. Therefore, electrodialysis is especially valuable for treating brackish water, industrial water preparation, and processes where the deepest purification isn't required.

However, electrodialysis isn't a universal solution. Removing organic substances, microorganisms, and mechanical contaminants requires additional treatment steps, and for very high mineralization, reverse osmosis may be more suitable. The choice of system depends on the source water composition, required output quality, and energy consumption considerations.