Electro De-Ionization or EDI
by Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM
Kremesti Environmental Consulting Ltd
Introduction
Electro-deionization (EDI) is an amazing water treatment process that uses a DC electrical field, electrodes, ion-exchange resins, and ion-selective membranes to remove ions from water, producing high-purity deionized DI water and a reject stream high in TDS. It works by forcing ions through the resins and membranes, which are then moved into a concentrated waste stream, making it a continuous. It is chemical-free alternative to traditional ion-exchange methods that require periodic chemical regeneration because the resins are regenerated by the H+ and OH- ions produced in the process. Its invention in the USA goes back to the 1950’s water treatment company US Filter which later became Evoqua and now Xylem.
Similar technologies based on electric-field separation of cations and anions are:
Electro-Dialysis and Electro-Dialysis Reversal
Capacitive De-Ionization CDI
Cation-selective membranes consist of sulphonated polystyrene, while anion-selective membranes consist of polystyrene with quaternary ammonia functional groups.
Original Patent
https://patents.google.com/patent/US4632745A/en
https://www.lalaw.com/vingettes/gary-ganzi/
Simple Schematic
A cathode and an anode are separated by Anion-exchange Membranes (AM) and Cation-exchange Membranes CM. Cations migrate to the cathode, anions migrate to the cathode. In the middle lane, Mixed Bed Exchange resin deionizes the water while being -continuously regenerated by in-situ H+ and OH- ions.
Schematic of Electro De-Ionization or EDI
Electro-deionization (EDI) achieves continuous resin regeneration by using a direct current (DC) electrical field to split water molecules into hydrogen (H+) and hydroxide (OH-) ions in situ, which then continuously recharge the ion exchange resins. This process eliminates the need for the batch-wise chemical regeneration (using acids and caustics) required by conventional ion exchange systems.
Difference Between EDI and Capacitive Deionization CDI
The main difference is their mechanism: EDI (Electrodialysis with Ion-Exchange Resin) uses an electric field to move ions through selective membranes, with ion-exchange resin in the diluate chambers to lower resistance and achieve ultra-purification. In contrast, Capacitive Deionization (CDI) uses an electric field to trap ions directly onto the surface of charged porous electrodes without membranes or resin. EDI is typically for high-purity water production, while CDI is more energy-efficient and ideal for brackish water desalination.
Capacitive Deionization (CDI) is a process system that removes charged species from water using an electrical potential difference (electrical driving force on the ions) between a pair of electrodes made often of porous carbon. One electrode which is positively charged, attracts anions (negatively charged ions) and the other electrode which is negatively charged attracts cations (positively charged ions).
The absence of hydraulic pressure means that OPEX can be reduced and fouling can be controlled in contrast to pressure-driven membrane processes. What’s more, a relatively low voltage is required (< 1.8 V) which means significant advantages in terms of low energy requirements with substantial water recovery.
A typical Capacitive Deionization system cycles between two phases: a) Adsorption, where ions are removed from the water and b) Desorption, where the electrodes are regenerated.
Schematic of Capacitive De-Ionization
It is also worthy of mention that CDI has moderate disinfection properties. The Mechanism of action is that Bacteria typically have a negative surface charge and are attracted to the positively charged anode during the deionization cycle. The primary killing mechanisms are the strong electric field, localized pH changes, osmotic effects, and the generation of short-lived reactive species (like hypochlorite ions, ClO⁻, or hydrogen peroxide, H₂O₂) at the electrode surfaces.
Modes of Operation of CDI
A Capacitive Deionization cell can be operated in either the constant voltage or the constant current mode.
- a) Constant voltage operation
When using constant voltage operation during the adsorption phase of a CDI the EDLs (carbon-based system) are uncharged at the beginning of an adsorption step, which results in a high potential difference over the two electrodes and the effluent concentration to decrease. When more ions are adsorbed in the EDLs, the EDL potential increases and the remaining potential difference between the electrodes decreases. Because of the decreasing ion removal rate, the effluent ion concentration increases.
- b) Constant current operation
Due to the ionic charge transported into the electrodes being equal to the applied electric current, by using a constant current allows for a better control on the effluent salt concentration. In order to have a stable effluent, salt concentration membranes should be used in the CDI cell (MCDI), as this operation mode does not only induce counter-ion adsorption, but co-ion depletion as well.
Advantages
Capacitive Deionization it has some unique advantages. First it enables salt removal at low (sub-osmotic) pressures and room temperatures, with the primary input being a small cell voltage (∼1 V) and an electric current whose magnitude depends on the system size. So, CDI does not need to be coupled to high pressure pumps (as in RO) or heat sources (as in MSF Distillation), which means that the system gets only superficial scaling.
Coupling CDI with RO
RO technology is the most widespread desalination process mainly due to its very high salt rejection but it still has some disadvantages such as membrane fouling & scaling and high energy consumption. CDI can potentially compliment RO and help with its limitations with RO-CDI hybrid systems achieving increased performance and higher energy efficiency.
There are two main RO-CDI hybrid systems:
- the RO-CDI pass system (CDI to treat RO permeate) for ultrapure water (UPW) production
- the RO-CDI stage system (CDI to treat RO brine) for maximizing water recovery rate in wastewater treatment.
- RO-CDI pass system
Recent advanced researches exhibited the great potential of the RO-CDI pass system for substituting the conventional RO-EDI system with satisfactory product water quality and improved energy efficiency.
Other Applications
Capacitive Deionization (CDI), particularly Membrane Capacitive Deionization (MCDI), is a viable and effective technology for treating both cooling tower make-up water and blowdown water. It is used to reduce dissolved solids and scaling potential, offering significant water, chemical, and energy savings compared to traditional methods like reverse osmosis (RO) for specific applications.
Difference Between EDI and Electro Dialysis
The main difference is that electrodialysis (ED) uses only electrical potential and ion-exchange membranes to separate ions, while electro-deionization (EDI) combines ED with ion-exchange resin, which is continuously regenerated by the electrical current. This means EDI can achieve higher purity water without the need for chemical regeneration of the resin.
Schematic of Electro-Dialysis
EDI, CDI and ED Technology Suppliers
DuPont
Veolia
GE
Xylem
Conclusion
Sometimes I have to pinch myself when I read about these technologies that can accomplish what Moses accomplished after the Exodus: turn salt water into potable water. I find it weird that there are still conspiracy theorists out there that doubt that man has landed on the moon…
References
Capacitive deionization in water treatment: A review of reactor dynamics, electrode materials, functional membranes, and modeling techniques. Desalination, Volume 600, 1 May 2025, 118459
https://www.sciencedirect.com/science/article/abs/pii/S0011916424011706
About The Author
Rami Elias Kremesti is a chartered water treatment specialist based in the UK. He is the MD of Kremesti Environmental Consulting Ltd which has its office in High Wycombe. He considers himself to be lucky to be working in one of the most exciting and purpose driven fields in applied chemistry, namely water treatment and recycling.
Rami Elias Kremesti Portrait