Capacitive Deionization (CDI)

Adsorption and Desorption Cycles

Capacitive deionization (CDI) is a water purification technology that uses electrodes to remove the charged ion species from aqueous solutions. Because CDI is so energy efficient, it can replace many of the water desalination technologies, particularly when used for low molar concentration streams such as brackish water, ground water, and surface water. The technology is based on ion electrosorption at the surface of a pair of electrically charged electrodes composed of highly porous carbon materials.

The technology works by passing the feed water between two porous electrodes, a cathode (+) and an anode (-). The charged particles separate from the feed water based on their charge (Adsorption Cycle). The porous electrode pair is charged with an applied voltage difference of typically 1-1.4 V, and charged particles (salt ions) present in the feed migrate into electrical double layers along the pore surfaces at the carbon/water interface, removing ions from the feed water. The charged particles are electrostatically held in the double layer until the discharging step, where the external power supply is shorted or its polarity reversed. During discharge, the release of the charged particles results in a brine stream (Desorption Cycle), and the charge leaving the cell can be leveraged to recover energy, analogously to the energy from a discharging electric capacitor.

Advances in the technology have occurred through optimizaiton of the electrode materials, the carbon materials (activated carbon, carbon nanotubes, and graphene), as well as in the architecture and operational modes of the systems.

Useful applications for CDI include desalination, wastewater remediation, and water softening. In addition, CDI can be used to remove contaminants such as lead and copper from public water (due to poor infrastructure) at the point of entry into homes.

CDI Advantages

When the water source is brackish water, ground water, river water, or municipal water, capacitive deionization is the most cost effective and energy efficient method of purifying water. Capacitive deionization removes the dissolved charged species such as total dissolved solids (TDS), inorganic contaminants, and radionuclides.

Capacitive Deionization (CDI) vs. Reverse Osmosis (RO)
  • CDI has a 95% water recovery when purifying to drinking water quality. RO has a 70% water recovery.
  • CDI costs approximately $0.06 per 1000 liters. RO costs $0.08 to $0.16 per 1000 liters.
  • CDI does not require water softening prior to purification. RO does require water softening prior to purfication.
  • CDI is a low maintenance system. RO is a high pressure system across membranes requiring routine maintenance.
  • CDI is capable of leaving some minerals in the water. RO removes everything, including the good minerals.
CDI Applications
  • Water Desalination
  • Water Softening
  • Wastewater Recovery
  • Water Purification

  • CDI is capable of removing the following:

    • Total Dissolved Solids (TDS), principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates.
    • Inorganic contaminants such as antimony, arsenic, barium, beryllium, cadmium, chromium, copper, cyanide, fluoride, lead, mercury, nitrate, nitrite, perchlorate, selenium, and thallium.
    • Radionuclides such as alpha and beta particles, radium, and uranium.
    • Disinfection byproducts such as bromate and chlorite.
    • Ammonia, nitrites, and nitrates
    • Organic contaminants dissolved in aqueous solution as a charged species.
EWCO manufactures capacitive deionization (CDI) systems which utitlize membrane CDI technology (MCDI). MCDI is a major variation of the basic CDI cell architecture which is a cell with two porous electrodes and a separator.

The MCDI architecture utilizes ion exchange membranes on the separator-side of each electrode. As in electrodialysis cells, the feed water channels in MCDI cells are bounded by an anion exchange membrane (AEM) and a cation exchange membrane (CEM).

EWCO MCDI builds a system with a free-standing CEM placed at the cathode and a free-standing AEM placed at the anode. Alternatively, using the porous electrode as structural scaffold, it is possible to directly coat the membrane onto the electrode, which can allow for a thinner membrane layer than when using a standalone membrane.

The main benefit of adding membranes to the CDI cell is the improvement of charge efficiency, as the membranes block co-ions from carrying parasitic current, and can increase the salt storage in electrode macropores. In addition, EWCO membranes may be tailored to have selectivity between different ions of the same charge sign which provides an additional level of tunability for complex multi-ion systems.

EWCO builds MCDI systems with materials that offer the longest life cycles for the capacitors. EWCO membranes are up to one-tenth the thickness of the standard membranes which help to lower operational pressures and increase ion removal rates. EWCO systems are reliable, high efficiency, and low maintenance therefore ideal for installing for large scale long-term applications treating drinking water, wastewater, and industrial process water. MCDI provides high contaminant removal efficiencies while achieving high water recoveries, with minimal waste volumes.