Capacitive Deionization

Capacitive Deionization (CDI) is an emerging technology that can be used for energy efficient removal of dissolved, charged species from water for example for desalination of brackish or sea water. Yet, CDI is much more than an attractive tool to generate potable water and has been applied also to wastewater remediation and water softening. The high energy efficiency of CDI for the desalination of water with a low salt concentration (typically below 10 g/l) is due to the fact that the salt ions, which are the minority component in the water, are removed from the mixture. By contrast, conventional water desalination systems (such as reverse osmosis, and distillation) instead remove the majority component, the water molecules, from salty feedwater. CDI is typically characterized by an intermittent operation between ion electrosorption (until the electrodes are fully saturated / fully charged) and electrode regeneration (which relates to discharging the electrodes to release the electrosorbed ions). Considering a full CDI cycle, the invested charge for ion removal can be largely recovered during electrode regeneration, enabling energy consumption significantly below 1 kWh/m3 for desalinated brackish water. As a result of the intermittent operation, sequential iondepleted and ion-enriched stream are generated, yielding a water recovery which can be significantly above 50%, and as high as 90%. Water recovery is an important performance metric for desalination technologies and is defined as the ratio of freshwater volume over inlet volume.