By Peter Chawaga
Capacitive deionization (CDI) is a process by which the ions are removed from water with the use of two electrodes. A positively charged electrode captures the water’s negatively charged anions while a negatively charged electrode captures the water’s positively charged cations.
“The technology can be best thought of as a tool which removes dissolved ionic species from a solvent using highly porous carbon electrodes charged to a small voltage,” explained Matthew Suss, assistant professor at the Israel Institute of Technology. “It works by a phenomenon known as electrosorption, where charging the porous carbon electrodes positively allows for dissolved ions of opposite charge to be brought to the pore surface and held there electrostatically. In this way, ions are removed from the water and held along the surface until the voltage is removed.”
This may seem like an abstract practice until you remember that salt is an ionic compound and that through CDI, it can be removed from water.
In the 2015 study “Water desalination via capacitive deionization: what it is and what you can expect” published by the Royal Society of Chemistry, Suss and a team of researchers take a long view at a field that has grown rapidly in the last few years, in large part because of its implications for the water industry.
“It is a highly scalable technique which does not require much energy for brackish water desalination,” he said. “To run it, you need mainly a low-voltage input, so no high-pressure pumps or heat sources are required.”
Probably the most popular desalination alternative to CDI is reverse osmosis (RO). That involves using high-pressure pumps to force water through semipermeable membranes which screen out the salt. These pumps need lots of energy to keep running and the process requires about 5 kWh to produce a cubic meter of freshwater, according to an online encyclopedia of desalination and water resources.
In contrast, a Chinese CDI operation featured in Suss’ study reported energy consumption around 1 kWh for every cubic meter of freshwater produced.
In their study, Suss and his research team put the water recovery ratio (the ratio of produced freshwater volume to feedwater volume) for a typical seawater reverse osmosis (SWRO) plant at 45 percent to 55 percent, while CDI systems have the potential to attain a ratio significantly higher than 55 percent.
However, as a relatively new technology there aren’t that many full-scale operations that utilize CDI. It’s hard to gauge how well it could perform if widely employed.
“It is a fast-emerging technology in the research world and so that is now translating to growth in the industry,” as Suss put it. “The main obstacle is the lack of demonstration plants and scaled-up systems at the moment. Most systems are lab-scale right now.”
With water scarcity propelling us to find creative solutions just so we have enough to drink, it won’t be long until CDI gets its time in the spotlight.