Although known for decades, capacitive desalination, or capacitive deionization, has not been widely adopted, but a redesigned process — created by Stanford University and Lawrence Livermore National Laboratory researchers — has reportedly greatly improved the existing method.
The new method is reportedly four to 10 times faster than traditional capacitive desalination and reduces the salt concentration by three to four times. It is also more energy-efficient, operates at lower pressures, and requires no membrane components than reverse osmosis.
Rather than the feed water flowing between electrodes, the redesigned system feeds the water through the electrodes’ pores. The method is called a flow-through electrode CD system. “Although this design was investigated back in the ’70s, the electrode material and its small pore size resulted in low performance, and there has been little research since then,” noted the researchers.
The system also uses a new electrode material called hierarchical carbon aerogel monoliths in lieu of carbon aerogels. The porous carbon material eliminates the limitations typically associated with conventional capacitive deionization. It also has less separation between electrodes to further increase the system’s performance. The entire process can be executed in only the time needed to charge the electrodes — a few minutes or less, but the system’s speed can also be optimized.
Capacitive deionization was initially touted as “the first new desalination technology in over 50 years.” Originally developed at Lawrence Livermore National Laboratories, capacitive deionization technology research intensified in the mid-1960s and, as of 1998, was still in the lab and had not scaled up to mass use.
According to the U.S. Department of Energy’s National Energy Technology Laboratory (NETL):
Capacitive deionization is based on an electrostatic process operating at low voltages and pressures. Produced water is pumped through an electrode assembly. Ions in the water are attracted to the oppositely charged electrodes. This concentrates the ions at the electrodes, while reducing the concentration of the ions in the water. The cleaned water then passes through the unit.
Once the capacity of these electrodes has been reached, the flow of water stops and the electrodes’ polarity is reversed, allowing the ions to move away from the electrodes. There is a concentrated brine solution that remains in addition to the clean water. This brine is purged from the unit and its disposal has to be considered.
The National Energy Technology Laboratory says capacitive deionization is more cost-effective than comparable desalination technologies such as reverse osmosis and is ideal for treating water that is not highly salty or used for drinking water. They cited several studies, which included the treatment of coal bed methane water.
In a 2005 overview of the technology, which appeared in the journal Desalination (PDF), T.J. Welgemoed and C.F. Schutte of the University of Pretoria explained that the technology has “the potential to be a ‘power tool’ in the desalination toolbox of the future.”
The technology is reportedly a competitor with Electrodialysis Reversal, another type of electrochemical process that has been used since the 1960s.
The Stanford University and Lawrence Livermore National Laboratory researchers made a prototype system for testing that was able to remove salt in water with a salinity of 250 mM at 1.25 V at a rate of 0.96 mg of NaCl per gram of electrode per minute. Typically, the rate is between 0.1 and 0.25. It can also reportedly reduce the salt concentration by as much as 70 mM per charge.
Matthew Suss, the study’s lead author says:
This translates into a reduced infrastructure cost for the electrode system. The flow-through geometry also has higher energy efficiency than the traditional flow-between geometry, as it ensures that all of the desalinated water in the system is used. In the previous systems, only the desalted water in the separator was used, but not that in the electrodes, and so the energy used to desalt the water in the electrode pores was largely wasted. Finally, the reduced spacer size reduces the overall resistance of the device, which further decreases energy cost.
Michael Stadermann, the LLNL’s principal investigator, stated:
FTE CD is capable of desalinating almost all brackish water concentrations in a single step. In some regions around the world, including North America, brackish water is projected to be the main source of water for desalination processes that provide drinking water. Brackish water also is the concentration range of effluent from some industrial processes, such as coal bed methane production — treating this water is essential for proper practical waste reduction. Further, FTE CD performs much better with high salinity streams than regular CD, and lends itself better to energy efficient multi-stage desalination, which allows us to tackle seawater with this method.
Maarten Biesheuvel, a CDI researcher based at Wetsus, the Netherlands’ Centre of Excellence for Sustainable Water Technology, also stated that “it is exciting to see how in about five years time the interest in the CD technology has exploded around the world, having established itself as the primary runner-up to the established technologies of reverse osmosis and electrodialysis.”