Water Online the Magazine gives Water & Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/164223
Technology A New Way To Desalinate — Government Tested, Real-World Approved Developed by the Department of Defense, capacitive deionization (CDI) removes salt from water, while conventional methods remove water from salt. By Patrick Curran W hat is easier, removing 96.5 parts of Technologies (November 2009, pages 35-37). Also something from 3.5 parts of something described in the report, the energy consumption is else or removing 3.5 parts of something very large and capital cost per gpm higher than statefrom 96.5 parts of something else? All else of-the-art alternatives. being equal, of course it is easier to remove the lesser CDI also removes ions from water with an electric component from the field (see diagram below). larger. But that is not how As ions pass through most water desalination the charge-specific CDI is another example of a technologies work today. membrane, they adsorb technology that was developed for Reverse osmosis (RO) and onto the surface of a one purpose, but found other market vacuum distillation (VD) high-surface-area carbon all work by removing the supercapacitor instead of segments where its advantages water from the salt water. pairing up with oppositely were greater than expected. So, how can you remove charged ions. Because of salt from water? this, anions and cations Removing ions from are kept separate during solution with an electric the purification cycle. field is a well-known method to desalinate water, and This enables CDI to remove low solubility species electrodialysis is a popular example. Because ions are without fouling because the cations and anions are disassociated in solution (Na+ separate from Cl-), they physically separated. can move independently of one another and can be After the supercapacitor fills with ions, the polarity pulled towards an oppositely charged plate. Removing is switched, and the ions are pushed back into the ions from solution as it is done with electrodialysis (ED) original flow channel. Water flow is stopped, and can be a much easier way to desalinate water versus the concentration of ions builds to more than 10X the standard reverse osmosis or brine concentrator the incoming solution. Because the electric field method of removing water from the salt water. is now opposite, the ions attempt to adsorb onto When the ions are removed the newly charged electrode. as in ED, they must be But because of the chargepaired up with an oppositely specific membranes, they charged ion supplied by are prevented from passing another flow channel within through and readsorbing. the device or the splitting of Although they reside in the water. This pairing causes same small space, the electric device, energy, and mass field minimizes mixing of the balance issues and limits the anions and cations, preventing effective range of ED to a any significant precipitation. maximum of 8,000 ppm and After all of the ions have been typical range of 1,200 ppm, discharged, they are removed per the Colorado School of from the device, and the cycle Mines' Technical Assessment repeats itself. By concentrating of Produced Water Treatment in this fashion, the recovery 28 wateronline.com ■ Water Online The Magazine