Technology A New Way To Desalinate — Government Tested, Real-World ApprovedDeveloped by the Department of Defense, capacitive deionization (CDI) removes salt from water, while conventional methods remove water from salt.
By Patrick Curran
What is easier, removing 96.5 parts of something from 3.5 parts of something else or removing 3.5 parts of something from 96.5 parts of something else? All else being equal, of course it is easier to remove the lesser component from the larger. But that is not how most water desalination technologies work today. Reverse osmosis (RO) and vacuum distillation (VD) all work by removing the water from the salt water. So, how can you remove salt from water? Removing ions from solution with an electric field is a well-known method to desalinate water, and electrodialysis is a popular example. Because ions are disassociated in solution (Na+ separate from Cl-), they can move independently of one another and can be pulled towards an oppositely charged plate. Removing ions from solution as it is done with electrodialysis (ED) can be a much easier way to desalinate water versus the standard reverse osmosis or brine concentrator method of removing water from the salt water. When the ions are removed as in ED, they must be paired up with an oppositely charged ion supplied by another flow channel within the device or the splitting of water. This pairing causes device, energy, and mass balance issues and limits the effective range of ED to a maximum of 8,000 ppm and typical range of 1,200 ppm, per the Colorado School of Mines’ Technical Assessment of Produced Water Treatment Technologies (November 2009, pages 35-37). Also described in the report, the energy consumption is very large and capital cost per gpm higher than state- of-the-art alternatives. CDI also removes ions from water with an electric field (see diagram below). As ions pass through the charge-specific membrane, they adsorb onto the surface of a high-surface-area carbon supercapacitor instead of pairing up with oppositely charged ions. Because of this, anions and cations are kept separate during the purification cycle. This enables CDI to remove low solubility species without fouling because the cations and anions are physically separated. After the supercapacitor fills with ions, the polarity is switched, and the ions are pushed back into the original flow channel. Water flow is stopped, and the concentration of ions builds to more than 10X the incoming solution. Because the electric field is now opposite, the ions attempt to adsorb onto the newly charged electrode. But because of the charge- specific membranes, they are prevented from passing through and readsorbing. Although they reside in the same small space, the electric field minimizes mixing of the anions and cations, preventing any significant precipitation. After all of the ions have been discharged, they are removed from the device, and the cycle repeats itself. By concentrating in this fashion, the recovery wateronline.com n Water Online The Magazine 28 CDI is another example of a technology that was developed for one purpose, but found other market segments where its advantages were greater than expected. of clean water can be as high as 95 percent. This is extremely beneficial in industries such as oil and gas that have wastewater disposal volume concerns. By using capacitors to capture the ions instead of another flow channel, the device construction costs and energy usage are much lower. And because the flux rates (moles/min/cm2 membrane) are significantly higher, a given device can process a much higher inlet of total dissolved solids (TDS). The latest system designs for 8,000 to 10,000 ppm applications have capital costs less than 1/3 that of ED. The cost of ownership (energy, maintenance, depreciation, and disposal) is up to 70 percent less than the leading state-of-the-art technology for that particular TDS range. The latest CDI devices can process up to and greater than 150,000 ppm of water including water with over 50,000 ppm of hardness and low solubility species such as barium(Ba)/strontium(Sr)/calcium(Ca) sulfates and calcium carbonates. Because any dissolved ions can be removed, it is also effective at removing low concentrations of heavy metals such as mercury, arsenic, selenium, uranium, etc. Because of the ability to process complicated high TDS waters, CDI is making inroads into industries with very difficult, highly saline waters such as oil/gas produced water, frack water, mining wastewater, flue gas desulfurization (FGD)/cooling tower blowdown, and other industrial wastewaters. This is very helpful when treating water for discharge with stringent limits such as power and greenhouse industries. In some cases, the concentrated solution is the desired product. CDI can concentrate a solution to recover high-value metals, salts, acids, and bases. The system basis is the grouping together of large supercapacitors, similar to how RO tubes are organized. This positions CDI to be able to build high-capacity systems simply by duplicating the supercapacitors. Pretreatment is very important for any membrane process, including CDI. The requirements for total suspended solids (TSS), organics, and iron are very similar to RO and ED due to the very thin flow channels and fouling nature of organics and ferric iron. But because of CDI’s ability to capture and remove low solubility salts safely, no antiscaling chemicals are needed. The origin of most current CDI devices is from a DARPA project from the DoD back in 2000-2004 to develop an alternative to desalination systems used to supply troops with fresh water. CDI is another example of a technology that was developed for one purpose, but found other market segments where its advantages were greater than expected. As system design and material properties continue to improve, the economics and performance capabilities of the CDI system will continue to improve and likely surpass all existing technologies in performance. These systems are starting to penetrate various markets and will continue to make inroads over the next decade.
n Technology wateronline.com n Water Online The Magazine 30 Patrick Curran, CEO and founder of Atlantis Technologies, has led the development and commercialization of many novel processes and products over his 25-year career, garnering seven issued and pending patents. He has a B.S. in chemical engineering from Drexel University. Removing ions from solution as it is done with electrodialysis can be a much easier way to desalinate water versus the standard reverse osmosis or brine concentrator method. Copyright © 1996 - 2013, VertMarkets, Inc. All rights reserved. To subscribe or visit go to: http://www.wateronline.com
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