Water. desalination + reuse

TECHNOLOGY Capacitive Deionization: Challenges and Opportunities _________ Lawrence Weinstein, FlexEl LLC Ranjan Dash, Dash & Associates LLC, USA ___ Editor's Note: When new technologies arrive on the scene, various companies and researchers publish their own particular view of their use and value. After a few years, some, like capacitive deionization, achieve a toehold in the industry and are deserving of the more considered look these authors have given them. CAPACITIVE DEIONIZATION (CDI), often called capacitive desalination, electrochemical desalination or flow-through capacitor, is a fairly new desalination method. Over the past couple of decades, CDI has been promoted as a cheap, low-energy, high-yield competitor to reverse osmosis (RO) and electrodialysis (ED), with applications ranging from water softening to seawater desalination. CDI electrochemically removes ions from salty water. A saltwater process stream flows between two electrodes held at a potential difference of around 1.2-1.5 V. Ions in the solution are attracted to the oppositely charged electrodes. The ions are electrosorbed onto the electrodes, removing them from the process stream, and the deionization cycle continues until the electrodes are saturated with ions. Then, during the regeneration cycle, the two electrodes are discharged or the polarity of the electrodes is reversed. This releases the ions into a waste stream, which has a much higher salt concentration than the process stream. The major market advantage that CDI currently has over competing technologies is its ability to remove a wide range of ionic contaminants with high recovery rates. CDI can remove nearly all ionic contaminants – sulphates, nitrates, iron, arsenic and fluorides, along with sodium, calcium and magnesium salts. RO, which forces salty water through a nanoporous membrane, removes water from salt. By contrast, CDI removes salt from water. CDI has a much higher water recovery (up to 90% or more) than RO, which normally has a recovery rate of 50% or less. While CDI's high recovery rate is advantageous, it can also cause brine disposal issues. Brine injection back to source water can gradually increase total dissolved solids (TDS) in underground water, causing long-term environmental damage, which is a growing concern in developing countries such as India and Bangladesh. Meanwhile, CDI-based water softeners are being developed to compete with conventional resin technology, which, in contrast to CDI, requires the use and disposal of large amount of salty water, which can be environmentally unfriendly. Unlike many, we believe that, at least in the short term, CDI will be used for low TDS applications (<5,000 ppm) and not for seawater desalination, due to high CDI unit costs, as discussed below, and the acceptability of low water-recovery in seawater desalination. EnERgy usE During CDI's initial development, the potential for very high energy efficiency was seen as a major selling point. The more optimistic energy-use projections, however, require recovering energy during the regeneration cycle of operation; this requires a slow discharge to minimize losses due to polarization, and the electronics needed increase the cost and complexity of a CDI unit. Because fewer charge/discharge cycles can take place in a given time with energy recovery than without, the unit cost increases when using energy recovery. No currently commercially available CDI system uses energy recovery. In fact, most CDI systems reverse polarity during the regeneration cycle, speeding up kinetics and increasing throughput while consuming additional energy. Cost ChaLLEngE The major challenge for CDI is its cost – both capital and operational costs are concerns. An RO system capable of treating 1,000 L/h costs between US$ 3,000 and US$ 4,000, whereas a similar capacity CDI system costs about US$ 10,000. It is possible for CDI units to be sold at a premium price over RO, primarily because of CDI's low life-cycle cost and its ability to remove a wide range of ionic contaminants with substantially higher water-recovery than RO. In the future, CDI may become more cost competitive. No components in CDI (apart from membranes) are expensive; today's high costs are due to | 34 | Desalination & Water Reuse | November-December 2013

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