Water. desalination + reuse

TECHNOLOGY low manufacturing volumes and immature manufacturing processes. SCaLiNG iSSuE Electrode scaling is one of the biggest issues encountered in CDI. Virtually all source waters contains calcium and magnesium ions, which are innocuous in concentrations normally seen but can create precipitates at high concentration. During operation, the negative electrode electrosorbs positive ions indiscriminately, including calcium and magnesium ions. When the unit is discharged, a buildup of magnesium and calcium compounds can form when high concentrations of magnesium and calcium are released. To date, mild acids (such as citric acid) have been the preferred descaling method; however, process monitoring to determine when to descale the unit adds to complexity. According to Idropan's CEO, Mariella Servida, CDI unit cleaning is a major technical challenge. Idropan claims to have solved this problem using a patent-pending microinjection system that injects a citric acid-based solution on a daily basis. Other companies including AquaEWP and Enpar also have product literature and/or patent applications noting the use of citric acid to clean CDI units. CDi aND mEmbraNES Historically, CDI has been touted as a membrane-free technology, and hence free from the issues facing membranebound processes such as RO and ED. Nonetheless, overcoming inefficiency and kinetic issues has generally required the use of membranes in practice. Marc Andelman of Biosource Inc first developed membrane CDI technology, and today most CDI units have ion-exchange (IX) membranes against their activated carbon electrodes to improve performance, while increasing cost. The IX membranes allow only positive ions to pass through to the negatively charged electrode, and only negative ions to pass through to the positively charged electrode. This solves two major problems: slow kinetics, and inefficiency due to counterion desorption with increased cost and decreased reliability. Counterion desorption refers to the expulsion of ions with the same sign as the electrode. When the electrodes are at the same potential, they have ions of both charges (positive and negative) adsorbed on their surface. Upon charging they expel same-charged ions (counterions) and Pentair's Hybrid-DI attract oppositely charged ions. For instance, the positive electrode expels positive ions and attracts negative ions. This causes a net transfer of positive ions to the negative electrode, and negative ions to the positive electrode, independent of the deionization, reducing efficiency. With membranes in place, the need to maintain electroneutrality necessitates that ions from solution cross the membrane to balance out the counterions, so that counterion desorption no longer causes inefficiency. Charge transfer membranes also dramatically improve device kinetics. When a CDI unit goes in the regeneration cycle without a membrane, it takes a relatively long time for the ions to diffuse out into the waste stream. With a membrane, the need to maintain electroneutrality at each electrode forces the ions to travel through the membrane quickly during the regeneration cycle, improving device throughput. Despite the advantages of adding membranes, they are prone to fouling and degradation, and are expensive. While the continual charge transfer across the membrane is believed to help maintain the surface, prefiltration before the water reaches the CDI unit is necessary. Historically, ED units, which use ion-exchange membranes like those used for CDI, have had device lifetimes limited by membrane longevity. However, electrodialysis reversal (EDR), where the polarity of an ED system is periodically reversed, has shown improved longevity. Since in CDI the polarity is reversed every cycle, this lends credibility to the idea that the membrane will be more robust than in ED. CDI units appear to decrease in water recovery over time due to engineering/ design and membrane issues. It is believed that the space between electrodes increases with time, decreasing the flow resistance of water between the electrodes. This may be from the membrane wearing away during the repetitive charge and discharge cycling, pointing to the need for further membrane development work and improved device engineering. There are very few companies that manufacture membranes specifically for CDI; one notable company is Fujifilm's Netherlands operation. Little published research has focused on characterizing and improving membranes for CDI. CDI's reliability problems are design-related, and therefore can be solved as the technology matures. NOvEL maTEriaLS While membrane development for CDI has seen little attention, much fundamental research has focused on novel carbon materials for use in CDI. Materials such as nanotubes, graphene and aerogel have been tested for use in CDI electrodes; the interest in novel carbon materials for CDI has been so high that Oak Ridge National Laboratories won an R&D 100 Award in 2011 for creating an ordered mesoporous carbon for CDI. However, activated carbon, at only US$ 4/kg for commodity carbon and US$ 15/kg for highly purified, specially selected supercapacitor carbon, remains much cheaper than the alternatives, which cost US$ 50/kg or more. Larger activated carbon electrodes are much cheaper than relatively small exotic carbon electrodes, and can remove just as much salt for a given current. The performance increase from novel carbons is insufficient to motivate their use at this point, especially since virtually all CDI applications under serious near-term November-December 2013 | Desalination & Water Reuse | 35 |

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