Membrane capacitive deionization (MCDI) is an emerging desalination technology that stores ions in the electrical double layer of porous electrodes. For most low-salinity water treatment technologies, fouling is a central issue, but few studies examine the its impact on MCDI. Fouling was evaluated using synthetic saline solutions with a combination of NaCl and CaCl2 and the model organic foulant sodium alginate, known to complex with calcium, in a lab-scale MCDI system. Fouling caused an increase in the system pressure as well as 10-15% declines in salt adsorption and charge efficiency, and a similar increase in desalination energy over 20 hours of operation. The pressure increased overall, but showed cyclical increases and decreases suggesting that fouling was worsened with increased salt concentration during the discharge/brine generation. To evaluate the reversibility of fouling, pH adjustment, chelating agents, and hydraulic cleaning were applied to the membranes. Cleaning with pH adjustment reduced the system pressure, but still showed declines in system performance, even with cleaning cycles every 20 hours for 80 hours of operation. Over 80 hours, salt adsorption declined by up to 30% and energy use per ion removed increased by the same. The addition of EDTA to the basic cleaning solution reduced pressure and showed significant recovery of pre-fouling salt removal and energy use, indicating that the fouling could be reversible. After each cleaning cycle, adsorption, energy use, charge, and charge efficiency recovered to within 5% of their pre-fouling values, indicating that EDTA was well suited to break up the fouling layer created by the alginate and calcium.
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Organic fouling in membrane capacitive deionization systems
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