Fact Sheet - Capacitive Deionization
This fact sheet describes capacitive deionization, another technology that removes salt and other inorganic chemicals from produced water.
Capacitive deionization is based on an electrostatic process operating at low voltages and pressures. Produced water is pumped through an electrode assembly. Ions in the water are attracted to the oppositely charged electrodes. This concentrates the ions at the electrodes, while reducing the concentration of the ions in the water. The cleaned water then passes through the unit.
When the electrodes' capacity is reached, the water flow is stopped and the polarity of the electrodes is reversed. This causes the ions to move away from the electrodes, where they had previously accumulated. The concentrated brine solution is then purged from the unit.
Capacitive deionization offers more cost-effective performance than other types of salt removal technology (e.g., reverse osmosis), especially when the produced water is not highly salty or the treatment goal is not geared towards achieving drinking water quality.
Christen (2006) describes tests conducted by Sandia National Laboratories. The researchers used coal bed methane (CBM) produced water from the San Juan Basin in New Mexico, where considerable CBM development has occurred. The levels of total dissolved solids (TDS) in the produced water ranged from 2,000 ppm to 5,000 ppm. Removal operations accomplished reductions from 75 to 90%. The highest recoveries were associated with the lowest TDS concentrations. Higher water salinity translates into increased energy requirements. Christen (2006) quotes Sandia's Mike Hightower, "There's a breakover point where you get your best energy efficiency with this technology at lower TDS levels, whereas reverse osmosis seems to work better at TDS levels of 6,000 ppm and higher." Christen (2006) also notes that capacitive deionization systems have a much lower tendency to foul than reverse osmosis systems. This means that the produced water requires less pretreatment.
Atlas (2007) ran a test to treat CBM water at flows of 5 gpm with two 5 gpm systems in series. Solids were filtered using a 30 micron filter. The incoming feed water had a TDS of 2,500 ppm. Following treatment in the unit, the TDS was 270 ppm (85% recovery). The sodium adsorption ratio (SAR) was reduced from 24 to 3. Atlas suggests that over a 10-year span the total cost of capital and operations will be $.05 per barrel of water processed.
The backwash stream from capacitive deionization treatment of produced water will contain high levels of sodium and chlorides. It is therefore advisable to consider disposal requirements for the concentrated brine when selecting ion exchange treatment.
References
Atlas, R., and J. Wendell, 2007, "Purification of Produced Water Using Hybrid CDI-ED Technology," presented at the 14th International Petroleum Environmental Conference, Houston, TX, November 5-9. Available at http://ipec.utulsa.edu/Conf2007/Papers/Atlas_77a.pdf.
Christen, K, 2006, "Desalination Technology Could Clean Up Wastewater from Coal-bed Methane Production," Environmental Science & Technology, January 11. Available at http://pubs.acs.org/subscribe/journals/esthag-w/2006/jan/tech/kc_desalination.html.
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