Technology▼Solutions Desalination technology could clean up wastewater from coal-bed methane production
SANDIA NATIONAL LABORATORIES
(15 psi). When the brackish water is pumped through a high-surface-area electrode assembly, ions in the water are attracted to the oppositely charged With the Bush Administration pushdeionization over reverse osmosis at electrodes. This concentrates the ions ing to double production of coal-bed low TDS levels. at the electrodes and thus reduces their methane (CBM) over the next decade, “This makes it an interesting techconcentration in the water. When the capacitive deionization is emerging nology for more remote settings far electrode capacity is exhausted, the waas a potential technology for cleaning from any power grid [where many ter flow has to be stopped to discharge up the large volumes of brackish waCBM wells are located], because in the capacitor, with the ions rejected ter extracted along with the natural theory, it could be run by photovolback into a now-concentrated solution. gas. The technology’s allure, Sandia’s lab tests used CBM researchers say, is that it may water from New Mexico’s San require less energy and mainJuan Basin, where much of tenance than competitive the CBM development has octechnologies. curred to date, with TDS levels CBM wells often produce of 2000–5000 ppm, Hightower as much as 100–400 barrels of says. Removal rates ranged methane daily. Because water from 75 to 90%, with the highis extracted with the methest recoveries associated with ane, this equates to 15–16 m3 the lowest TDS concentraof water that typically contions. As the water salinity intains salts and dissolved solcreases, so does the energy ids. The concentration of total requirement. “There’s a breakdissolved solids (TDS) in the over point where you get your water ranges from 500 to 7000 best energy efficiency with this Desalinating the large volumes of brackish water extracted ppm, according to researchers technology at lower TDS levels, with coal-bed methane would reduce the amount of salty at Sandia National Laboratowhereas reverse osmosis seems wastewater injected back into wells, such as this one in ries and the Colorado School of to work better at TDS levels of New Mexico’s Raton Basin. Mines. By comparison, seawa6000 ppm and higher,” he exter contains ~35,000 ppm TDS. Drilltaic cells,” says Jörg Drewes, an enviplains. However, many of the CBM ing companies dispose of these salty ronmental engineer at the Colorado wells are in the 3000-ppm range. waters via deep-well injection or by School of Mines. He presented preOther potential advantages that cadischarging them directly into surface liminary findings of bench-scale tests pacitive deionization could offer over waters; this practice—although leon brackish groundwater at the Wareverse osmosis include far less pregal—poses a significant environmenteReuse Association’s annual meettreatment of the water, a much lower tal hazard. However, researchers say ing in September that showed that tendency to foul, and because the systhat if the TDS concentrations could the technology can reduce TDS to the tem’s operation involves reversing the be removed in an economical fashion 1000-ppm level appropriate for irrigachange on the electrodes, the technolto levels of ~1000 ppm, then the water tion water and to the 500-ppm federogy creates energy that can be colcould be used for irrigation and rehaal drinking-water standard. lected. Hightower estimates an 80% bilitating overgrazed rangelands. Michael Hightower, a civil engienergy recovery. The lab experiments at Sandia and neer at Sandia, agrees. “Reverse osThe technology is still in the dethe Colorado School of Mines looked mosis gives you a very high quality velopment stage, and both Drewes at different types of capacitive dedrinking water with TDS levels on and Hightower caution that it’s not ionization systems. The researchers the order of 10–100 ppm, but that’s yet clear how practical or economical found that for brackish waters with not necessary for rangeland rehabilicapacitive deionization could be if it TDS concentrations of
