Oil and gas companies send millions of gallons of pressurized water—along with chemicals and sand—down wells to fracture shale. But the liquid, called flowback, returns to the surface filled with contaminants, making it unusable for other purposes—even fracturing new wells. Water-treatment technology is still in its "45rpm phonograph" stage says David Burnett, a Texas A&M petroleum engineer who oversees pilot studies of prototype systems for the Department of Energy. Of various methods now in development, membrane-filtration technology is especially energy efficient and site adaptable: Engineers can fine-tune a series of membranes to remove different substances from water as it passes through. Such on-site systems could reduce the need for freshwater, critical in arid regions of the U.S.