The hydraulic fracturing (fracking) industry doesn’t have a lot of positives to show for itself. While the technology has helped to wean the US from dependence on foreign oil, it is responsible for the low oil and natural gas prices that keep the American public addicted to gas guzzling vehicles and that continue short term fossil fuel competitiveness with renewable energy.
Besides the known side effects of earthquakes and water table contamination, one lesser known fact about fracking is the sheer volume of wastewater generation. How is so much wastewater produced?
Wastewater Handling: A Massive Expense and Environmental Liability
According to the US Energy Information Agency, for every new single well site, 3 to 6 million gallons of water are used used, per frack, for the rock fracturing process – water that includes added chemicals such as benzene, anti-corrosion fluids, and additives that kill microorganisms. And typical fracking operations may consist of 20 wells per “pad” and dozens of pads – so multiply 3 to 6 million gallons times 20, and then by, say, two dozen, for total water used for a standard operation. It’s no wonder that water handling constitutes one of the largest operating costs of the wells. And we are talking trucked-in water here, which means 400-600 tanker trips for the fracturing. For just one well.
After the water is injected to break up the rock, wastewater is brought back to the surface. It is highly concentrated brine with chemical residue, too concentrated for standard desalination using reverse osmosis. It must either be trucked away for disposal in treatment facilities, or more commonly, transported elsewhere for injection back underground into injection wells, away from water tables used for drinking wells. Wastewater transport can represent as much as 200-300 tanker trips.
New Research on Wastewater Mitigation
As economic and policy forces will likely enable fracking to continue into the near future, and because “successful” fracking techniques aren’t going to change, some technology teams have turned their sights to mitigation of fracking byproducts, in particular, the wastewater problem.
Concentrated Solar for Water Evaporation
Pittsburgh-based start-up Epiphany Water Solutions is trying a hybrid renewable approach using wastewater evaporation. They have developed a self contained, shippable module (it actually uses a shipping container) that can be deployed at the wellpad. The unit, called E5H, is a combination of concentrated solar power (CSP) dishes, natural gas (for electric power), and mechanical vapor recompression (MVR).
The unit uses the three 8-foot dishes of the CSP system and the natural gas-generated electricity to heat the wastewater to 1500 degrees F. The MVR recaptures the steam and compresses it to turn it into distilled water while also recapturing the thermal energy. The distilled water is sprayed into the air at the wellpad. The remaining concentrated salt slurry can be recycled or disposed of. The whole process drastically reduces the need for large volumes of high-saline wastewater to be transported away from the site.
Nanotube Membranes Increase Yield of Recovered Water
At University of California Riverside, a team of researchers has developed a means to recover almost 100% of the salt solutions from highly concentrated brines such as those generated by the fracking industry. It is also hoped the technology will help alleviate water shortages in arid regions.
One way to treat brine is using membrane distillation, a thermal desalination technology in which heat drives water vapor across a membrane, allowing further water recovery while the salt stays behind. However, hot brines are highly corrosive, making the heat exchangers and other system elements expensive in traditional membrane distillation systems. Furthermore, because the process relies on the heat capacity of water, single pass recoveries are quite low (less than 10 percent), leading to complicated heat management requirements.
David Jassby, an assistant professor of chemical and environmental engineering in UCR’s Bourns College of Engineering, led the project. “In an ideal scenario, thermal desalination would allow the recovery of all the water from brine, leaving behind a tiny amount of a solid, crystalline salt that could be used or disposed of,” Jassby said. “Unfortunately, current membrane distillation processes rely on a constant feed of hot brine over the membrane, which limits water recovery across the membrane to about 6 percent.”
To improve on this, the team developed a self-heating carbon nanotube-based membrane that only heats the brine at the membrane surface. The new system reduced the heat needed in the process and increased the yield of recovered water to close to 100 percent.
In addition to the significantly improved desalination performance, the team also investigated how the application of alternating currents to the membrane heating element could prevent degradation of the carbon nanotubes in the saline environment. Specifically, a threshold frequency was identified where electrochemical oxidation of the nanotubes was prevented, allowing the nanotube films to be operated for significant lengths of time with no reduction in performance. The insights provided by this work will allow carbon nanotube-based heating elements to be used in other applications where electrochemical stability of the nanotubes is a concern.