Taking out the Salt
The large amounts of water required in the hydraulic fracturing of shales for oil or gas is a pressing concern. In hydraulic fracturing, water is sent down underground at high pressures to fracture the rocks below, allowing gas or oil to be released and captured. When water returns to the surface it often contains large amounts of salt that were dissolved from the rocks down below. These large quantities of salt have made it difficult to re-use the water for further fracturing processes - making it difficult to reduce water use by adopting recycling strategies. Water produced from an operating oil or gas well, usually very salty after contacting underground rocks, can be cleaned of its salts and other contaminants using electrodialysis, and then reused to reduce the amount of freshwater needed. This diagram illustrates the process, with salty water in dark blue and fresh water in light blue. The electrodialysis process, using membranes and electric charges, is illustrated inside the circle.
Illustration: Jose-Luis Olivares/MIT Earlier this month, my co-authors, Adam Weiner, Lige Sun, Chester Chambers, Prof. Syed Zubair and Prof. John Lienhard and I published an article describing how electrodialysis can be used to remove salt from these waters to facilitate greater reuse. We were able to show that, compared to the evaporators are currently available to purify these waters, the system we proposed was of similar energy efficiency but more cost effective. We feel that electrodialysis, whereby salt is removed from water by means of an electrical current, is promising for this application - the next steps are to see how the system will fair in real world rather than lab conditions. The author's copy of the manuscript may be downloaded here. The article published in Applied Energy is available here and the MIT news office article appears here.