Getting the salt out

McGovern, a postdoc in MIT’s Department of Mechanical Engineering and lead author of the paper, says another advantage of the proposed system is “flexibility in the amount of salt we remove. We can produce any level of output salinity.” The costs of installing an electrodialysis system, he says, appear to compare favorably to other widely used systems for dealing with produced water.

It’s not clear at this point, McGovern says, what the optimal salinity is for fracking fluids. “The big question at the moment is what salinity you should reuse the water at,” he says. “We offer a way to be able to control that concentration.”

Filtration first
Before reaching the desalination stage, the researchers envision that chemical impurities in the water would be removed using conventional filtration. One remaining uncertainty is how well the membranes used for electrodialysis would hold up following exposure to water that contains traces of oil or gas. “We need some lab-based characterization of the response,” McGovern says.

If the system works as well as this analysis suggests, it could not only provide significant savings in the amount of fresh water that needs to be diverted from agriculture, drinking water, or other uses, but it would also significantly reduce the volume of contaminated water that would need to be disposed of from these drilling sites.

“If you can close the cycle,” Lienhard says, “you can reduce or eliminate the burden of the need for fresh water.” This could be especially significant in major oil-producing areas such as Texas, which is already experiencing water scarcity, he says.

While electrodialysis technology is available now, Lienhard explains that this application would require the development of some new equipment.

Jack Gilron, who studies desalination and water treatment at Ben Gurion University in Israel and who was not involved in this research, says the team has “created an impressive demonstration of their systematic approach to analysis of [electrodialysis] performance [in] the produced water system.” However, he adds, “The authors correctly point out that the issues of scaling and organic fouling must be answered to actually put [electrodialysis] into practice for this system.”

The work was supported by KFUPM through the Center for Clean Water and Clean Energy, and benefitted from a Hugh Hampton Young Memorial Fellowship and undergraduate student support from the MIT Energy Initiative.

— Read more in Ronan K. McGovern et al., “On the cost of electrodialysis for the desalination of high salinity feeds,” Applied Energy 136 (31 December 2014): 649-61 (DOI: 10.1016/j.apenergy.2014.09.050)

Reprinted with permission of MIT News