A by-product of the fracking process is millions of gallons of water that’s much saltier than seawater, following the leaching of salts from rocks deep below the surface.
Now researchers at MIT and in Saudi Arabia say they have found an economical solution for removing the salt from this water. The method they propose for treating the “produced water” that flows from oil and gas wells throughout their operation is one that has been known for decades, but had not been considered a viable candidate for extremely high-salinity water, such as that produced from oil and gas wells.
The technology - electrodialysis - is generally thought of as being advantageous for relatively low-salinity water, generally with salinity around one-tenth that of seawater. But electrodialysis also turns out to be economically viable at the other end of the salinity spectrum, the new analysis shows.
Produced water from fossil-fuel wells can have salinity three to six times greater than that of seawater; the new research indicates that this salt can be effectively removed through a succession of stages of electrodialysis.
The idea would not be to purify the water sufficiently to make it potable, the researchers say. Rather, it could be cleaned up enough to enable its reuse as part of the hydraulic fracturing fluid injected in subsequent wells, significantly reducing the water needed from other sources
Before reaching the desalination stage, chemical impurities in the water could be removed using conventional filtration. One uncertainty, however, is how well the membranes used for electrodialysis would hold up following exposure to water that contains traces of oil or gas.
The researchers say that if the system works as well as their 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.