BLOG: Shock electrodialysis could be the next step in desalination technology

With an increase to the demand for global freshwater projected to be 64 billion cubic metres per year,[1] finding access to freshwater has become more and more of a challenge. Now, a potentially ground-breaking advancement in desalination (salt removal), called shock electrodialysis, could make desalinating seawater and brackish water into clean, potable water much easier.

Currently, the main way for salt water to be desalinated is through reverse osmosis (RO). RO desalinates saltwater by pushing it through a membrane (that only allows water to flow through) at high pressure and thus separating the water from the dissolved salt. This pressurisation requires a substantial amount of energy, however.[2] Furthermore, the membranes must be replaced as they get clogged with salt. These factors contribute to making RO practically accessible only to public water companies and large industries (due to its high cost).[2]

To meet the increasing demand for freshwater, cheaper and simpler alternative solutions must be found. This is where shock electrodialysis comes in. Discovered by Massachusetts Institute of Technology (MIT), shock electrodialysis does not use a membrane to separate the salt from the water. Instead, it uses electric current to generate a shockwave within the water to push the salt ions to one side of the stream, where it can be separated by a physical barrier:[3] see Figure 1.

Figure 1: The shock electrodialysis process. The cation exchange membranes (CEM) act as electrodes and the shockwave (red line) pushes the salt ions (the blue and yellow lines) to one side of the saltwater stream, before being separated by a physical barrier.

Since the saltwater does not flow through the membrane (as in RO) but rather across it, the membrane itself is not prone to being clogged by filtered materials or damaged by high water pressure.[3] Thus, one of the costly components in traditional desalination techniques is eliminated.

A similar technology that is currently being used to some extent is standard electrodialysis (ED). This technology, invented in the 1950s,[4] works by using electricity to separate the ions from saltwater by attracting negative ions to the anode and positive ions to the cathode of the system, thereby producing a separated freshwater and brine stream.[5] However, this process also uses membranes to prevent the ions from re-entering the freshwater stream. These membranes are also susceptible to the accumulation of salts and other organic material. Additionally, this process is unsuitable for producing drinking water from seawater since it is not cost effective enough and cannot filter out pathogens and other biological contaminants.[6]

Shock electrodialysis overcomes these problems by using a shockwave — generated by an electric current — to push the salt ions to one side of the stream.[3] The electric current allows for much faster ion transport compared to diffusion in ED, due to the presence of surface charges on the cation exchange membranes (CEMs) that are present in the shock electrodialysis process.[7] In addition, shock electrodialysis performs other functions, such as filtration and disinfection, both of which are essential in drinking water production.[8][9] Results published in 2015 found that the shock electrodialysis process was capable of eliminating approximately 99% of Escherichia coli bacteria that were present in the saltwater input.[7]

In addition to producing drinking water, shock electrodialysis could also have potential in industrial applications, such as purification of waste water in hydraulic fracturing.[10] Hydraulic fracturing (or fracking) is a method of extracting natural gas by fracturing oil shale, which is carried out by pumping freshwater deep down into wells at high pressures. When the water is extracted along with the natural gas, the water is extremely salty (up to 192000 parts per million)[10] due to dissolved minerals. This saltwater cannot currently be reused for fracking but, with the help of shock electrodialysis, the water could be desalinated and therefore reusable. This would not only lower the cost of fracking, but also reduce the amount of freshwater that is used and waste water that is produced.

Unfortunately, the technology is still in early research stages, but does show very high potential in the field of desalination. Currently only small models exist but scalable prototypes of “continuous desalination and water purification”[7] are being developed.

In the future, shock electrodialysis could potentially replace many existing methods of desalination — such as reverse osmosis and standard electrodialysis — in the production of freshwater for both domestic and industrial use. Shock electrodialysis simplifies the pre-treatment that is required and eliminates post-desalination disinfection, thereby cutting costs. This will allow smaller companies and communities to access this technology more easily.

References

  1. New report highlights crucial role of water in development, UNESCO. Accessed on 26th November 2017.
  2. Desalination by reverse osmosis., Organisation of American States. Accessed on 26th November 2017.
  3. D. Chandler, Shocking new way to get the salt out., Massachusetts Institute of Technology, Massachusetts. Accessed 21 October 2012.
  4. W. Juda and W. A. McRae, Coherent ion-exchange gels and membranes, Journal of the American Chemical Society. 72, p. 1044, 1950.
  5. How does electrodialysis (EDR) work, SUEZ - Water Technologies & Solutions. Accessed 26th November 2017.
  6. F. Valero, A. Barceló and R. Arbós, Electrodialysis Technology. Theory and Applications, Aigues Ter Llobregat (ATLL). Accessed 26th November 2017.
  7. D. Deng, W. Aouad, W. A. Braff, S. Schlumpberger, M. E. Suss and M. Z. Bazant, Water purification by shock electrodialysis: Deionization, filtration, separation, and disinfection, Desalination. 357, pp. 77–83" url = "http://www.sciencedirect.com/science/article/pii/S001191641400602X#bbb0020, 2015.
  8. B. Sauvet-Goichon, Ashkelon desalination plant — a successful challenge, Desalination. 203, pp. 75–81, 2007.
  9. L. F. Greenlee, D. F. Lawler, B. D. Freeman, B. Marrot and P. Moulin, Reverse osmosis desalination: Water sources, technology, and today's challenges, Water Research. 43, pp. 2317–2348, 2009.
  10. D. Szondy, Electrodialysis identified as potential way to remove salt from fracking waste water, New Atlas, 27 October 2014.

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