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Published on Nov 13, 2015
As the availability of clean, potable water becomes an increasingly urgent issue in many parts of the world, researchers are searching for new ways to treat salty, brackish or contaminated water to make it usable. Now a team at MIT has come up with an innovative approach that, unlike most traditional desalination systems, does not separate ions or water molecules with filters, which can become clogged, or boiling, which consumes great amounts of energy.
Instead, the system uses an electrically driven shockwave within a stream of flowing water, which pushes salty water to one side of the flow and fresh water to the other, allowing easy separation of the two streams.
According to the researchers, this approach is a fundamentally new and different separation system. Unlike most other approaches to desalination or water purification, this one performs a “membraneless separation” of ions and particles.
Membranes in traditional desalination systems, such as those that use reverse osmosis or electrodialysis, are “selective barriers”.
They allow molecules of water to pass through, but block the larger sodium and chlorine atoms of salt. Compared to conventional electrodialysis, “This process looks similar, but it’s fundamentally different,”
In the new process, called shock electrodialysis, water flows through a porous material —in this case, made of tiny glass particles, called a frit — with membranes or electrodes sandwiching the porous material on each side. When an electric current flows through the system, the salty water divides into regions where the salt concentration is either depleted or enriched. When that current is increased to a certain point, it generates a shockwave between these two zones, sharply dividing the streams and allowing the fresh and salty regions to be separated by a simple physical barrier at the center of the flow.
Even though the system can use membranes on each side of the porous material, the water flows across those membranes, not through them. That means they are not as vulnerable to fouling — a buildup of filtered material — or to degradation due to water pressure, as happens with conventional membrane-based desalination, including conventional electrodialysis.
The underlying phenomenon of generating a shockwave of salt concentration was discovered a few years ago by the Stanford University.
But that finding, which involved experiments with a tiny microfluidic device and no flowing water, was not used to remove salt from the water.
The new system, by contrast, is a continuous process, using water flowing through cheap porous media, that should be relatively easy to scale up for desalination or water purification.
One possible application would be in cleaning the vast amounts of wastewater generated by hydraulic fracturing, or fracking