Researchers say
the new desalination method could be useful for cleaning the
contaminated water
generated by hydraulic fracturing, or fracking.
Shown here is a
holding pit for fracking water.
MIT team invents efficient shockwave-based process for
desalination of water.
(November 12, 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.
The new approach is described in the journal Environmental Science and
Technology Letters, in a paper by professor of chemical engineering and
mathematics Martin Bazant, graduate student Sven Schlumpberger, undergraduate
Nancy Lu, and former postdoc Matthew Suss.
This approach is “a fundamentally new and different
separation system,” Bazant says. And unlike most other approaches to
desalination or water purification, he adds, this one performs a “membraneless
separation” of ions and particles.
Diagram of the new
process shows how a shockwave (red line) is generated in salty
water flowing
through a porous medium, with a voltage applied to membranes (green)
at each side of
the vessel. The shockwave pushed the salt ions off to one side of the flow,
leaving fresh
water at the other side, where it can be separated out.
Membranes in traditional desalination systems, such as those
that use reverse osmosis or electrodialysis, are “selective barriers,” Bazant
explains: 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,” he says.
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.