From: Tsegai Emmanuel (emmanuelt40@gmail.com)
Date: Mon Mar 22 2010 - 02:00:06 EST
Device to convert seawater offers hope to parched lands
by Richard Ingham Richard Ingham
Sun Mar 21, 10:39 pm ET
.PARIS (AFP) – Scientists said on Sunday they had made a nanotech
device to strip salt from seawater, paving the way to small-scale or
even battery-powered desalination for drought-hit regions and disaster
zones.
The tiny prototype is reported on the eve of the UN's World Water Day,
which aims to highlight the worsening problems of access to clean
water.
Conventional desalination works by forcing water through a membrane to
remove molecules of salt.
But this process is an energy-gobbler and the membrane is prone to
clogging, which means that de-sal plants are inevitably big,
expensive, fixed pieces of kit.
The new gadget has been given a proof-of-principle test by Jongyoon
Han and colleagues of the Department of Electrical Engineering and
Computer Science at Massachusetts Institute of Technology (MIT).
It works through so-called ion concentration polarisation, which
occurs when a current of charged ions is passed through an
ion-selective membrane.
The idea is to create a force that moves charged ions and particles in
the water away from the membrane.
When the water passes through the system, salt ions -- as well as
cells, viruses and micro-organisms -- get pushed to the side. This
saltier water is then drawn off, leaving only de-salted water to pass
through the main microchannel.
The tiny device had a recovery rate of 50 percent, meaning that half
of the water used at the start was desalinated. Ninety-nine percent of
the salt in this water was removed.
Energy efficiency was similar to or better than state-of-the-art
large-scale desalination plants.
"Rather than competing with larger desalination plants, the methods
could be used to make small- or medium-scale systems, with the
possibility of battery-powered operation," their paper, published by
the journal Nature Nanotechnology, suggests.
In an email to AFP, Han said the experiment entailed a tiny
microfluidic chip, just a few millimetres (fraction of an inch)
square, that desalted just 10 microlitres per minute.
"The idea toward the real-world application is that we would make many
of these devices, thousands or tens of thousands of them, on a plate,
and operate them in parallel, in the same way semiconductor
manufacturers are building many small electronic chips on a single
large wafer," explained Han.
"That would bring the flow rate up to around 100 millilitres (three
fluid ounces) per minute level, which is comparable to typical
household water purifiers and therefore useful in many applications."
A patent has been filed for the device. However, it may be a matter of
years before the invention reaches a commercial scale.
At such early days, the costs of the future system are unknown.
But, said Han, overheads may be significantly reduced because gravity
can be used to put the water through the device, as opposed to forced
it through by pumps, and there is less of a problem of membrane
fouling.
The theme of Monday's World Water Day is "Clean Water for a Healthy
World," touching on the growing problem of water contamination in
countries grappling with water stress and fast-rising populations.
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