<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content="text/html; charset=iso-8859-1" http-equiv=Content-Type>
<META name=GENERATOR content="MSHTML 8.00.6001.18928">
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff><FONT size=2 face=Arial>
<DIV class=article-heading>A new, more energy-efficient seawater distillation
membrane yields more potable water, and less briny discharge.</DIV>
<DIV class=article-heading><FONT size=4>New desalination technique yields more
drinkable water</FONT></DIV></HGROUP>
<DIV><SPAN class=vcard><A class=fn
href="http://www.nature.com/news/new-desalination-technique-yields-more-drinkable-water-1.10702?WT.ec_id=NEWS-20120529#auth-1">Larry
Greenemeier</A></SPAN></DIV>
<DIV class=pubdate-and-corrections><TIME datetime="2012-05-23" pubdate>23 May
2012</DIV>
<DIV class=pubdate-and-corrections><A
href="http://www.nature.com/news/new-desalination-technique-yields-more-drinkable-water-1.10702?WT.ec_id=NEWS-20120529">http://www.nature.com/news/new-desalination-technique-yields-more-drinkable-water-1.10702?WT.ec_id=NEWS-20120529</A></TIME></DIV></HEADER><SECTION>
<DIV class=section>
<DIV class=article-tools> </DIV>
<DIV class=article-tools>More than a third of the world already suffers from
shortages of potable water—with a rise to 50 percent expected by 2025.
Desalination of seawater can help coastal communities can address local
shortfalls, although the process is costly, and releasing leftover brine back to
the ocean has environmental implications. Now a new system promises to produce
more drinkable water with less salty effluent.</DIV></DIV>
<DIV class=section>
<P class="content no-heading cleared main_content">Kamalesh Sirkar, a New Jersey
Institute of Technology (N.J.I.T.) distinguished professor of chemical
engineering, says he has devised<STRONG> a direct-contact membrane distillation
(DCMD) system</STRONG> that can efficiently wring drinking water out of up to 20
percent-salt-concentrated brine. (After about 25 percent, salt precipitates out
of the solution in the membrane distillation system and could damage the
membranes, pumps, lines and other components, Sirkar says.)</P>
<P class="content no-heading cleared main_content">Normal seawater has a salt
concentration of about 3.5 percent, which means the new system can reprocess the
same seawater several times. "More water can be recovered with less residue,"
Sirkar says.</P>
<P class="content no-heading cleared main_content">In Sirkar's system, heated
seawater flows across a membrane strung with a series of hollow tubes made of a
porous, yet hydrophobic, fiber—meaning only water vapor can be osmotically
transferred. Cold distillate water runs through each of the tubes in a direction
perpendicular to that of the seawater. The temperature difference between the
heated seawater and cold distillate water causes vapor to form on the tubes.
This vapor diffuses through the pores and condenses again inside the tubes,
joining the flow of cold distillate water. The salt cannot penetrate the tubes
and is carried away; with each cycle, more fresh water is drawn off, leaving
more highly concentrated brine behind.</P>
<DIV class="related-stories-box box"><U>Sirkar's recently patented system can
deliver about 80 liters of drinking water per 100 liters of seawater, he says. A
comparable reverse-osmosis system—which relies on pressure to force seawater
through a salt-filtering membrane—would reclaim 41 liters from that same amount
of saltwater, according to Sirkar.</U></DIV>
<P class="content no-heading cleared main_content">Membrane distillation's
advantages include its ability to produce drinking water with very low salinity.
In addition, seawater can be distilled at a range of temperatures—from 30 to 100
degrees Celsius—reducing the amount of heat typically needed for desalination,
an energy savings, Sirkar says. Prolonged use may decrease a typical membrane's
efficiency, but Sirkar says his system adds an ultrathin layer of a highly
porous silicone–fluoropolymer coating to extend membrane lifetime.
Fluoropolymer—a polymer that contains fluorine atoms—has a high resistance to
the solvents, acids and bases found in ocean water. As for the environmental
impact of desalination, Sirkar says dumping concentrated brine back into the sea
creates a "minimal" disturbance to sea life. He adds, "Seawater is a very large
volume with enough turbulence to dilute [the brine] very quickly."</P>
<P class="content no-heading cleared main_content">That's not to say membrane
distillation is without problems. It requires a steady, inexpensive source of
heat to prevent the temperatures of the water on either side of the membrane
from equalizing, which would impede the vaporization/condensation process. For
DCMD to be practical it needs to be easier to use, more cost-effective and able
to take advantage of available heat sources, including waste heat produced by
places such as shore-based factories and offshore drilling operations, Sirkar
says.</P>
<P class="content no-heading cleared main_content">Although membranes are
getting better, reverse osmosis is more common and has been used in desalination
plants since the late 1960s. Recent improvements in reverse-osmosis
technology—including more efficient membranes made from carbon nanotubes and
energy-recovery devices that boost output while cutting energy consumption and
costs—have made it a feasible option for even small communities such as the Sand
City, Calif., on the Monterey Peninsula, with a population of less than 350.</P>
<P class="content no-heading cleared main_content">Whichever technology is used
for desalination, the price tag remains a wild card, dependent on the cost of
energy necessary to build and maintain the facility, run the process, and
transport seawater in and desalinated water out. A recent study by the WateReuse
Association indicates that costs for seawater desalination projects vary widely
from about $2 to $12 per 3,785 liters. Smaller capacity units, which produce
less than 3.8 million liters daily, are at the higher end of that cost range, in
part because they cannot leverage the same economies of scale as larger
facilities.</P>
<P class="content no-heading cleared main_content">As a result, desalination
does not figure prominently in the U.S. Environmental Protection Agency's
National Water Program 2012 Strategy: Response to Climate Change report,
released in March as a draft for public comment. (pdf) The report notes that
"desalination is energy intensive and there may be risks and costs associated
with disposing of waste brines from the treatment." Still, the agency does
acknowledge that rising sea levels over time may increase saltwater intrusion on
coastal freshwater aquifers and notes that desalination is one way to ensure
those aquifers remain usable.</P></DIV></FONT><BR>
<BR>
__________ Información de ESET NOD32 Antivirus, versión de la base de firmas de virus 7179 (20120529) __________<BR>
<BR>
ESET NOD32 Antivirus ha comprobado este mensaje.<BR>
<BR>
<A HREF="http://www.eset.com">http://www.eset.com</A><BR>
</BODY></HTML>