Over
one-third of the world’s population already lives in areas struggling
to keep up with the demand for fresh water. By 2025, that number will
nearly double. Some countries have met the challenge by tapping into
natural sources of fresh water, but as many examples?such as the
much-depleted Jordan River?have demonstrated, many of these practices
are far from sustainable.
A
new Yale University study argues that seawater desalination should play
an important role in helping combat worldwide fresh water shortages
once conservation, reuse and other methods have been exhausted. The
study also provides insights into how desalination technology can be
made more affordable and energy efficient.
“The
globe’s oceans are a virtually inexhaustible source of water, but the
process of removing its salt is expensive and energy intensive,” said
Menachem Elimelech, a professor of chemical and environmental
engineering at Yale and lead author of the study, which appears in the
Aug. 5 issue of the journal Science.
Reverse
osmosis?forcing seawater through a membrane that filters out the
salt?is the leading method for seawater desalination in the world today.
For years, scientists have focused on increasing the membrane’s water
flux using novel materials, such as carbon nanotubes, to reduce the
amount of energy required to push water through it.
In
the new study, Elimelech and William Phillip, now at the University of
Notre Dame, demonstrate that reverse osmosis requires a minimum amount
of energy that cannot be overcome, and that current technology is
already starting to approach that limit. Instead of higher water flux
membranes, Elimelech and Phillip suggest that the real gains in
efficiency can be made during the pre- and post-treatment stages of
desalination.
Seawater
contains naturally occurring organic and particulate matter that must
be filtered out before it passes through the membrane that removes the
salt. Chemical agents are added to the water to clean it and help
coagulate this matter for easier removal during a pre-treatment stage.
But if a membrane didn’t build up organic matter on its surface, most if
not all pre-treatment could be avoided, according to the scientist’s
findings.
In
addition, Elimelech and Phillip calculate that a membrane capable of
filtering out boron and chloride would result in substantial energy and
cost savings. Seventy percent of the world’s water is used in
agriculture, but water containing even low levels of boron and
chloride?minerals that naturally occur in seawater?cannot be used for
these purposes. Instead of removing them during a separate
post-treatment stage, the scientists believe a membrane could be
developed that would filter them more efficiently at the same time as
the salt is removed.
Elimelech
cautions that desalination should only be considered a last resort in
the effort to provide fresh water to the world’s populations and
suggests that long-term research is needed to determine the impact of
seawater desalination on the aquatic environment, but believes that
desalination has a major role to play now and in the future.
“All
of this will require new materials and new chemistry, but we believe
this is where we should focus our efforts going forward,” Elimelech
said. “The problem of water shortage is only going to get worse, and we
need to be ready to meet the challenge with improved, sustainable
technology.”
