Global Challenges/Chemistry Solutions
Water Desalination: Freshwater from the Sea
Combating disease . . . providing clean water and safe food . . . developing new sources of energy . . . confronting climate change. Hello, from Washington, DC, this is “Global
Challenges,” a special podcast from the American Chemical Society — whose 160,000
members make up the world’s largest scientific society. Today’s headlines are a drumbeat of dilemmas that affect the everyday lives of people everywhere. “Global Challenges” takes you behind those headlines for eye-opening glimpses of how chemistry
is responding to those challenges — improving and sometimes saving people’s lives.
You’ll hear the stories and meet the scientists whose discoveries are helping to make life
longer, healthier, and happier for millions of people. Today’s global challenge in this ongoing saga of chemistry for life: Turning the oceans into a drought-proof source of freshwater.
―Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.
Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink.‖
Samuel Taylor Coleridge penned the “Rime of the Ancient Mariner” almost 210 years
ago. At one point, parched by thirst, the mariner muses on a dilemma that ranks among today’s great scientific challenges. Water is everywhere, covering more than 70 percent of Earth’s surface. And yet at least 1.2 billion people face shortages of water to drink. Water to irrigate farm fields and orchards. HO for the mills and factories that produce 2
food, medicine, computers, cars, clothing, and thousands of other products.
Our great dilemma — and science’s challenge — involves that “nor any drop to
drink.” About 97 percent of Earth’s water is salt water in the oceans and brackish water on land. People cannot drink saline water, nor can it quench the thirst of agriculture or industry. However, a technology called desalination — literally de-salting — promises
to further tap this enormous resource as a source of freshwater.
In June, the Global Innovation Imperatives project convened experts on water in New Delhi, India. Dr. Mark Shannon, of the University of Illinois at Urbana-Champagne, was among speakers in this collaboration between the American Chemical Society Committee on Corporation Associates and the Society of Chemical Industry. It’s a
collaboration that seeks science-based solutions for global health, environmental and societal problems.
―So the world‘s clean water supply is in crisis now – and
its only getting worse… If one could desalinate that water
affordably, effectively, and in an environmentally sound
way, then much of the world‘s problems with water
A Dash of Salt
So what is salt, or saline, water? That’s water with high amounts, or concentrations, of
dissolved salts. One is sodium chloride — common table salt. Saline water also has salts
made from calcium, magnesium, and other chemical elements. Seawater is unfit for human consumption because it contains too much salt. About 3.5 per cent of its weight is salt. Drinking water normally quenches your thirst. But drinking salt water will make you thirstier. The salt draws water out of cells in your body by a process called osmosis. We will hear more about osmosis a little later. That water winds up in your urine. Drink enough salt water, and you’re dead from dehydration.
Water desalination expert Dr. James Birkett told Global Challenges that desalting is a very ancient technology:
―Desalination probably was recognized prior to Aristotle,
but that‘s first reliable, recorded piece of history. Aristotle,
probably among others, noted that if you took salt water
and evaporated it, that the vapors thereof were pure, and
when condensed, they would produce pure water.‖
Two ancient desalination methods — distillation and what might be described as
“filtering” — remain our mainstays today. More than 13,000 water desalination plants now operate worldwide. They produce at least 12 billion gallons of freshwater each day.
About 80 per cent of that water comes from distillation. In modern distillation, we no longer hang sponges over pots of boiling seawater, catch the steam, and wring fresh water out of the sponges. We use high-tech processes, like the multi-stage flash distillation at the Jebel Ali Desalination Plant. Located in the United Arab Emirates, Jebel Ali is the world’s largest desalination facility. In flash distillation, seawater heats in
chambers where it evaporates or flashes into steam at about 160? Fahrenheit, rather than 212? F. The remaining water flows to the next stage where it flashes again, and to the next stage and the next. That approach saves energy. The freshwater condensate from each stage goes into storage tanks.
Water’s Salt-Free Diet
And methods for filtering salts out of saline water have come a long way from the ineffective wool and cloth filters used in ancient times. Most modern desalination plants in this genre use high tech membranes in a process called reverse osmosis. Chemists and chemical engineers who developed reverse osmosis borrowed the idea from Mother Nature.
Osmosis is a natural process. The body uses it to keep cells in liquid chemical balance. In osmosis, water molecules flow from less concentrated salty solutions to areas that are more concentrated. Osmosis would dehydrate anyone drinking seawater, with water flowing out of cells and into the urine.
In reverse osmosis, water molecules move out of a more highly concentrated salt solution — that’s seawater — so they can be collected as freshwater. This process filters water at the molecular level, by forcing it through high-tech membranes. The largest
water desalination plant in North America uses reverse osmosis. That’s the Tampa Bay
Seawater Desalination Plant in Florida. That facility can desalt up to 25 million gallons of water per day, providing drinking water for 2.4 million residents of the Tampa Bay area. Salty Problems
Both modern methods — flash distillation and reverse osmosis — have serious
drawbacks that limit the use of desalination. Here is Dr. Julie Beth Zimmerman, of Yale University. Zimmerman was a guest editor of a special June edition of Environmental
Science & Technology devoted to global water problems. It is one of the American
Chemical Society’s 36 peer-reviewed scientific journals.
―The big challenges with it are the energy demands, which
obviously have their own environmental impacts in terms of
using fossil fuel-based energy and CO emissions. The 2
other big challenge has to do with the concentrated salt-
water brine that you get as a result of desalination. On top
of that — because so much energy is required — there is
the economic cost. Unfortunately, the people who need the
water the most live in poor, developing countries where this
technology is not feasible for those reasons.‖
First, the dollars and cents that Zimmerman mentioned. Desalination is the most expensive kind of water supply. It takes a lot of energy to heat water for distillation. In the United Arab Emirates, it costs more to produce a gallon of freshwater than a gallon of crude oil. The pumps at reverse osmosis plants slurp up electricity to push seawater through those filters. At Tampa Bay, for instance, it takes a pressure of 1,000 pounds per square inch to push salt water through reverse osmosis filters. Your car tires have about 30 pounds of pressure.
As a result, it may cost $1 to produce 1,000 gallons of freshwater with desalination. Drinking water produced from lakes and rivers sells for less than 30 cents per 1,000 gallons. Water for irrigation costs less than 5 cents per 1,000 gallons.
The environmental concerns about desalination stem from the leftovers of distillation and reverse osmosis. Both produce large volumes of brine — concentrated
salt water that can endanger fish and other marine life. Disposing of brine is a big problem, especially for inland desalination facilities. They currently have no cost-effective, environmentally sustainable way to get rid of brine.
A Matter of Membranes
Some of the solutions are a matter of membranes — of making it easier for water
molecules to pass through reverse osmosis membranes. Getting water to pass at lower pressures means less energy and lower costs. Those costs can be lowered further through renewable sources of energy for electricity, such as wind turbines that partially power the Perth, Australia, desalination plant.
Dow Water Solutions produces the membranes that are currently in use at Perth, Tampa Bay and many other large desalination facilities. These thin-film polyamide FILMTEC? Reverse Osmosis (RO) membranes represent a major improvement over the cellulose acetate membranes used in the 1950s and 1960s. Polyamide membranes are more permeable. They need about 50 per cent less pressure and use up to 40 percent less
energy. More advances are occurring in the next-generation membranes, including larger-diameter membrane modules to drive down the cost of producing potable water from seawater. Dr. David Klanecky, global director of research & development for Dow Water Solutions, told Global Challenges that while these technological leaps are important, they’re just one element to making desalination a viable solution:
―Dow believes that the world‘s water problems can be
solved through technology, sound management and pricing
policies, cooperation of the public and private sectors, and
education. The company‘s commitment to deploy science
and technology to provide pure water for both industrial
applications and human consumption, globally, is part of
its 2015 Sustainability Goals.‖
While most agree that desalination is only one solution for helping to bring fresh, potable water to whose who need it, innovation continues. We need membranes that are more permeable and let water pass with even lower pressures and reduced energy consumption. We need membranes that are stronger and more durable, less prone to getting clogged with minerals and requiring less frequent replacement.
Big Challenges/Small Solutions
Some desalination researchers are responding to those challenges in a small way with huge implications for providing a drought-proof source of water. That response involves nanotechnology — technology built from atoms and single molecules where dimensions are measured in nanometers, or billionths of a meter. One nanometer is about 50,000 times smaller than the width of a human hair.
The next generation reverse osmosis membranes may be nanocomposite membranes. These polymers contain small particles that produce major improvements in the polymer’s ability to transport and separate salt water. Nanocomposites may be the key to solving another problem with existing membranes. That is the tendency to foul, or clog up, with salts so that more pressure is needed to force seawater through the membrane. Here is Dr. Benny D. Freeman, of the University of Texas at Austin, who reported on nanocomposites this year in ACS’s peer-reviewed journal, Macromolecules:
―Fouling is recognized to be one of the most ubiquitous
and highest priority problems facing membranes in
virtually any water purification application that is
currently practiced or envisioned. Membrane fouling is the
process by which small particulates and other components
in water that is being treated deposit on the surface of
membranes. They act to impede the flow of water through
the membranes. As a result, the amount of membranes that
are needed to process a given amount of water is increased.
One of the examples of fouling is the flux, or amount of
water coming through a unit surface area per unit time of a
membrane such as an ultra filtration membrane, can
decrease more than a thousand fold as a membrane
becomes fouled with particulate matter (or proteins or
emulsified oil droplets, depending on the application).
Researchers are discovering that another product of nanoscience — carbon nanotubes —
show great promise as a new filter membrane. These hollow tubes of pure carbon have an inner channel or pore 100,000 times smaller than a human hair. Water can pass through, but not salt. And water molecules move with amazing speed through these tubes, which have been called “water wires.” In a study in ACS’ Journal of Physical Chemistry
B, Dr. Ben Corry and colleagues at the University of Western Australia, reported on the potential of these “water wires” in nanotube membranes that require less pressure and
―Indeed, a membrane made from nanotubes could be
expected to obtain 95 percent desalination with a flow rate
over 1500 times that of existing membranes. Lower
pressures could be used in carbon nanotube membranes,
provided they overcome the osmotic pressure difference
between seawater and freshwater.‖
Scientists are taking one step forward toward that challenge with a new desalination process called . . . well . . . forward osmosis. Forward osmosis saves energy and money because it does not need the high pressures for reverse osmosis. Rather than pushing water through membranes with pumps, forward osmosis draws it though with the chemical energy of osmotic pressure.
―Its all about chemistry – in forward osmosis desalination,
the main principle is the use of ammonia and CO that 2
dissolves in water. They form ammonium salts, ionic
species, which create such huge osmotic pressure that it
can drive the water through the membrane.‖
That was Dr. Menachem Elimelech, who leads of a Yale University team working on a forward osmosis pilot plant that has reported at ACS National Meetings. Imagine a membrane with seawater on one side and a highly concentrated solution of dissolved ammonia and carbon dioxide gases on the other. The ammonia and CO create osmotic 2
pressure that draws the water on the other side through the membrane. Freshwater can then be recovered from the draw solution by heating it to about 136 ?F so that ammonia and carbon dioxide bubble out. They are captured and recycled to the next batch. Greener Desalination
In addition to offering potential savings on energy, forward osmosis produces less brine than existing desalination processes. Desalination brine is about 1.5 times as salty as seawater, and has raised environmental concerns. The challenge to chemists and other scientists: Forward osmosis requires membranes that are very thin, very porous, and resistant to highly alkaline water.
Scientists are pursuing many other solutions to the energy and environmental drawbacks of desalination. Solar energy, for instance, could substitute for fossil fuel. And the environmental impact of returning brine to the ocean can be reduced by planning and smart desolation plant placement. One approach: Locating desalination plants near wastewater treatment or electric power plants so that brine can be diluted in the outfall streams from those plants before entering the ocean.
Meeting the Demand
These solutions will be more crucial in the future as water shortages intensify demand for desalinated water. Dr. Wolfram Kloppmann, of French Geological Survey BRGM, described future demand for desalination in a paper in June in Environmental Science &
Technology on fingerprinting of desalination derived freshwater in the environment.
―Over the next decade, we will probably double
desalination capacity worldwide, with the most steep
increase in the Mediterranean area and in the Middle East
with a growth rate of more than one hundred percent in the
The challenge of meeting that demand is fostering new ways of thinking and encouraging new approaches. Dr. Mark Shannon and his colleagues have presented some of those at ACS National meetings, and Shannon emphasized it in an address at the U. S. Department of State in June and to Global Challenges:
―We need to innovate. We need to change the paradigm.
We can‘t just keep on doing what we‘re doing. I even
invoked Einstein, ‗Doing the same thing again and again
and expecting a different outcome is the definition of
insanity.‘ We do the same thing over and over again to try
and solve the world‘s problems – and its not getting better.
Let‘s think of something else.‖
Yes, let’s think of something else. Let’s think of these and other 2008 research advances stin chemistry paying off in the years ahead. Paying off in ways that foster a 21 Century
rewrite of Samuel Taylor Coleridge’s script. Water….water everywhere…and every drop to drink. Please join us at the American Chemical Society for the next chapter in this ongoing saga of chemistry for life. In our next special Global Challenges podcast, we’ll examine solutions to global warming. Today’s podcast was written and edited by
Michael Woods. I’m Adam Dylewski at the American Chemical Society in Washington.