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Will Salt Water Quench the World’s Thirst?

Fresh water shortages are likely to plague two-thirds of the world's population in the next decade and processes like desalination are still very costly. Luckily, a new material called graphene may be able to help.  

By
Sabrina Stierwalt, PhD,
Episode #189

Friday, April 22nd is Earth Day, so in honor of Mother Earth, let’s discuss one of the biggest issues facing our home planet—fresh water shortages—and how a material called graphene may be able to help.

Our global thirst for fresh water is rising by an estimated 170 billion gallons each year. By 2025, two-thirds of the world’s population are predicted to have problems accessing fresh water, a problem being exacerbated by global warming and continued population growth. The regions hardest hit are expected to be the Middle East, northern China, northern India, and the western United States.

Already, an estimated 2.7 billion people have trouble accessing fresh water for at least one month out of the year. Countries like Yemen and Syria are already facing civil unrest due in part to a lack of access to fresh water. In 2014, Saudi Arabia began farming more water intensive crops like hay on land purchased in Arizona to avoid tapping their own dwindling water supply.

But fresh water makes up only 3% of the Earth’s water supply (and roughly 2/3 of that is trapped in glaciers). What about the salt water that makes up the other 97%? Historically, removing salt from salt water, a process called desalination, has been too expensive to be a reliable method for meeting our rising fresh water needs. However, scientists and engineers may have found a new way to use graphene to make the process easier and more affordable.

Desalination

If you’re thirsty (and desperate), drinking salt water may seem like a good idea, but it actually only serves to dehydrate you further. When you eat (or drink) salt, your body will attempt to dilute it by drawing moisture from your cells—a process called osmosis that ends in a net loss of water. Left unchecked, this dehydration can eventually cause kidney failure and even brain damage.

So how can we remove salt from water in order to make it drinkable? Solar stills have been used by the U.S. Navy as far back as the 1700s, and in fact, you can easily create your own. Place an empty glass inside a larger bowl of salt water outside in the Sun and cover the entire thing with saran wrap with a tiny hole poked in the top.  As the sunlight evaporates the salt water, the salt will stay encrusted on the bowl while the water begins to form condensation on the plastic wrap. That condensation will eventually drip into the empty glass providing fresh water. Drink up!

But if the process is that easy for an individual with the time to wait, why can’t we use desalination on a large, more time-efficient scale? Most desalination plants employ a process called reverse osmosis. Basically, a large amount of pressure is applied to salt water to force it through a semi-permeable membrane, a sort of filter with holes so tiny that water molecules can pass through it but larger salt molecules cannot. The thickness of these filters, called thin-film composite membranes, requires that the water reach very high pressures. Thus, a large input of energy to run the fairly powerful pumps to do it, and so producing about 325,000 gallons of fresh water via desalination (about the amount used by a family of five in a year) costs about $2,000.

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About the Author

Sabrina Stierwalt, PhD

Dr. Sabrina Stierwalt is an extragalactic astrophysicist at the California Institute of Technology and Adjunct Faculty at the University of Virginia.

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