Ever Forward

With a little bit of waste energy, a technology called forward osmosis could make salty water drinkable

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Meny Elimelech: In principle, if energy cost nothing, then you can have unlimited supply of water. You can have seawater desalination endlessly.

[sounds of the harbor, water, boats]

Lisa Raffensperger: As the winds pick up over Boston Harbor, engineer Rob McGinnis stands on the dock. He sweeps his arm toward the bustling waterfront.

Rob McGinnis: …container ships loading up not far from here, cruise ships coming in. We have power plants which use the water for cooling purposes.

Lisa Raffensperger: But for all the things this water is good for—

Rob McGinnis: It’s not good for drinking.

Lisa Raffensperger: But, as chief technology officer of a water company, McGinnis will tell you, that’s not strictly true. Desalination makes seawater drinkable for millions of people every day. But existing technologies consume energy—mostly fossil fuels—that could be used for other things. Not so for the desalination technology McGinnis developed as a Ph.D. student at Yale, in the lab of Meny Elimelech. The revolutionary part of it is the energy source.

Rob McGinnis: We can take this very, very low temperature energy, the kind of temperature that you would find in, say, a hot bath, say, 40 degrees Celsius. This is the temperature at which power plants often will reject heat to the atmosphere. We can use that energy source to drive a desalination process.

Lisa Raffensperger: The process uses “forward” osmosis—so called to distinguish it from reverse osmosis, the primary desalination technology today. Existing reverse osmosis requires energy to push water through a membrane, against its natural flow. Forward osmosis, on the other hand, doesn’t apply pressure. Seawater goes on one side of a membrane, and even saltier water goes on the other. The saltier side is called the “draw” solution because it pulls water to that side. But the salts in the draw solution perform a chemistry trick: When heated to low temperatures, they bubble out as gases…

[brief bubbling sound]

…leaving behind pure water.

Meny Elimelech: So, the invention here was to come up with a draw solution that you can really separate relatively easy and inexpensively by means of waste heat.

Lisa Raffensperger: Waste heat, like the billows of steam you see coming from power plants. If waste heat is available, the only energy input is for pumping. Which means—

Rob McGinnis: —that all the water we produce by this method would not require additional fuel, and this is a huge difference in terms of sustainability.

Lisa Raffensperger: The systems could be installed alongside power plants to use their discharged heat. Ultimately, the water produced in this way could cost half as much as water produced by reverse osmosis and use just a tenth of the electricity.

[sounds of the laboratory]

Lisa Raffensperger: It’s at a much smaller scale that testing continues at Elimelech’s lab at Yale. Ph.D. student Laura Hoover is studying the membrane. Traditional membranes are too thick for forward osmosis. She’s trying to make thinner membranes to allow more water through.

[sound of machine turning on]

Lisa Raffensperger: The test unit is just two big jars of water, connected by tubes and pumps—on one side, seawater; on the other, a “draw” solution of the special ammonium carbonate salts. The water streams pass on either side of a tiny membrane.

Laura Hoover: So we have the draw solution here on mass balance so we can measure the weight that’s in this container over time, and so we can see how much water has moved into the draw side of the system from the feed side.

Lisa Raffensperger: We watch the numbers climb. Today’s best forward-osmosis membranes can produce the same flow as the older reverse-osmosis ones, and no pumping is required.

[harbor sounds]

Lisa Raffensperger: Meanwhile, back in Boston, Rob McGinnis at Oasys Water is looking at the big scale. The company is close to commercializing the technology. And there are applications besides seawater. Forward osmosis could be used for municipal water treatment or water recycling. But desalination is the company’s first goal. And, says Rob McGinnis, there’s really only one question that matters.

Rob McGinnis: And the question is, what do we use to do that? Do we use fossil fuels or electricity that can be used for so many other things, or do we find some way to use less resources to do it, and we think that’s what we can do.

Lisa Raffensperger: And that’s something all of us can raise a glass to. I’m Lisa Raffensperger, in Boston.

[harbor sounds, music]

To Probe Further

Check out the rest of the special report: Water vs Energy.

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