Amy Zander on turning salt water to fresh

EarthSky spoke with water expert Amy Zander about desalination – the process of removing salt from water. She said that rising demand for water will mean increased use of desalination in the United States, in the coming years.

Amy Zander: We’ve been turning sea water into fresh water for centuries. Now the state of the science appears to be reverse osmosis, which is where we push the salty water through a membrane, and clean water comes out the other side and we have a salty concentrate solution remaining.

Zander, who led an assessment in 2008 by the U.S. National Research Council on desalination, said she’s concerned by desalination’s possible environmental effects – effects both on sea water and on inland aquifers of brackish, or naturally salty, water.

Amy Zander: We have worries about what to do about that concentrate, both inland and in sea water. We need to know what kind of environmental effects there are from gathering the water from those places. And we’re also worried about potential greenhouse gas emissions from the energy that goes into desalting water.

Amy Zander: Probably we’ll have to supplement it with other ways like conservation and water reuse in order to get the full water supply portfolio.

Today, less than one half of one percent of the world’s drinking water comes from desalination – still about a billion gallons per day.

Dr. Zander also expressed concern about how desalination can affect aquatic life.

Amy Zander: Of course, the Academies did have concern with entrainment and impingement of microorganisms and organisms from sea water. The impingement would be catching small organisms against the intake screen, and entrainment would be actually bringing organisms all the way to the plant, and of course that’s not very good for the organisms in the end. So what needs to happen is figuring out ways to mitigate that.

Zander told EarthSky about some possible ways to protect against impingement and entrainment of organisms.

Amy Zander: And some ideas there that we saw that seemed to be working are to not take your seawater directly from the sea, but instead take it from beneath the sea floor. So use the sand of the sea floor as somewhat of a filter there and not entrain or impinge organisms. So just put an extra little barrier between your intake and the ocean. Or put your intake far enough out into the middle ocean, not the coastal ocean, where you see lower numbers of organisms in that area than you do in the highly productive coastal region. Or another idea is to put a much greater area of screen around the intake so that the velocity of the water that is pulled through the screen is locally much smaller. So we would see far less impingement and entrainment of organisms. So there are ways to technologically avoid impingement and entrainment. They just need to be locally designed and put in place in order to avoid it.

Zander spoke more about the potential of desalination to meet the world’s need for more water.

Amy Zander: What we once thought was a technological problem is not the state of the problem right now. Right now, desalinization is constrained by financial, social, and environmental factors. Permitting of desalination plants is somewhat difficult because we don’t know what are the environmental constraints of withdrawing seawater and discharging the concentrate, and managing the inland concentrates. The social factors are that there’s a pushback on permitting these plants when we don’t understand the environmental factors. And the financial constraints come primarily from the high-cost interest of building the plants, capital interests, and on energy, the operating costs of the plants, because it an energy intensive process.

It might be possible to use waste heat from industries as a way to get energy, said Zander.

Amy Zander: So one way around the energy problem is to look at not using so much primary energy, by primary energy I mean electrical energy is to look instead at, perhaps we do want to do a desalination-type thermal removal of salt from water, and use instead of primary energy, use waste heat from industries as a way of getting that energy into the system. That, or waste heat from power plants. So co-locating a desalination plant near a source of waste energy is another way that we could potentially bring down the financial cost. But the technology itself is not what’s holding back the movement of desalination for water provision.

Climate change might accelerate the use of desalination in the future, added Zander.

Amy Zander: There will be, definitely, a bigger role for desalination. And it has to be carefully planned. If we see rising sea waters, we’re going to see brackish or salty water coming inland quite a bit farther. Or a lot more of our area is going to be affected by seawater intrusions. We’re going to see areas now that have a fairly good ground water source, clean drinking water source from ground water. We may soon see that to become much more salty with climate change. We will then be seeing desalination be far more used in order to get the water supplies that we once had. So yes, climate change will definitely change the impact of desalination.

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