Desalinating seawater on a massive scale

HostI was at the coast last week, and it's such a strange feeling to look at all that water and know you can't have a drop of it. We have these massive cities right next to the sea that are running out of water, which feels like it should be a simple fix. Why is it so hard to just take the salt out so we can drink it?

GuestIt's hard because salt and water really like being together. When salt melts into water, it doesn't just sit at the bottom like sand. It breaks apart and hooks onto the water at a very small level. To pull them apart, you have to break very strong ties. For a long time, the only way we knew how to do it was to boil the water. You turn it into steam, the salt stays behind, and then you catch the steam as it cools. But boiling that much water takes a massive amount of heat and fuel. These days, we have a way to do it that's a bit more clever, but it still feels like we're fighting a war against the sea.

HostSo we're not just boiling it in big pots anymore? What's the new trick?

GuestMost of the big plants use something called reverse osmosis. Think of it like a very, very tight screen. We take a thin sheet of plastic that has holes in it so small that a single water piece can just barely fit through. Salt is just a little bit bigger, so it gets stuck on the other side. The problem is that the water doesn't want to go through that screen on its own. In fact, if you just let it sit there, fresh water will actually try to move toward the salt side to balance things out. To make it go the way we want, we have to use giant pumps to shove the sea water against that screen with a huge amount of force. It's like trying to squeeze water through a solid brick.

HostThis feels like we're just burning more fuel to solve a problem we could fix by just using less water. If it takes that much power to move the water, is that not just making the world hotter and our water problems worse?

GuestThat's a huge worry. For a long time, these plants were energy hogs. But the tech has gotten much better. We now have tools that catch the high pressure of the water as it leaves the pipes and use it to help push the next batch in. It's like a car that saves power when it slows down. Even so, it's still a very power-heavy way to get water. In some places, they're starting to use big fields of sun panels to run the pumps. If you can use the sun to pull water from the sea, you start to win that battle. But the power is only half the story. The other half is what happens to the stuff you don't keep.

HostYou mean the salt? I guess you just end up with a big pile of it, like a mountain of table salt.

GuestI wish it were that simple. You don't get dry salt. You get a thick, heavy soup of super-salty water. We call it brine. For every gallon of fresh water we make, we usually have a gallon of this salty sludge left over. It's twice as salty as the ocean. And it's not just salt. Because we have to clean the pipes with soap and chemicals so they don't get clogged with tiny sea life, all those chemicals end up in that sludge too.

HostSo we're just making a huge batch of poison and putting it back in the water? That doesn't sound like a win for the planet.

GuestThat's where the friction is. If you just dump it back through one big pipe, it sinks to the bottom because it's so heavy. It spreads out like a thick blanket over the sea floor and chokes out the life there. There's no air in it, and the salt is too much for the fish or the weeds. Now, some plants use big sprayers to toss the sludge over a wide area so it mixes faster. It's better, but it's still a lot of stress on one spot. Some people are trying to find ways to dry that sludge out and turn it into stuff we can use, like road salt. But right now, it's mostly just a waste problem we haven't fully solved.

HostWhy do we not do this everywhere if the tech is getting better? It seems like it would end the water wars for good.

GuestIt's not a magic fix because of the price tag. Building one of these plants costs billions of dollars. Then you have to pay for all that power and the upkeep of those tiny screens, which get dirty very fast. If a city has a river or a well, that water will almost always be cheaper than the sea. This is really a tool for places that have no other choice. We're also finding that sucking in all that sea water kills a lot of tiny life. The intake pipes act like a giant vacuum, pulling in fish eggs and small bugs that the whole food chain needs to eat.

HostThose little screens are basically the only thing standing between a thirsty city and a whole ocean of water.

GuestThe biggest hurdle left is finding a way to deal with that salty sludge so we can drink from the sea without harming the life that's already there.

HostThe ocean is still that same vast tease it was when I was standing on the sand, but we're finally learning how to look at those waves as more than just salt.