How cruise ships make fresh water for thousands daily

HostWhen you think about those massive cruise ships, they're basically floating cities with thousands of people taking hot showers, splashing in pools, and drinking water all day long. It's easy to assume they just fill up some giant tank at the dock and hope it lasts until the next stop, but the math on that just doesn't work out. How do they actually keep the taps running when they're out in the middle of the ocean for weeks?

GuestYou're right that they can't just carry it all. A large ship might use hundreds of thousands of gallons of fresh water every single day. If they tried to carry all of that from the start, the ship would be so heavy it would barely move, and they would've to give up half the guest rooms just to make space for the tanks. Instead, these ships are basically giant, moving water treatment plants. They make almost all their own water right under your feet while you're sleeping or eating dinner. They take the salt water from the ocean and strip it down until it's pure enough to drink. It's a constant, high-speed process that happens in two main ways, depending on whether the ship is moving fast or sitting still.

HostThat sounds like it would take a massive amount of energy. Are they just burning extra fuel just to boil the ocean?

GuestWell, that's the clever part. For a long time, the main way ships made water was by using heat, but they weren't burning extra fuel for it. See, those massive ship engines get incredibly hot, and most of that heat just goes to waste. Engineers figured out they could catch that heat and use it to boil seawater. But you hit on a problem, boiling that much water normally takes a ton of energy. To get around that, they use a vacuum. Inside a special tank, they suck out the air to drop the pressure. When the pressure is low, water boils at a much lower temperature, like it does on top of a very high mountain. So, they can use the leftover warmth from the engine to turn seawater into steam without needing a massive furnace. The salt and gunk stay at the bottom, and the clean steam is cooled back down into pure water.

HostBut what happens when the ship slows down or pulls into a port? If the engines aren't working hard, you lose that free heat. It seems like the whole system would just grind to a halt the moment you want to go for a swim while docked.

GuestThat's exactly when the second system kicks in. It's called reverse osmosis, and it doesn't need heat at all. Instead of boiling the water, they use brute force. They take the seawater and pump it at incredibly high pressure against a special screen. This screen has holes so tiny that only the water bits can get through, while the salt, minerals, and bacteria are too big and get stuck on the other side. It's like trying to push sand through a fine mesh, but on a molecular level. This setup is great because it can run on electricity alone, so as long as the ship has any power, they can keep making water. Most modern ships use a mix of both systems to stay efficient.

HostI have to wonder about the leftovers, though. If you're pulling all that clean water out, you're left with a lot of really salty sludge. If they're just dumping that back into the sea, isn't that a huge problem for the fish and the reefs around the ship?

GuestIt's a real concern and one the industry gets pushed on a lot. When you take the fresh water out, what's left is called brine. It's basically water that's way saltier than the ocean. If you just dumped it in one big glop, it would sink to the bottom and could hurt sea life because it's so thick and salty. To stop that, ships have to be very careful. They usually only dump it while they're moving at a good clip, and they use special nozzles to spray it out so it mixes with the wake of the ship immediately. The goal is to thin it out so fast that the ocean barely notices a change. There are also very strict rules about where they can do this. They can't just dump brine near a coral reef or in a quiet bay.

HostSo once they have this pure water, is that what actually comes out of the faucet? I have heard that water that's too pure actually tastes a bit strange, or that it can even be bad for your pipes.

GuestYou have a good ear for the details. Pure water from those systems is actually a bit of a problem. Because it has no minerals in it, the water is technically hungry. It wants to grab minerals from whatever it touches, which means it can actually eat away at the metal pipes in the ship. Plus, it tastes flat and a bit hollow. To fix this, they actually have to make the water a little less pure before it reaches your glass. They run it through beds of crushed limestone or add minerals back in to give it that crisp taste we expect. They also have to treat it with a bit of chlorine or blue light to make sure no bugs grow in the storage tanks.

HostIt's funny to think there's a whole chemistry lab working just to make a glass of water taste normal. Is there anything they can't filter out? Like, if the ship is in a really dirty harbor, can they still make water safely there?

GuestMost captains will actually turn the systems off if the water around the ship is too dirty. While the filters are amazing, things like oil or certain chemicals can ruin those expensive screens very quickly. They prefer to make the bulk of their water when they're out in the deep, blue parts of the sea where the water is already pretty clean. The highest tech ships are even starting to look at ways to recycle the water they have already used for things like laundry or even the toilets, though that's usually kept for things like the engine cooling systems rather than the drinking taps.

HostThe most surprising part is that they're basically making a tiny, perfect version of the earth's rain cycle inside a metal box.

GuestThe ship is really just a closed loop where every drop has to be accounted for and cleaned until it's ready to go back through the system one more time.

HostThat glass of water on the nightstand starts to look a lot more impressive when you realize it was a piece of the deep ocean just a few hours before.