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Comment by nayuki | Hacker News Reader

> They used a new nanofiber membrane distillation process that could desalinate water with 99.99% efficiency.

The article doesn't say much about the process, but it seems like membrane distillation does involve water vapor in some way: https://en.wikipedia.org/wiki/Membrane_distillation

> Unfortunately, reverse osmosis uses a tremendous amount of energy, and in the process of desalting water in this way, harmful waste is generated, which is usually discharged back into the sea or ocean.

I believe every desalination process, including this one, inherently needs to discharge salty wastewater back to the ocean.

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Every desalination process uses a lot of energy, too! However fancy a membrane they're using, they need to force the salt water through it, and push hard enough to beat the osmotic pressure.

This is a reversible process, as you can get the energy back by using the osmotic pressure to drive a piston. A desalination process that used a lot less energy would let you build a perpetual motion machine, and is seriously against the laws of physics.

I was thinking about this today,

What if we use the oceans water pressure in our benefit?

For example, deploy this membrane 100m under the ocean, and have a pipe from the membrane going into a tank on the beach nearby.

As we pump fresh water out of the tank on the beach, the oceans own water pressure, pushes water through the membrane and up into the tank on the beach. That way the pump on the beach requires “very little” energy to move the water horizontally on the surface.

The excess brine build up would still be an issue but that’s always an issue.

The membrane consumes pressure, so you need to provide some pressure to get the desalinated water the full 100 m back up to the surface.

> the oceans own water pressure, pushes water through the membrane and up into the tank on the beach

Unfortunately, that's not how pressure works. There's a 100m column of water above the salt side of the membrane, but there's also a 100m column of water in the pipe on the fresh side. Those columns generate an enormous pressure, but it's pushing from both sides, which doesn't help you get water through the membrane.

That’s why you have a pump to move the water horizontally into different tanks. Then there is a 100m column and a 95m column forcing the water through the membrane.

So the pumps still need energy by “very little” energy because they are doing relatively little work.

> Then there is a 100m column and a 95m column forcing the water through the membrane

In that scenario, why do you think it helps to put the membrane 100m deep, instead of 5m deep?

That is the equivalent of having a 5m column on one side and being open to air on the other side.

There is no free lunch - for the membrane to experience 100m of water pressure on one side only, something else has to be reducing the pressure by the same amount on the other side.

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