[GhostVII]

Wouldn't pumping from 230 meters be exactly equivalent to just pumping through the membrane at sea level?

If you have a pump 230 meters below sea level trying to push water up a pipe, to get water to the surface that pump must be exerting the same amount of pressure as you'd be experiencing 230 meters below sea level (since otherwise it wouldn't be strong enough to push up the water column). So you might as well just take the pump up to sea level and have it pump water through the membrane, since it would be exerting the same amount of pressure.

Or alternatively, you could just find a hill that is 230 meters high, pump sea water to the top of it, and then have the sea water come down in a pipe and go through a membrane in the bottom. Should be totally equivalent without requiring anything underwater.

[kelnos]

I don't think that's correct. Pumping water from below the surface of the ocean is about overcoming gravity. The pipes themselves would need to be constructed to withstand the pressure of the water around it, of course, so the pipes don't collapse.

The reason you don't need to "pump" to force the water through the reverse-osmosis filter when you're down that low is because the overall pressure at that depth is sufficient to push the water through as-is. Merely raising water up 230m in the air and "dropping" it through a pipe into a filter sitting on the ground would not give you the same amount of pressure.

Put another way, the pressure of the water in a 230m-tall pipe on the filter on the ground is much lower than the pressure of all the water 230m under the surface of the ocean pushing on that submerged filter. While the pipe ensures that the water stays confined, it is not putting pressure on the water in the same way all the water in the ocean is putting pressure on the water that's being pushed into a 230m-submerged filter.

I imagine there is some way to do what you describe above-ground, my my intuition is that it would require essentially recreating a large ocean, suspended in the air (not as large as the Atlantic, say, but still fairly large). Much more efficient to just use the ocean we already have.

For the submerged filter approach, I think the biggest challenges are probably maintenance and keeping things stable and functioning at the pressures present at >230m below the surface. Those challenges might make it infeasible. I don't think the need to pump the desalinated water back to the surface is all that large a problem in comparison.

[leovailati]

I think you are incorrect. If you build a 230-m tall pipe and fill it up with water, the water at the ground-level end of the pipe will be at the exact same pressure as the water 230 m deep in the ocean. Hydrostatic pressure only depends on the depth, not the container shape.

[lazide]

Uh no?

The water at the ground level on the ocean will have the same pressure as the water at ground level in the pipe. The water at 230m down in the pipe will be the same as the water 230m down in the ocean.

Even if the pipe is capped on both ends.

[jherico]

a pipe 230m tall with the bottom at ground level would experience the same hydrostatic pressure as water 230m under the ocean. the pressure comes from the weight of water above it, not distance from sea level. this is literally how water towers work.

[lazide]

That is literally what I said, yes.

The post I replied to said the opposite - that the pressure at the surface of the ocean would be the same as the pressure at the end of a pipe going under water, if there was a pipe.

Which clearly isn’t true or we’d have a trivial perpetual motion machine.

Edit: it looks like they changed their post?

[lazide]

To be clear - unless there is work done to reduce the pressure at -230m below mean sea level, there is zero pressure gradient there. Anything sitting at -230m will feel a pressure gradient of essentially zero.

So you couldn't run a filter or anything, unless it's literally magic.

If someone put a pump at -230m and pumped out the water on one side of the membrane so that on one side the pressure was 'ocean' at -230m, and the other side was mean sea level, then yeah the membrane would work.

Notably, this requires roughly the same amount of work and energy as pumping water from the ocean at sea level into a filter so it has roughly the same pressure you would find if you did the first thing.

There is no free lunch.

[jlawson]

Yes, this is how normal reverse osmosis filters work. However, pumping water at that pressure obviously takes a motor and consumes energy. What's interesting about the OP's concept is that in theory it can be performed with no energy expenditure at all.

[lazide]

Except it can't because of thermodynamics, if you pencil it out.