Best case scenario, pumping 1 kg of water 230 meters costs 2.3kJ.
Elsewhere in this thread it was mentioned that typical desalination approaches need 10-15kJ/kg.
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.
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.
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.
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.
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?
All of these sorts of repairs are also required for land-based water treatment facilities, of course, but now you have to perform them all at 230m underwater. That’s gotta be mega-expensive. Furthermore, all of your equipment needs to be able to survive at those depths, with saltwater attacking exposed surfaces and seals, requiring much more expensive materials and tighter tolerances.
It really doesn’t seem very economical to me!
I think a good analogy for this is oil extraction. These days we extract oil from places where it's more expensive and difficult to do so, because we can no longer meet demand with the cheaper options (many of which have simply dried up over time). But we'd never go after the expensive-to-extract oil 40 years ago (or whatever) when cheaper options were available that could meet demand.
You add your osmosis filter at the end of a long flexible hose. For maintenance, you reel it back in.
From what I can gather, oil rigs have profit margins below 20% (at best) and margins decline as the pressure in the well head drops, until they become totally unprofitable and shut down to wait for oil prices to go up. Water prices would have to increase by more than 2 orders of magnitude to make this strategy begin to approach profitability!
A cubic meter is ~264 gallons.
Here in my area of California (CA has lots of water agencies, so YMMV) 1000 gallons costs $38.66 (varies based on total consumption).
So that's $10.21 for a cubic meter.
...straws surrounded by air don't work beyond 10 meters, because the external pressure at the bottom of the pipe doesn't exceed 1 atmosphere.
Obviously this isn't true if the pipe does down into the ocean.
which really puts into perspective how insane desalination, with it's order of magnitude higher energy costs is. why spend all that energy when we could simply build more infrastructure to capture more peak runoff from the sierras