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Comment by ChuckMcM | Hacker News Reader

FWIW there is a really fun "disruptive" desalination option that could work in places like Monterey Bay which is just dropping a really really long pipe down into the ocean that has the desalination membrane at the bottom of it. Top of the pipe is above the surface. Water pressure at depth drives water through the membrane and up the pipe. At the surface you just pump fresh water out of the pipe like an ordinary fresh water well.

Maintenance is basically pulling the pipe up every couple of years and replacing the membrane which will become fouled over time, but generally it would be deep enough that it would not get barnacles and other mollusks on it.

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The pressure at the bottom of the ocean will need to push thru the column of water as well as the membrane - there's not enough energy to do that (otherwise, there'd be spontaneous water sprouts in the ocean!).

Not the full column.

You pump out the top x feet of water from the pipe. The equalizing levels of inside and outside the pipe is what drives the upward flow through the membrane.

Pumping x feet of water creates x feet of pressure difference. How is that better than doing reverse osmosis at the surface?

Perhaps the advantage is that you can lift in multiple stages, so each pump doesn't bear the full column pressure. Also the pumps aren't exposed to salt.

Why would you need to lift in different stages? If you take the top 3 feet out of the pipe, the water pressure will push three more feet into the pipe.

Am I missing something here?

The membranes require hundreds of feet of head worth of pressure, not 3 ft.

The pressure at the membrane will only ever be the difference. It has nothing to do with the depth the membrane is actually under the water.

That makes sense, but then, why put it on a pipe at all?

Near as I can tell, it offers no actual advantage to do so, and creates several serious disadvantages.

In theory, wouldn’t you be able to just keep pumping from the surface of the pipe (increasing x) until x provided a sufficient differential pass through the membrane? It would then stay at x even while you continue pumping. The pump would merely need to lift the water from x below sea level.

Pumping from the surface, like with a vacuum? The maximum height of a drinking straw is about 10 meters. It's generally easier to lift water by pushing from the bottom than pulling from the top.

10 meters at sea level. A lot more when you have an ocean’s weight of water pushing on the other end.

That's irrelevant, because the pressure at the water surface is ~1 atm regardless of its depth below sea level.

Air pressure increases slightly as you descend a well, but not enough to change the outcome.

The site pressure at a bottom of a well is irrelevant. You do realize air pressure is not what matters, but pressure as a whole? That includes water pressure. The bottom of the straw is not in a well; it’s more akin to a cassion, so you need to factor in the water pressure as well.

What you’re saying would imply that the Titan submersible would be at the same pressure whether under an ocean of water, or in a well the same depth. No. The former is 375atm, the latter is a bit more than 1atm.

You only need to lift the top layer of water a little bit. The rest of the water column would come up after it. Of course, you cannot lift the top of the siphon more than 10m above sea level.

In order for the reverse osmosis membrane to function, you need a rigid pipe at atmospheric pressure, without water in it. The pressure difference inside/outside the pipe is what powers the desalination process.

While operating, it's basically a deep open well with a bit of water at the bottom. You need to pump that water out to keep the process going.

I mean the “pump” probably isn’t a vacuum pump. Maybe it’s a water screw. Maybe it’s a rope and bucket. The effect is the same, no? The water must be lifted X.

Yes, the effect _is_ the same. Regardless of the mechanism, you can only lift water from above with a sufficiently short hose.

It is incontrovertible that this system would have to push the water up from below.

Maybe the thought is that if you drop a 4000m-deep-to-surface pipe into Monterey Canyon, there will be a pressure differential at the base of the pipe (since sea water is denser than fresh water).

I don't think the math works out all that spectacularly though. The pressure difference would be a about 1.1 MPa, and desalination of seawater takes ~6MPa, so by running a pipe down to the bottom of Monterey Canyon you could... save almost 20% of the energy costs.

Which makes sense. You can't build a perpetual motion machine by sticking a pipe with a membrane at the bottom and a turbine at the top into the ocean. Even if it's a very long pipe.

Seawater has a higher density, about 2,5% according to Wikipedia. So 40m of water column fresh water vs seawater would be equal to 1m water column fresh water. 400m would result in a 1 bar pressure difference.

Reverse osmosis systems (like the one speculated with a membrane) has operating pressures between 40 and 82 bar when using seawater. The ocean is pretty deep but having a 16 kilometer pipe straight down seems like it has its own engineering problems.

Have you heard of oil?

If we can run those for hundreds of miles through water, pumping thick sludge that is crude oil....

On paper this sounds like a really obvious great idea

Have we ever built a working prototype/plant that does this?

There was a startup pursuing a test setup using an oil derrick of Santa Barbara if I recall. These guys[1] were proposing a different system (basically providing fresh water to an underwater habitat) however the paper goes over the basics pretty effectively.

[1] https://www.hindawi.com/journals/ijce/2021/8898472/

It does sound clever at first glance. It caught me at first, too.

The problem is, your fresh water is now several hundred feet below the surface. To raise it up to where you are, you need pumps to push up the same exact pressure differential that you got by putting the membrane so deep. And if you need to do that anyway, just use surface level water and pumps for ease of maintenance and access.

Have you seen a "wave" pump?

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