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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 in

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.

cgb223 3y ago
On paper this sounds like a really obvious great idea Have we ever built a working prototype/plant that does this?
- ChuckMcM 3y ago
  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/ https://www.hindawi.com/journals/ijce/2021/8898472/
- Baeocystin 3y ago
  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.
  - ChuckMcM 3y ago
    Have you seen a "wave" pump?
chii 3y ago
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!).
- jojohohanon 3y ago
  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.
  - p1mrx 3y ago
    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.
    - jethro_tell 3y ago
      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?
      - lazide 3y ago
        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.
        jethro_tell 3y ago
        That makes sense, but then, why put it on a pipe at all?

cgb223 3y ago

On paper this sounds like a really obvious great idea Have we ever built a working prototype/plant that does this?

ChuckMcM 3y ago
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/ https://www.hindawi.com/journals/ijce/2021/8898472/
Baeocystin 3y ago
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.
- ChuckMcM 3y ago
  Have you seen a "wave" pump?

chii 3y ago

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!).

jojohohanon 3y ago
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.
- p1mrx 3y ago
  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.
  - jethro_tell 3y ago
    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?
    - lazide 3y ago
      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.
      - jethro_tell 3y ago
        That makes sense, but then, why put it on a pipe at all?