4 ms·

MIT Technology Review wrote a great article on this: http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/ http://www.technology

by xasos 12y ago

MIT Technology Review wrote a great article on this: http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/ http://www.technologyreview.com/featuredstory/533446/desalin...

ChuckMcM 12y ago
That is a good summary. A friend of mine is looking at putting a passive desalinator on the ocean floor off the coast of Monterey. Basically its a pipe with the permeable membrane surrounding the submerged collector. The pipe carries fresh water to the surface and as you pump out fresh water, the pressure of the ocean at depths of up to 1000 meters pushes in more water through the membranes, desalinating it in the process. The challenge (other than just building the darn thing) is whether or not the current flow is sufficient to keep the brine levels down around the intake port. I have no idea how practical that idea really is (he's pretty enthused of course) but I liked the notion of using the pressure difference between the ocean surface and at depths to do the reverse osmosis work for you.
- cma 12y ago
  How does the membrane stay clean of marine growth?
  - weaksauce 12y ago
    hiring a diver or two to do that is probably cheaper than the alternative ways to desalinate.
  - mattgrice 12y ago
    there is very little marine growth at 1000 meters depth. A bigger concern would be marine snow: http://en.wikipedia.org/wiki/Marine_snow http://en.wikipedia.org/wiki/Marine_snow
- colechristensen 12y ago
  You can do it, but it turns out the work to pump water up to 1400 psi in a pressure vessel before it goes through a reverse osmosis membrane is about the same as the work to pump desalinated water up from the bottom of the ocean at 1000 meters. There are different inefficiencies in each situation, but I'd assume maintaining equipment 1km below the sea would trump them all.
  - cma 12y ago
    I think he really means you bail the water from the top rather then pump it, leaving an airgap at the top that the pressure from below gradually fills with more water through the membrane. But then again unless you bail all the water, that leaves only a small pressure differential, and if you bail it all, it gets less efficient as it fills back up and the pressure differential gradually reduces and lifting it from the bottom takes a lot of energy :P. Yeah, I don't understand what he's getting at with the design.
    - ChuckMcM 12y ago
      As described in the paper the top 20 meters of the pipe to the surface are emptied (the paper refers to a 30 meter 'pump head' which I believe is the distance below the surface that the water has been pumped out of the pipe). The part on the surface of the ocean is what looks like a giant bucket of sand on the outside (it is supposed to be below the level at which coral grows). The pipe fills with fresh water at some rate (unspecified by the paper) and you can pump it out at the top when you need it. The part on the bottom of the ocean requires no maintenance.
- manachar 12y ago
  Sounds like the MaHaLo Hawaii Deep Sea Water project[1]. They use that water in Ocean Vodka interestingly enough[2], which is where I found out about it. On the tour they indicated this project seems to be working out quite well. [1] http://hawaiideepseawater.com/source http://hawaiideepseawater.com/source [2] http://oceanvodka.com/our-vodka/ingredients/ http://oceanvodka.com/our-vodka/ingredients/
  - ChuckMcM 12y ago
    Interesting that their "3000 feet" corresponds to the 1000 meters I read in the paper. I could not find anything on their site though that actually talked about the extraction process.
- jefurii 12y ago
  What does all that extra salt do to the chemistry of the water in that area?
  - ChuckMcM 12y ago
    Well depending on rate it can be a pollutant or not. Most desalinating plants have a mixing pond where the mix fresh seawater with partially desalinated water (which has a higher salt content) to produce an effluent that is saltier than the unprocessed water but not so salty that it causes issues. That same mixing could, in theory, be done by ocean currents if the water motion was enough.

by xasos 12y ago

ChuckMcM 12y ago

That is a good summary. A friend of mine is looking at putting a passive desalinator on the ocean floor off the coast of Monterey. Basically its a pipe with the permeable membrane surrounding the submerged collector. The pipe carries fresh water to the surface and as you pump out fresh water, the pressure of the ocean at depths of up to 1000 meters pushes in more water through the membranes, desalinating it in the process. The challenge (other than just building the darn thing) is whether or not the current flow is sufficient to keep the brine levels down around the intake port. I have no idea how practical that idea really is (he's pretty enthused of course) but I liked the notion of using the pressure difference between the ocean surface and at depths to do the reverse osmosis work for you.

cma 12y ago
How does the membrane stay clean of marine growth?
- weaksauce 12y ago
  hiring a diver or two to do that is probably cheaper than the alternative ways to desalinate.
- mattgrice 12y ago
  there is very little marine growth at 1000 meters depth. A bigger concern would be marine snow: http://en.wikipedia.org/wiki/Marine_snow http://en.wikipedia.org/wiki/Marine_snow
colechristensen 12y ago
You can do it, but it turns out the work to pump water up to 1400 psi in a pressure vessel before it goes through a reverse osmosis membrane is about the same as the work to pump desalinated water up from the bottom of the ocean at 1000 meters. There are different inefficiencies in each situation, but I'd assume maintaining equipment 1km below the sea would trump them all.
- cma 12y ago
  I think he really means you bail the water from the top rather then pump it, leaving an airgap at the top that the pressure from below gradually fills with more water through the membrane. But then again unless you bail all the water, that leaves only a small pressure differential, and if you bail it all, it gets less efficient as it fills back up and the pressure differential gradually reduces and lifting it from the bottom takes a lot of energy :P. Yeah, I don't understand what he's getting at with the design.
  - ChuckMcM 12y ago
    As described in the paper the top 20 meters of the pipe to the surface are emptied (the paper refers to a 30 meter 'pump head' which I believe is the distance below the surface that the water has been pumped out of the pipe). The part on the surface of the ocean is what looks like a giant bucket of sand on the outside (it is supposed to be below the level at which coral grows). The pipe fills with fresh water at some rate (unspecified by the paper) and you can pump it out at the top when you need it. The part on the bottom of the ocean requires no maintenance.
manachar 12y ago
Sounds like the MaHaLo Hawaii Deep Sea Water project[1]. They use that water in Ocean Vodka interestingly enough[2], which is where I found out about it. On the tour they indicated this project seems to be working out quite well. [1] http://hawaiideepseawater.com/source http://hawaiideepseawater.com/source [2] http://oceanvodka.com/our-vodka/ingredients/ http://oceanvodka.com/our-vodka/ingredients/
- ChuckMcM 12y ago
  Interesting that their "3000 feet" corresponds to the 1000 meters I read in the paper. I could not find anything on their site though that actually talked about the extraction process.
jefurii 12y ago
What does all that extra salt do to the chemistry of the water in that area?
- ChuckMcM 12y ago
  Well depending on rate it can be a pollutant or not. Most desalinating plants have a mixing pond where the mix fresh seawater with partially desalinated water (which has a higher salt content) to produce an effluent that is saltier than the unprocessed water but not so salty that it causes issues. That same mixing could, in theory, be done by ocean currents if the water motion was enough.