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gravity powered desalination

 Use waste brine from coastal desalination plants to desalinate more water
 
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Concentrated brine that is produced by modern desalination plants can be used as deadweight to drag pressure vessels containing reverse osmosis chambers to suficient depths offshore. The constant supply of sea water passing through the vessel under pressure supplies the RO process and when the chamber is full of fresh water the concentrated brine is replaced with sea water and the vessel returns to the surface. Normally, the concentrated brine is pumped offshore creating a polution source of unkown effect, with this scenario the brine is released at random locations throughout a broad geographic region.
—sutheric, Mar 13 2003

(?) "Technology Properly Applied" http://www.rimnetics.com/osmotek5.htm
Or so they say... [pluterday, Oct 06 2004, last modified Oct 21 2004]

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       I don't think dropping the concentrated brine at random locations would be very efficient, owing to the expense of moving all that water. If this worked at all, I would suggest some kind of long column structure in which a pressure vessel alternately rises and falls.   

       Having said that, if you're doing your desalination deep underwater, where do you get the concentrated brine at sea level? (This system seems to be powered by the falling weight of the brine you put in at the top, and thus depends on having a net movement of mass downwards.)
—pottedstu, Mar 14 2003
   

       //Having said that, if you're doing your desalination deep underwater, where do you get the concentrated brine at sea level? //   

       //...modern desalination plants...//
—LoneRifle, Mar 14 2003
   

       If you're going to do your desalination at sea, there is no need to produce concentrated brine. Just sink a long pipe from the surface down about twenty feet, with a reverse osmosis membrane at the end. The water pressure relative to the air-filled pipe will drive water across the membrane. All that remains is to pump the fresh water out of the pipe.   

       I have never figured out why this type of desalination is not more popular. Maybe it is the difficulty of maintaining apparatus at sea? Nahh...
—bungston, Mar 14 2003
   

       perhaps it's offshore, so the pipeline is more expensive?
—LoneRifle, Mar 14 2003
   

       It could be at the end of a dock, weighted with a brick.
—bungston, Mar 14 2003
   


 

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