Forward Osmosis for Desalination

Technocrat

Forward Osmosis for Desalination

zogger

Mon Jun 05 20:56:18 -0700 2006

Chemistry.

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Large areas of the planet have plenty of water for all the normal uses — if only it wasn't salty. Removing the salt is an intricate and energy expensive process, so anything that can be done to speed it up and make it cheaper is welcome. Reserchers are working on a technique now called forward osmosis, obviously different than reverse osmosis. How it works is the flow-to side of the osmosis membrane contains a water solution that has been super saturated with ammonia and carbon dioxide, which then allows the salt water to pass through with little energy expended, leaving most of the salts on the other side. The ammonia and carbon dioxide are then removed from the fresh water, with moderate heating, to be recycled into the next batch. The result is much cheaper fresh water accumulation, and a higher brine concentrate on the original side, making disposal easier. Net energy savings are considerable, as no high pressure is needed like in conventional reverse osmosis setups.

"One-third of the world’s population lives in countries with insufficient freshwater to support the population, according to the UN Environment Programme. For that reason, desalination plants that extract drinking water from seawater are increasingly popular across the world. But many water-poor countries cannot afford the conventional desalination technology"....more sweet water there

Forward Osmosis for Desalination

Technocrat.net Editor

Tue Jun 06 06:34:02 -0700 2006
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Forgive me for cutting chem class, but I had to look up what NH3 was. I edited the text in pity of others in a similar bind :-)
Bruce

Forward Osmosis for Desalination

rubycodez

Tue Jun 06 12:31:51 -0700 2006
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in water, it'll make ammonium hydroxide, NH4+ and OH-. I'm having a hard time believing that just heating to 140 degrees F will drive enough of the stuff away so the water doesn't taste like dilute glass cleaner. Anyone care to heat up some janitor's ammonia solution for awhile and then take a swig? Let us know, or have your next of kin drop us a line if you're indisposed.....

Forward Osmosis for Desalination

danielwebb

Wed Jun 07 12:46:17 -0700 2006
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That's a great article. I wish more tech articles were written that way, with the challenges clearly explained. A lot of articles like that leave me asking "So why aren't they using that technology commercially already?"
In this case, the idea is great. The theoretical driving force of this system is much higher than a pressure-based system. I won't bore you with the details (and I've forgotten most of them anyhow :) ), but pressure is a really crappy way to drive reverse concentration gradients due to the thermodynamics of it.
You could divide the energy used in desalination into two basic kinds: one is the chemical energy required to remove the salt from the water, and the other is the system losses. You can't do anything about the first kind, so only if system losses are significant, or if membrane cost is significant compared to energy cost, then I could see their idea being feasible. However, they have plenty of challenges left. I suspect they're going to need a larger distillation column than they think, and the fact that their "fresh" water is coming out so salty has them very concerned I'm sure. If they have to decrease the driving force to increase the separation they may end up losing a lot of the advantage.

Forward Osmosis for Desalination

Paul Jackson

Fri Jun 09 07:57:40 -0700 2006
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well lets see, to be suitable for drinking, freshwater should contain <2 mg-NH₃/L, or 2 ppm;

Test for toxic ammonia in your fresh or saltwater tank. Reads from 0 to 8 parts per million. @$5.99 for 130 tests
so it shouldn't be too hard for some geek puttering aroung in the garage.
The hard part is the membrane, you want the pores big enough for water to go through, but not big enough for salt and enough of them so the the membrane almost isn't there