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A Clever New Approach To Desalination 128

Posted by kdawson
from the pass-the-salt dept.
jbeaupre writes "The Economist reports on progress by a company called Saltworks on using saline gradients to do the heavy lifting of desalination. In essence, Saltworks uses solar energy or waste heat to concentrate sea water. They then use the ionic gradient between the concentrated brine and two sea-water streams to pull ions from from a 3rd sea-water stream. It appears to work with entropy by trading the reduced entropy of the desalinated water against the increased entropy of 'mixing' the brine and the other sea-water streams. The article only discusses Na and Cl, but even just removing these ions is a step in the right direction."
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A Clever New Approach To Desalination

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  • Making Dew (Score:3, Interesting)

    by lyinhart (1352173) on Saturday October 31, 2009 @03:54PM (#29936563)
    Thinking about desalination makes me remember that episode of "The Voyage the Mimi [wikipedia.org]" in which they used the process to make drinking water:

    http://video.google.com/videoplay?docid=-524069894840499801# [google.com] (A/V's not synced)
  • Maybe (Score:2, Interesting)

    by clang_jangle (975789)
    From TFA:

    The process begins by spraying seawater into a shallow, black-bottomed pond, where it absorbs heat from the atmosphere. The resulting evaporation increases the concentration of salt in the water from its natural level of 3.5% to as much as 20%. Low-pressure pumps are then used to pipe this concentrated seawater, along with three other streams of untreated seawater, into the desalting unit. As the diagram explains, what Mr Sparrow and Mr Zoshi create by doing this is a type of electrical circuit. In

    • by jeffstar (134407)

      what if they collected the fresh water vapour that is evaporating off the salt water as well?

      TFA says they make fresh water by heating salt water with electricity so why not just heat it mostly with the sun and then a bit of electricity.

      TFA is a bit light on details: why do Na+ ions go to one stream and CL- to the other? Have they got membranes that are impervious to CL- and NA+?

      • Re:Maybe (Score:5, Informative)

        by Timothy Brownawell (627747) <tbrownaw@prjek.net> on Saturday October 31, 2009 @04:17PM (#29936743) Homepage Journal

        what if they collected the fresh water vapour that is evaporating off the salt water as well?

        I'm guessing this would require active refrigeration unless they're in a colder climate?

        TFA is a bit light on details: why do Na+ ions go to one stream and CL- to the other? Have they got membranes that are impervious to CL- and NA+?

        Yes. From TFA:

        Each of the four streams of water is connected to two neighbours by what are known as ion bridges. These are pathways made of polystyrene that has been treated so it will allow the passage of only one sort of ion—either sodium or chloride.

        • by TubeSteak (669689)

          I'm guessing this would require active refrigeration unless they're in a colder climate?

          You just put a roof over it and collect whatever condenses.
          But unless you have a lot of surface area, the water you collect will be negligble in comparison to the main distillation process.

          • Re: (Score:2, Troll)

            by Rei (128717)

            Yeah, you "just" put a roof over it. As though that wouldn't 10x the construction and maintenance costs of the evaporation ponds. :P

            • by Hucko (998827)

              We should release carbon dioxide into the atmosphere! It is a greenhouse gas so it will help us make the earth one big greenhouse! Then everyone will have lots of fresh water! Damn, I need more exclamation points to qualify...

          • by pbhj (607776)

            I'm guessing this would require active refrigeration unless they're in a colder climate?

            You just put a roof over it and collect whatever condenses.
            But unless you have a lot of surface area, the water you collect will be negligble in comparison to the main distillation process.

            One fractal green house coming up ...

        • what if they collected the fresh water vapour that is evaporating off the salt water as well?

          I'm guessing this would require active refrigeration unless they're in a colder climate?

          If it's on Earth, then it is a colder climate. Colder than 100 degrees centigrade. Or as you probably call it, 212 degrees-F.

          • by Culture20 (968837)

            what if they collected the fresh water vapour that is evaporating off the salt water as well?

            I'm guessing this would require active refrigeration unless they're in a colder climate?

            If it's on Earth, then it is a colder climate. Colder than 100 degrees centigrade. Or as you probably call it, 212 degrees-F.

            Exactly. Just get a really long Aluminum pipe for the "waste steam", slightly angle it down, and you've got a water condenser. Run the "waste" pipe next to the input water pipe, and you've got increased efficiency on both heating and cooling.

          • what if they collected the fresh water vapour that is evaporating off the salt water as well?

            I'm guessing this would require active refrigeration unless they're in a colder climate?

            If it's on Earth, then it is a colder climate. Colder than 100 degrees centigrade. Or as you probably call it, 212 degrees-F.

            Very funny, please go read up on "vapor pressure".

            All that's needed to get some condensation is a surface that's colder than the pool, and in the same enclosed area. The trouble is, the rate of condensation and evaporation depends on the temperature differential between the condenser and the pool. So if your ambient temperature is 90 F and your pool is heated to 100 F you won't see very much evaporation compared to if you just exhaust your waste humidity into the (presumably comparatively dry) environment.

        • by Avin22 (1438931)
          "what if they collected the fresh water vapour that is evaporating off the salt water as well?" I was thinking that might actually be a good idea, with a slight tweak. Instead of collecting the evaporated water as a byproduct of this process, why not combine this with the process of reverse osmosis. One of the major difficulties with reverse osmosis (in addition to the energy requirement) is that it produces a highly concentrated brine that must be disposed of. Instead of dumping it back in the ocean, why
          • Unfortunately, RO systems still require high pressures to work. That defeats the benefit of this new system, wherein no high pressure system (thereby no steel piping, no expensive pumps) is required.
      • Because these 4 "streams" conatin a medium that only allows positivly or negativley charged particle to travel through it depending on what is required.
      • by Langolier (470727)

        And what if they let the water vapour, that is lighter than air, rise, and cool off by rising, to the point where it would condense again?
        And if the rising force of the water vapour was used to drive some fans or turbines?

        Or they could just release enough water vapour, letting it rise, so that there would be more precipitation downwind of the site. All of these would
        generate power, and more fresh water, as well.

      • what if they collected the fresh water vapour that is evaporating off the salt water as well?

        Or better yet, collect the billions of gallons of condensate that falls out of the sky every day. Then you don't have to worry about all those silly ions, electrons and membranes.

    • Re:Maybe (Score:5, Funny)

      by samkass (174571) on Saturday October 31, 2009 @04:18PM (#29936753) Homepage Journal

      Dr. Flammond: "A year ago, I was close to perfecting the first magnetic desalinization process. So revolutionary, it was capable of removing the salt from over a million gallons of sea water a day! Do you realise what that could mean to the starving nations of the earth?"

      Nick Rivers: "My God, they'd have enough salt to last forever!"

      • by herojig (1625143)
        I liked this joke because it points out the futility of desalinization - if the seas are so polluted with bio-hazards that they will make the water unfit for consumption in the first place...all that's left is some dirty salt.
    • Current is by definition the flow of charge. In the case of your muscles, there is an electrical current along the muscle cell membrane that is caused by a change in the amount of cations allowed into the cell.

  • Desalinization is most likely the wave of the future, given the rise in sea levels and melting ice. We might as well put the extra water to good use, rather than just let our low-lying lands drown.
    • Re: (Score:3, Interesting)

      by Absolut187 (816431)

      Agreed. We (homo sapiens) should get to work on massive desalination efforts and fill up some of the lakes that have been going dry for years. Like lake Mead [nytimes.com], the sea of Galilee [encyclopedia.com], etc.

      Maybe if we fill up enough large lakes/seas with desalinated water we can make a small dent in sea level rise.

      Some new manmade lakes in Africa wouldn't hurt either. They seem to be short on potable water all the time.

  • by Yergle143 (848772) on Saturday October 31, 2009 @04:18PM (#29936747)
    OK between this and the General Fusion guys http://www.generalfusion.com/ [generalfusion.com] Canada has got water and energy completely licked. http://www.saltworkstech.com/ [saltworkstech.com] OK actually I'm still trying to run the numbers on the both of them (and waiting for some peer reviewed publications.)
    • If you have the first one, the second one becomes redundant. Loads of problems just go away if you have cheap, abundant, clean energy.

      -Peter

      • by TheLink (130905)
        If it's too cheap and abundant, the whole earth might start to glow due to waste heat. People would be using lots of energy and having to pumping the waste heat skywards.

        But most people might be on space colonies before that scenario becomes likely.
        • But most people might be on space colonies before that scenario becomes likely.

          If we're lucky it might stave off the next Ice Age, due to begin this millennium. Then we have 10,000 years to figure the rest out.

    • General Confusion (Score:2, Interesting)

      by epine (68316)

      Thank you for that link to General Confusion. Made my day. Check out the freshman T-rex with his lava lamp and the sordid diatripe:

      http://www.generalfusion.com/fossil_fuel_crisis.php [generalfusion.com]

      The planet was covered with dense clouds and the atmosphere contained a high concentration of carbon dioxide, producing tropical conditions north of the 45th parallel. For example, many dinosaur fossils were excavated in Alberta, Canada. As the earth's crust cooled down, volcanic activity reduced.

      Riddle of Burgess Shale's fossil-rich deposits solved [canada.com]

      The site, close to the B.C.-Alberta border, is considered crucial to understanding the so-called Cambrian "explosion" of life - a time when the future Canadian land mass was drifting in tropical climes close to the Earth's equator.

      In my historical atlas, the equator is considerably south of the 45th latitude. The dinosaur fossils in Alberta are equatorial in origin. But hey, if you can't get that right, no obstacle to solving the fusion problem. Like it's not a hard problem or anything. The typ

      • I'm fairly certain that the oil industry has a particularly good understanding of plate tectonics; the term "fossil fuels" isn't lost on them.
      • by EL_mal0 (777947)

        You're right that the equator is considerably south of the 45th parallel. However, you forget that continents move around, given enough time. So in the Cambrian North America (Laurentia) was near the equator - here's a map [scotese.com]. Fast forward 320 million years to when the dinosaurs began to rule the earth, and North America is approaching where it is today - here's another map. [scotese.com] Note how Alberta is approaching the 45th parallel, where it is found in your atlas.

        You need to get yourself a different atlas if you'r

      • Hey man, they have patents pending. Therefore the technology must be solid. Write them a check now before it's too late!
  • by 140Mandak262Jamuna (970587) on Saturday October 31, 2009 @04:46PM (#29936945) Journal
    The key piece of the work is an ion bridge. This has to permit the travel of one kind of ion but not the other, i.e. Na+ or Cl-. Looks like this material could be expensive. It might plug up need to be periodically replaced. How expensive these are? How non toxic these are? What is needed to manufacture them? These are the questions we need to ask.
    • Re: (Score:2, Insightful)

      by Vesvvi (1501135)
      More important than the cost is the question of effectiveness.

      In their diagram, they have this schematic in the critical location:

      [Salt water]<----(+)----[Brine]----(-)----->[Salt water]

      Chemically, that "equation" just doesn't balance without an input of energy. It doesn't matter what kind of "ion bridges" they put into place between the brine and salt water reservoirs, or what the concentration of salt exists in the brine or salt water, it will require some energy to offset the entropy increase
      • Re: (Score:3, Informative)

        by klaun (236494)

        Chemically, that "equation" just doesn't balance without an input of energy.

        The energy was input by the sun before the different solutions were brought together.

        it will require some energy to offset the entropy increase

        delta S > 0 for a closed system as a consequence of the second law of the thermodynamics. No need for additional energy. The entropy of a the concentrated solution is less than the entropy of the dilute solution, hence dilution happens spontaneously, much like osmosis.

      • The energy comes from the Sun. The concentrated brine has higher mobility ions. Even if ion bridge allows ions to move in both directions, the higher concentration on one side will send more ions down to lower concentration side purely by diffusion. At some point the concentrations should equal and the flow should stop but the low pressure pumps keep pumping out the water with altered concentrations and keep the ion gradient active all the time.
      • The brine is created from a portion of the sea water. They use the sun to heat it and evaporate some of the H2O. This is the primary energy input.

      • by whit3 (318913) on Saturday October 31, 2009 @05:40PM (#29937255)

        More important than the cost is the question of effectiveness.

        In their diagram, they have this schematic in the critical location:

        [Salt water]<----(+)----[Brine]----(-)----->[Salt water]

        Chemically, that "equation" just doesn't balance without an input of energy.

        The article DOES explain this, the salt imbalance makes
        a kind of battery.

        It's brilliant! Solar energy concentrates a brine, which
        then (just as dissimilar metals make a thermocouple)
        causes current and builds an electric potential
        when connected via a membrane (impermeable
        except to Na+ ions) to a less-concentrated brine.

        So, the difference in concentration of ions between two
        channels results in a diffusion from more-concentrated to
        less-concentrated, OF A CHARGED ION. That means
        electric current flows, until the charge buildup raises
        the electric potential enough to stop the diffusion.

        The solar input concentrates the brine, the resulting
        (small) voltage then is electrically applied to the to-be-desalinated
        channel, and (in the absence of a concentration difference)
        the electric field causes the ions to leave the
        to-be-desalinated stream.

        Thus, it's a solar-concentration-of-salt that makes
        the desalination occur. The electricity caused by the
        diffusion is active ALL NIGHT until the concentration
        of salt goes down, so the concentrated brine is
        an effective load-leveling device for the whole plant.

        The 'electric input' part of the process is entirely for
        pumping the brines around, so it can be a small fraction
        of the brute-force desalination energy requirement.
        Heck, you could use wave or wind power for that.

        Solar collectors for this kind of gizmo are just open-air
        trays of brine. Can't get any more cost-effective than
        THAT.

      • by Rich0 (548339) on Saturday October 31, 2009 @05:42PM (#29937271) Homepage

        You missed the other side of the reaction. It is charge neutral because the +/- charges in the two side pools are then balanced with -/+ charges from the water to be desalinated.

        The charges flow apart in the first place because the central pool is highly concentrated - so it contains far more + and - charges than anything else in the system.

        This kind of approach would never yield completely drinkable water, but that isn't the point. The goal is to get rid of a lot of the ion load before using more expensive processes to get rid of the rest.

        Desalination is a marvel of process optimization. Multiple stages of purification are used - each one being more expensive than the last but more effective. The early steps get rid of a huge mass of dissolved matter for dirt cheap, so even if their product isn't drinkable it GREATLY reduced the cost of the later stages.

        If you don't care about cost then desalination is trivially easy. Just run any kind of water you like through a H+ exchange resin followed by an OH- exchange resin, and then run it trough activated charcoal. The resulting water will be as clean as clean can be and the system would be remarkably simple. The catch is that those resins cost a small fortune to make, and if you run seawater into them then they're probably going to last all of 5 minutes. It might be a good approach for a camper to use to obtain water (the resin is a lot lighter than the amount of water that it could clean), but it is not a cost-effective method overall. Also - the purity it would achieve would be massive overkill. This is drinking water - we're not manufacturing CPUs.

      • by Timothy Brownawell (627747) <tbrownaw@prjek.net> on Saturday October 31, 2009 @05:50PM (#29937305) Homepage Journal

        [Salt water]<----(+)----[Brine]----(-)----->[Salt water]

        Chemically, that "equation" just doesn't balance without an input of energy. It doesn't matter what kind of "ion bridges" they put into place between the brine and salt water reservoirs, or what the concentration of salt exists in the brine or salt water, it will require some energy to offset the entropy increase.

        This is exactly backwards; energy input is required in order to decrease entropy of (part of) a system. Entropy increases come for free. Consider if you fill one half of a fish tank with fresh water and the other half with brine, do you get a fish tank full of somewhat salty water or do the fresh and salt water separate out?

    • by MoellerPlesset2 (1419023) on Saturday October 31, 2009 @05:49PM (#29937299)
      The key piece of the work is an ion bridge.

      No, the key piece of work is the idea. Ion bridges have been around forever.

      This has to permit the travel of one kind of ion but not the other, i.e. Na+ or Cl-. Looks like this material could be expensive.

      So you use, for instance, a polymer electrolyte (ionomer) with negatively charged side-chains for one bridge and a polymer with positively charged side-chains on the other. Only the counterions are mobile. The article says they're using modified polystyrene. This is not new, or terribly expensive. Similar things are already being used in industrial desalination technology for ion exchange columns.

      It might plug up need to be periodically replaced.

      Plug up with what? You naturally would have a mechanical filter to keep the crap out. It's not a major problem.

      How expensive these are? How non toxic these are? What is needed to manufacture them? These are the questions we need to ask.

      No, they're the questions asked by someone who doesn't know s--t about chemistry/chemical engineering. I happen to have a degree in the subject, but damnit, I learned about (used, even) polymer ion exchange columns in high school. If you want answers to your questions, go get Coulson & Richardson or some other chemical engineering textbook, and find the relevant section.
      This technology is certainly very clever, but it does not make use of any new technology. The only question I think is worth asking here is whether or not it turns out to be more efficient or not.
      • by lawpoop (604919)

        The only question I think is worth asking here is whether or not it turns out to be more efficient or not.

        I detect a little unnecessary redundancy there.

      • by AcidPenguin9873 (911493) on Saturday October 31, 2009 @07:01PM (#29937705)

        Not everyone reading Slashdot has a degree in chemistry or chemical engineering. I appreciated OP's questions since I had the same ones. I appreciate your answers but not the attitude that I had to endure when reading your post.

        Plug up with what? You naturally would have a mechanical filter to keep the crap out. It's not a major problem.

        You answered the dumb question but failed to answer the smarter one. Does the ion bridge ever somehow lose its effectiveness after a good amount of use? If it does, it will need to be replaced. How often does this happen? How much water can one of them desalinate before needing replacement? If it never needs replacement because of *use* (not mechanical crap getting in the way), then that's great, but I don't know the answer. Again, I do not have a degree in any of this stuff, so please enlighten me.

        • by nutshell42 (557890) on Saturday October 31, 2009 @08:17PM (#29938051) Journal
          Not everyone reading Slashdot has a degree in chemistry or chemical engineering. I appreciated OP's questions since I had the same ones. I appreciate your answers but not the attitude that I had to endure when reading your post.

          The attitude of the GP was the problem. "These are the questions we need to ask", as if they were non-obvious and revolutionary. Whenever there is a post about an invention on /. the easiest way to get "+5 (Group-Wank)" is to write that it will never work because the inventors overlooked an issue a drunk chimpanzee could come up with. Then a thread ensues where everyone congratulates themselves on saving the world yet again.

          You are right, the GP's questions were interesting and should have been answered in the article (which is for laypersons) and because they weren't it's good that someone answered them here on /.

          The problem is that the GP posed the question in a way that implied he knew what he was talking about and was making a statement about the invention, instead of admitting that he had no idea and was asking for clarification. jm2c

      • by drinkypoo (153816)

        This technology is certainly very clever, but it does not make use of any new technology. The only question I think is worth asking here is whether or not it turns out to be more efficient or not.

        If it's miniaturizable technology and sturdy enough to be permanently mounted on the deck of a ship unlike this [landfallnavigation.com] then I think it's got a future.

  • From the way they describe it, it sounds a lot like Maxwell's Demon. Since there is energy going into the system, however, it's clearly not that.
  • Reverse osmosis? (Score:2, Interesting)

    by macraig (621737)

    Isn't this a large-scale demonstration of the same principle used in home reverse-osmosis systems? It sure sounds familiar.

    • by stevelinton (4044) <sal@dcs.st-and.ac.uk> on Saturday October 31, 2009 @08:14PM (#29938041) Homepage

      No. It does look a bit similar but it isn't. In reverse osmosis the water has to pass through the membrane, driven by high pressure pumps, leaving its impurities behind.

      In this version the impurities pass through the membrane (two separate membranes in fact) driven by an electrical current. Cleverly, the electrical current itself is generated by the salt passing through other membranes out of the highly concentrated brine that you made in your solar ponds.

  • ... is if it would be possible to combine solar-thermal power generation with desalination.

    Build Fresnel-lens solar concentrators and stick them near a source of seawater. Boil the water using the sunlight, and use it as the working fluid in an ordinary steam-turbine-type power generator. But instead of recycling the same water once the steam recondenses, realize that you've just made a giant distiller: drink the water and use "new" seawater.

  • So I read the thing.

    The process concentrates sea water to brine by an evaporation method. So why waste this low grade stream it is still has high in moisture content. There is already a condenser in this system. I'm thinking this can somehow boost output of clean water.

    • by tsotha (720379)

      I think the answer is cost. Near where I live a company makes table salt. They have acres and acres of these shallow ponds, and when the tide comes in they open the valves, filling up the ponds. Then they close the valves and wait for a few days for it all to dry into this muddy slush that gets scooped up by heavy machinery and, I hope, purified.

      Aside from the land itself the entire operation is so cheap it's almost free. Sure, you could somehow trap that water vapor and cool it enough to change it bac

  • This idea of ion bridges has been around a long time. The application here is basically misrepresented. All it is doing is replacing a small amount of commercial electric power with solar-generated potentials. But the process isn't feasible when run on commercial power, even if the power is free, so replacing the commercial power with solar (the germ of the "idea" here) is just disguising the dead horse. Reminds me of the algae gambit: the solar constant crossed by photosynthesis is dismal, so no biofue
  • Just like it takes energy to desalinate water the opposite is also true: energy can be produced from salinating water.
    See http://en.wikipedia.org/wiki/Osmotic_power [wikipedia.org]

    What this system does is to concentrate seawater by evaporation and then use the salinity gradient between that concentrated brine and normal seawater to produce energy. This energy is then used to desalinate another stream of seawater. In principle, there is no reason to use this energy specifically for desalination. It could also be fed to the

  • I've long wondered about a few things when it comes to desalination, desert areas etc.

    If it's cheap to make water with this process and remove the salt - would it be a good idea to create a huge bunch of these machines in desert-areas, pumping desalinated water into fields to promote vegetation to grow, fighting back the desert? I would think this wouldn't require the water to be further cleaned, after removing the salts..

    Am I completely off my rockers, or is it a maybe-sort-of workable idea?

  • This process depends on evaporation to concentrate the brine. Can someone more familiar with the process costs explain why it's cheaper to use ion exchange rather than distilling the water that was evaporated?

    The source of the energy to evaporate the water is irrelevant, solar works just as well for either process. Assuming an essentially unlimited supply of seawater for cooling to the distillation step, I don't see how they can make enough concentrated brine to filter the seawater more cheaply.

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