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A Clever New Approach To Desalination

Posted by kdawson on 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."

17 of 128 comments (clear)

  1. Re:Anyone else think... by bcmm · · Score: 5, Informative

    anyone else think this looks suspiciously like: http://en.wikipedia.org/wiki/Perpetual_motion

    Yeah, pretty much, for all practical purposes, but not quite, because sooner or later the fucking sun will in fact burn out.

    You didn't need to read TFA. It's in the summary. Second sentence.

    Saltworks uses solar energy or waste heat

    --
    # cat /dev/mem | strings | grep -i llama
    Damn, my RAM is full of llamas.
  2. Re:Maybe by Timothy+Brownawell · · Score: 5, Informative

    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.

  3. Vancouver saves the world? by Yergle143 · · Score: 3, Interesting

    OK between this and the General Fusion guys http://www.generalfusion.com/ Canada has got water and energy completely licked. http://www.saltworkstech.com/ OK actually I'm still trying to run the numbers on the both of them (and waiting for some peer reviewed publications.)

  4. Re:Maybe by samkass · · Score: 4, Funny

    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!"

    --
    E pluribus unum
  5. Re:Maybe by klaun · · Score: 4, Informative

    The ions are not a substitute for electrons, they're the source. There is no electricity without electrons. :)

    Electricity is the flow of charge, not electrons.

    If your statement was accurate, your computer would not work as it depends upon semiconductors which function in part based on the flow of positively charged holes in the electron structure of the material. (see p-n junctions, etc.) The Hall effect can be used to verify the charge of the moving carrier within a current. It can be either positive or negative.

    Note that this desalinization mechanism works very similarly to a fuel-cell which also involves ion flow as part of an electric circuit.

  6. Re:Anyone else think... by Jah-Wren+Ryel · · Score: 4, Funny

    Yeah, pretty much, for all practical purposes, but not quite, because sooner or later the fucking sun will in fact burn out.

    Or get bought out by Oracle after giving away all its energy for Free.

    --
    When information is power, privacy is freedom.
  7. ion bridges cost? Consumable? by 140Mandak262Jamuna · · Score: 3, Insightful

    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.

    --
    sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
    1. Re:ion bridges cost? Consumable? by whit3 · · Score: 3, Informative

      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.

    2. Re:ion bridges cost? Consumable? by Rich0 · · Score: 4, Informative

      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.

    3. Re:ion bridges cost? Consumable? by MoellerPlesset2 · · Score: 5, Informative

      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.

    4. Re:ion bridges cost? Consumable? by Timothy+Brownawell · · Score: 3, Informative

      [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?

    5. Re:ion bridges cost? Consumable? by AcidPenguin9873 · · Score: 4, Insightful

      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.

    6. Re:ion bridges cost? Consumable? by nutshell42 · · Score: 5, Insightful
      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

      --
      Don't think of it as a flame---it's more like an argument that does 3d6 fire damage
  8. Re:Maybe by AdamHaun · · Score: 4, Insightful

    No, it's not inaccurate, unless you're claiming that protons don't have a charge. The ions here are nothing like wires. In a wire, the atoms (nuclei and nonconductive electrons) are fixed in position while the conduction band electrons are free to move from atom to atom. But in this desalinization process, the nuclei themselves actually move -- that's what makes it desalinization. The sodium and chlorine ions are true charge carriers. Ion conduction is not uncommon. Here's some more info on that:

    http://amasci.com/amateur/elecdir.html

    --
    Visit the
  9. Re:Maybe by maxfresh · · Score: 5, Informative

    Be that as it may, atoms are not ions, which is what the attempt at an article states.

    The article doesn't state that atoms are ions. Rather, it states that ions are electrically charged atoms, which is totally correct. Here is the exact quote, in context:

    As the diagram explains, what Mr Sparrow and Mr Zoshi create by doing this is a type of electrical circuit. Instead of electrons carrying the current, though, it is carried by electrically charged atoms called ions.

    Salt is made of two ions: positively charged sodium and negatively charged chloride. These flow in opposite directions around the circuit. 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. Sodium and chloride ions pass out of the concentrated solution to the neighbouring weak ones by diffusion though these bridges (any chemical will diffuse from a high to a low concentration in this way).

    I don't find any incorrect statement in the above quote regarding ions.

  10. Re:Maybe by gandhi_2 · · Score: 4, Funny

    True. Too bad greenhouses are impossible.

    --
    THL phish sticks
  11. Re:Reverse osmosis? by stevelinton · · Score: 4, Insightful

    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.