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treadwater

put seawater in drum, pull drum on long walk, obtain fresh water.
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looking at those pull along water drums, I wondered if desalination was possible.

The mechanism would be two chambers separated by a osmotic membrane. One of the chambers would be a large threaded piston. This chamber would be weighted or sprung so as to wind the piston on the walking rotation of the drum. Seawater is placed in this chamber, pure water collected in the other side. Appropriate gearing could be used but this idea would totally be down to whether the energy numbers stack up.

Oh, probably not for the boat, more for long walks on a desert island.

wjt, Sep 05 2021

Mellon-baked https://www.cmu.edu...-barrel-filter.html
[pertinax, Sep 05 2021]

RO Desalination https://www.science...nce/reverse-osmosis
800 - 1000 psi [Frankx, Sep 05 2021]

Hand-powered water desalinator https://dkutenx65dk...550277Datasheet.pdf
"Katadyn SURVIVOR 06" [Frankx, Sep 06 2021]

Yacht-sized desalinator https://www.rainman...lectric-watermaker/
1250W, output of the order of 100 litres per hour. [Frankx, Sep 06 2021]

Reverse osmosis - apparently contravening laws of thermodynamics Supersimple_20reverse_20osmosis
[bungston] May 07 2004 [Frankx, Sep 09 2021]

The Perpetual Salt Fountain https://www.science...56900958?via%3Dihub
[a1, Sep 09 2021]

[link]






       //looking at those pull along water drums//   

       Could you provide a link so that we could look at them, too?
pertinax, Sep 05 2021
  

       Good idea - I don’t know if it exists or not. Years ago on “Dragons’ Den” there was someone with a very similar concept to this. It was for refugee camps and similar places where there is often no source of clean water. Someone could collect water from a dirty or polluted water source and then drag the drum back to the camp and, by the time they got back, the rotation of the drum and the action of the filters etc. inside the drum would have purified the water
hippo, Sep 05 2021
  

       So I looked for a link myself and, apart from ads, the first thing to come up was [link].   

       This is certainly baked.
pertinax, Sep 05 2021
  

       Hmm nice idea! [hippo] - Hippo Roller - are you being unduly modest?
Frankx, Sep 05 2021
  

       If you're on a desert island you're much better off with a more traditional design that doesn't require the expenditure of calories. Or if you're spending calories pumping the water to and fro, one that doesn't also need you to move yourself to and fro. [+/-]
Voice, Sep 05 2021
  

       As [pertinax] says, baked - except maybe that the linked article mentions only filtration and not desalination. Is it a simple thing to add reverse osmosis membrane to the filtering?
a1, Sep 05 2021
  

       //not desalination// True - I think the desalination aspect is enough to label this as a new idea. For reverse-osmosis desalination you need a fairly high pressure ~ 800 psi [link], so you’ll need a mechanically driven pump, driven by the rolling motion. Not particularly difficult to derive.   

       Would need prototyping to find whether flow rates were practical. It might need to be a *rather long* walk.
Frankx, Sep 05 2021
  

       That’s around 5 or 6 MPa
Frankx, Sep 05 2021
  

       As others have noted, the pressure require for desalination is MASSIVE, so this will either take a long time & lots of energy, or only do a small amount at a time.
neutrinos_shadow, Sep 05 2021
  

       [link] So, it's certainly feasible. That one provides only 0.89l/hr at 40 pumps per minute. It weighs just over a kilo, but costs £1400. I'm guessing that means RO membranes are expensive, although there's probably a hefty mark up on low-volume yachting survival equipment.   

       In terms of the roll-along RO desalinator - yes, feasible. Manual-powered, you could play with mechanism design, gearing etc to optimise output.   

       A yacht-sized 1200w unit looks to be able to produce 100l/hr [link].   

       An adult of good fitness is likely to average between 50 and 150 watts for an hour of vigorous exercise.   

       Perhaps then 5l/hr as an output for "vigorous exercise".   

       Looking at economics, it's going to be challenging to come up with a design that's adequately robust, simple in construction, and affordable in the kinds of context where lack of clean water is life-critical. Perhaps most suitable in disaster relief efforts where [over £1000 per unit] might be acceptable.   

       I seem to remember RO membranes are susceptible to poisoning by some common water pollutants, so again, usage context might dictate design changes.   

       Finally, it's only going to be useful in those areas where seawater is near. So, as you say - desert island.   

       But yes, good idea [+]
Frankx, Sep 06 2021
  

       Your drum is also going to have to be rather heavy to hold the pressure, and the gearing needed to generate that pressure.   

       I think, in most places where sea water is readily available, solar distillation is a better option.   

       If you happen to have a brine spring significantly uphill from your settlement, than the self delivering version might might have an application.
MechE, Sep 07 2021
  

       // If you happen to have a brine spring significantly uphill from your settlement //   

       Make your own hill. Someplace by the coast, sink two deep* wells next to each other. Link them at the bottom with a reverse osmosis system between them. Pump salt water into the top of one - let it pass through the membrane and seek its own level to fil up the other from the bottom up. It won't come ALL the way back up to the top of the desalinated well, due to the pressure drop from the RO process.**   

       * How deep depends on how much pressure you need the water column to have to drive the RO.   

       ** And therein lies the flaw. Even though the fresh water comes almost to the top, you still have to use some energy to pump it up the last part. Nothin' is free.
a1, Sep 07 2021
  

       Filtration seems Baked, that's fantastic. Hope it has vital function function in the world and those whom worked on it get the credit. Sounds like it is just waiting for an osmotic membrane advancement.   

       The photo says concept but I would argue that having a thing, it has been prototyped. Well, unless the photo has been doctored.
wjt, Sep 08 2021
  

       A sea water drinking Stranndbeest?
wjt, Sep 08 2021
  

       Howabout another method? Would converting that many calories to steam do the trick?
RayfordSteele, Sep 09 2021
  

       [a1] - //Even though the fresh water comes almost to the top, you still have to use some energy to pump it up the last part. Nothin' is free//   

       Actually, in [bungston]'s Supersimple Reverse Osmosis this was discussed at great length, much calculation done, and generally agreed that, given a sufficiently deep ocean you could have an "energy free" freshwater fountain. Many didn't like this, because it looks like a perpetual motion machine. It's not, but it looks like one. And the depth of ocean required - I think 24km was calculated, but others have said 11km.
Frankx, Sep 09 2021
  

       I don't like the prejudice against perpetual motion. "Perpetual" must imply "infinite" to some idealists, or the word must be meaningless. An otherwise brillant system may break down when the earth's gravity or its oceans disappear, but so what? There really needs to be a naming system to describe degrees of perpetual.
4and20, Sep 09 2021
  

       I think it's normally accepted to mean a hypothetical device which has an energy output but no energy input, breaking the laws of thermodynamics.   

       In that sense a hypothetical "perpetual" machine could run for an unlimited duration without energy input (assuming a constant surrounding environment, ignoring wear and tear etc)   

       Devices that are demonstrated claiming to be "perpetual energy machines" might have, for instance, very low friction bearings and high-energy flywheels; might have a deliberately-hidden energy source (perhaps batteries); or might be using a novel or poorly understood source of energy - like the freshwater fountain.
Frankx, Sep 09 2021
  

       [FrankX], thanks for the older link. Many of the annos point out the same issue I did here. Anyone who thinks it would work (whether or not it’s “perpetual motion”) are overlooking the energy required to pump the water out of the desalinated side. At a very minimum, that’ll be at least as much as the amount of pressure used to force the water through the RO membrane, plus friction, pumping losses, etc…   

       Your suggestion of a brine source on top of a very tall hill and letting gravity do the work may be the only way to go. But it has to already be there and fed / replenished by some natural process. Do you know of any place in the world?
a1, Sep 09 2021
  

       //overlooking the energy required to pump the water out of the desalinated side//   

       ...that's the funny thing (which caused so much disagreement).   

       If you had a sufficiently deep ocean (and it was sufficiently mixed in terms of salt concentration), the fresh water would actually 'spout' out of the desalinated column without any pumping.   

       Cool, huh?
Frankx, Sep 09 2021
  

       // if //   

       A lot hinges on that word. But it really wouldn’t “spout” from any configuration you could actually build.
a1, Sep 09 2021
  

       Yes, it could. The trick is the difference in density of fresh and salt water at an equal temperature. If you run a pipe 10 or so km deep, the difference in the weight of the water column between the salt side and the fresh side is sufficient to perform reverse osmosis and spout the fresh out the top of your pipe.   

       In still water, this stops work rapidly, because the salinity of the salt side increases until equilibrium is reached, and the energy recovered in water flow is less than required to sink the membrane. But in a free flowing situation with active currents that both maintain salinity and flush the membrane, it can be ongoing.   

       Of course there are some complicating factors with pressure and salinity with depth, so the actual numbers may be deeper than that 10km. Of course if you go deep enough, it warms back up again. So put the Kola borehole at the bottom of the Marianas trench, and you've got a continual fountain.
MechE, Sep 09 2021
  

       Oh, there's that "if" word again, coupled with the ever popular "the trick is..." disclaimer.   

       Build it. You don't even need a 10KM pipe- Stommel's original proposal in 1953 suggested you might only need to go 2KM dep and he even presented a lab experiment setup you could try at home with some glass tubing and coloured water. <link>   

       Have you ever come across a writeup from anyone actually DOING the experiment (on table-top or other scale) as opposed to numerical modeling and/or saying "it should be possible to..."?
a1, Sep 09 2021
  

       Since RO membranes are weird, would an "overpressure" on the salty side result in more pressure on the fresh side as well, or would the membrane block up & only let the fresh water get to a particular pressure & no more?
neutrinos_shadow, Sep 09 2021
  
      
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