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RO-on-a-Rope
A semi-permeable membrane on a vessel held at depth on a weighted rope. | |
If you had one of these on your life raft you could lower it to a depth that would replicate the pressures created by surface RO systems at great expense, and use the differential to force fresh water from the sea.
The AI bot sez: “The ideal water pressure for most reverse osmosis (RO) systems typically
ranges from 40 to 80 psi, with 60 psi being optimal for efficient filtration. Insufficient pressure can lead to slow water production and reduced system performance.” I don’t know how much to trust the bot yet.
Not being too concerned with high volume; just a pint or so a day at a minimum.
You only need to quadruple the 14.7psi surface pressure to get an ideal 60psi and that would be at only 100.’ (Check me; math is not my forte.) More efficient membranes that work at MUCH higher pressures like municipal scale installations would be more efficient at greater depths, at the cost of rope.
The size of the orifice feeding the membrane(s) is TBA and will determine the output. It seemed so simple that someone must have thought of this before. They did at scale but not as a survival tool. If this rig could make 2L/day you could survive indefinitely, and 7-14 days on just a pint a day.
[Props to bungston’s pier pipe.]
Possible?
https://physics.sta...smosis-station-work
We'll see. [minoradjustments, Mar 23 2025]
Human water needs.
https://preppersapp...r-survive-each-day/
Just a little goes a long way. [minoradjustments, Mar 23 2025]
[link]
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(+) I have no input, but eagerly await the discussion. |
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60psi sounds a bit low...
The RO plant I worked on (industrial scale) was (IIRC; a while back now...) 70bar (~1000psi).
Still achievable though; 1bar is 10m so 700m; a fishing reel with 1km of line isn't un-heard-of or particularly large.
(Membrane technology may have improved since my involvement.) |
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[neutrino] I suspect the 60psi may be citing the delivery pressure from the common municipal supply, i.e. your tap pressure. Some of the stuff I've read cites 1200psi and above. Membranes are critical. |
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//use the differential// between what and what? |
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If you have a membrane at the surface, there is 1atm pressure on each side, so zero differential. |
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If you attach the membrane to a large and heavy rock, and lower it to 300m depth, there is still zero differential on each side. |
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One side has to be attached to a void that remains at low pressure. This could be a solid pipe up to the surface, or it could be a large pressure vessel capable of withstanding 30atm of pressure. |
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So we need: 300m rope <check> 5 litre pressure vessel with membrane on it = guess perhaps 10kg weight. That should be heavy enough to sink itself I guess? You should get a litre or so of fresh water into this before the air pressure rises enough to stop the osmosis? |
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Question: Why is this better than a long-handled high pressure pump to force the sea water through the membrane at the surface? |
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// I don’t know how much to trust the bot yet.// Given that the LLM text generator bot is pretty much the definition of a bullshit generator, I would suggest "not at all" is the appropriate amount. |
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Okay, here's what I have after chatting with Gemini. An RO filter needs to maximize surface area. A smooth sphere insert minimizes it, and also has no support against the vacuum. Instead what we need is a loosely folded membrane with a sponge inside it, like cacti hold water while maintaining structure against the pressure. This should have an external shape that roughly maximizes surface area. A shape like a christmas tree seems ideal: the fresh water can fill the sponge over time at the trunk. This can be brought up and drained at the "trunk". The outside of the device can be whatever shape best contains it. |
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//Why is this better than a long-handled high pressure pump to force the sea water through the membrane at the surface?//
fewer moving parts, no need for extra energy beyond lowering and raising it. The down side is the risk of catching on the bottom and the fact this device won't work in shallow water. |
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[pocmloc] It's a bottle with the membrane between the 1atm internal surface pressure and whatever pressure it experiences at depth. At 700M there's still 1atm inside until it starts to fill with fresh desalinated water. |
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I don't know what topology would be most efficient for this process. Maybe a double-walled bottle with the membrane on the outside of the inner porous one, to maximize surface area, per Voice. Maybe a tree, or a stack of plates. |
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The differential exists and it's free. We don't need no stinking handles. |
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For stuff the LLM comes up with that can be verified, they're great. They are getting more and more like humans in their ability to bullshit. Bullshitting is what separates humans from the rest of the animal kingdom. It's what conquered the Neanderthals and all the other nascent humanoids. That's what we're really afraid of. We've seen it before. |
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Bullshitting is Imagining |
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So not only lying or dissembling is possible (which we hate), but bullshitting also makes possible creative expression of all types, mathematics, music, visualization, and fiction (which we love). |
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It makes the halfbakery possible. Duh. |
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<More thinking>
Given that the vessel starts with an air-filled cavity, it will require a lot of mass (for we want a lot of water at the end) to sink it, against it's natural buoyancy.
That mass (+ the new fresh water) then needs to be hauled back up.
Has anyone done a quick check to see just how much energy is required to do so? It might be that the only advantage is time. |
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[neutrino] We only want a litre or two. I think the unit could be The size of a small fire extinguisher, stowed on the life raft or handy for abandoning ship. A pony tank for SCUBA emergencies will hold about 1.5L. |
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Whatever the final form of the RO pressure vessel, it will start at 1atm and as the pressure increases with depth, salt water will be pushed against the membrane. At great depth the desal water will be pushed into the vessel and start to replace the air at 1atm surface pressure. At some point the pressure across the membrane will equalize due to the fresh water pushing back against the membrane, and it will stop working. Haul it up, take a drink and drop it back. I'm thinking a real engineer could design it so that the weight and pressure would remain constant using the sea water in channels and silicone reed valves. |
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//fewer moving parts// Well, it depends if you consider the 300m line and reel a moving part. Certainly the lid with a 30atm pressure seal has to move otherwise you can't get at the fresh water to drink it. |
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//no need for extra energy beyond lowering and raising it// as already pointed out here and elsewhere, thermodynamically these are basically the same amount of energy are they not? Possibly the reeling in requires a greater amount of energy because you are also working to raise the pressure vessel as well as the fresh water. With a pump you merely pressurise the amount of water you need. |
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To be clear, I think this idea would work, but I think it is a stupid way to implement the desired result so instant [+] |
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I respectfully disagree. This replaces a complex pump with a line and reel. It still takes a lot of energy, but it's a lot simpler and therefore more efficient. |
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