Could we make a deep sea elevator similar to a space elevator?
And if so, could we use deep sea pressures to facilitate cheaper desalination?
Why is desalination so difficult?
261–270 of 644 posts
Re: Why is desalination so difficult?
#262Earlier quoted context omitted.
Its always been my understanding that any treatment to remove stuff from water is going to produce waste which needs disposing, just look at the Brita water jug filters, they need disposing. I've often wondered why dont we have more pure water pumped through the water mains in various countries, and I think after reading about Super K the Japanese Neutrino detector [1] and how the water in the tank was so pure it had…
> the water in the tank was so pure it had dissolved a spanner/wrench that was left in the bottom "Apparently somebody had left a wrench there when they filled it in 1995," he said. "When they drained it in 2000 the wrench had dissolved." I dunno, I think if you left a wrench soaking in regular water for five years there wouldn't be much left of it after that either.
What's the process, though? The "dissolving" is presumably rusting and then motion of water washing away the rust, but rust requires an oxygen source for the chemical reaction, and apparently Super Kamiokande has dissolved oxygen specifically removed using a vacuum degasifier to prevent interference and growth of bacteria.
I'm not feeling particularly convinced by this anecdote. It sounds a bit urban legendy. Still, I won't claim more than a high-school knowledge of Chemistry so I'm eager for someone to correct me and supply an explanation.
Re: Why is desalination so difficult?
#263I had a dream the other day that we started generating electricity with steam engines using ocean water, then collected the steam for potable water
Re: Why is desalination so difficult?
#264Is it just me, or did this article dance around the question? I am not a physicist but let me give it a stab: except for a few specialized steps like UV or oxidizing heavy metals, most filtration is mechanical. A series of filters with smaller and smaller pores capture more and more of the mess in the water like bacteria and particulates while UV breaks down viruses, the oxidizer precipitates out metals, and so on. N…
This is correct.
But we seem to have colossal amounts of essentially free solar energy, and that energy already evaporates large amounts of sea water. We just don't capture it well.
Imagine building a pipe that stands above shallow tropical coastal waters. Make the bottom of it into an almost flat funnel to cover more water surface, using transparent plastic or even glass. Now all the evaporated water and hot air go into the pipe.
Build the pipe a kilometer tall. Humans have adequate technologies already, and the pipe does not need to be bearing much internal load, unlike Burj Khalifa or World Trade 1.
At 1km, the air is cool enough. The hot air will shoot upwards, cooling on its way up and releasing fresh water. Lightweight collector pipes will bring it down into a reservoir. The remaining dampness of the air will help it produce clouds, and thus shadow, over the land.
With a tall enough pipe, we could even generate electricity by putting a turbine inside.
Why are we not building it? It's expensive, and most (sub)tropical countries that lack water are poor. They are also politically unstable, and such an installation would be a high-value military and terrorist target.
Maybe Singapore or Dubai would some day dare and build it. (California, unlikely; it would never pass an environmental review.)
Re: Why is desalination so difficult?
#265Earlier quoted context omitted.
There are potentially some good reasons to just spray the brine into the air. Seawater sprayed into the air becomes tiny salt crystals, which in turn help clouds to form, and cause increased rainfall. The rain produced has negligible levels of salt. In places with dry climates, this often can turn desert land into farmland across an area hundreds of kilometers wide.
It might just cause earlier rainfall, not more of it. There's x amount of water in the atmosphere, and you can't add more by spraying salt crystals.
You can though. The amount of precipitation (averaged over a long enough period of time) is inversely proportional to the amount of evaporation and other water entering the atmosphere (averaged over a long enough period of time). Note that the two things being compared are _rates_ not _masses_. If all you do is cause water to fall sooner then:
1. The humidity in the air drops, increasing evaporation rates because of the lower partial pressure of water vapor in the air.
2. The humidity on the surface increases (dusty areas becoming moist, plant leaves uncurling to expose more surface area for other processes but incidentally increasing evaporation rates, reservoirs having more surface area, ...), increasing the evaporation rate.
There are limits of course, and that back-of-the-napkin analysis ignores 2nd-order changes in temperature and all of the other hairier bits of climate modeling, but it illustrates that things are more complicated than they appear anwyway.
Edit: "inverse" here just meaning a multiplication by -1
Re: Why is desalination so difficult?
#266Earlier quoted context omitted.
You know this is how the Phoenecians became the dominant culture, its also where the term "salary" comes from and "worth his weight in salt" -- as salt was the only known preservative of the massive amounts of Tuna the phoenecians were catching and shipping throughout the mediterrainian - and made them a super-power - they had control of the preservance of food over shipping distances... Salt was used as money. EDIT:…
> worth his weight in salt Never heard this saying before. I've heard "worth his weight in gold".
"worth his salt" is common, at least.
Re: Why is desalination so difficult?
#267Earlier quoted context omitted.
In other words, you want to salt the land? Isn't that what marauders do when they want to destroy an area permanently?
seems you may as well put nuclear power plants in the desert
Re: Why is desalination so difficult?
#268Is it just me, or did this article dance around the question? I am not a physicist but let me give it a stab: except for a few specialized steps like UV or oxidizing heavy metals, most filtration is mechanical. A series of filters with smaller and smaller pores capture more and more of the mess in the water like bacteria and particulates while UV breaks down viruses, the oxidizer precipitates out metals, and so on. N…
> As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of.
There are no pores, so to speak. Polymer materials form amorphous solids with transient voids which open and close randomly due to thermal motion. They're not "pores" because they aren't permanent over long time scales. Rather, the polymer+water is modeled as a single fluid phase, the same as if you were modeling ethanol+water. The fact that the polymer is a "solid" doesn't affect the fact that it's actually a tangle of vibrating molecules just like any other mixture.
Other materials do have well defined pores, like MOFs and zeolites. In this case, the water does sorb as a liquid in the pore space, but is gated by transport between the pores in a similar manner.
This is made apparent because water does enter into polymers (even those which desalination) freely, with or without the presence of salt. It is not the case that "the pores are too small for water to cross normally". I can take a polymer that will swell with 50% of its own weight in water, and which has no "free" liquid water (as evidenced by the inability of the water in the polymer to form ice), yet make it reject >90% salt at very high pressures (>3000 psi). If you just let salt water sit on one side without pressure, salt and water will make their way through non-selectively. So it can't be that the water is being physically sieved from the ions to enter into the membrane. Rather, the pressure creates a change in the activity of water (due to the mechanical forces acting on the polymer near the low pressure/support material interface). Since the water is more soluble and more mobile in the polymer, it transports at a more rapid rate than the salt, resulting in desalination.
Re: Why is desalination so difficult?
#269Earlier quoted context omitted.
If people needed as much salt as was contained in the water to begin with, we wouldn’t need to remove it in the first place.
No reason to keep the 1:1 ratio. Use the salt to replace our current salt mines/outtakes and then use the water as an addition to our current freshwater usage.
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Re: Why is desalination so difficult?
#270Earlier quoted context omitted.
The people who need that water tend to shed it after some time. Discarding waste water into the oceans via rivers is a huge idiocy. You essentially rely on the environment to "magically" sort it all out. Naively so and fraught with huge inefficiencies. Proper treatment of that waste in the sense of recovering usable matter streams is the logical way to go.
I know that generally speaking, disposing of waste in the ocean and expecting it to disperse enough to be harmless is foolish and wrong. But in the case of salt, It would seem to me that the ocean can handle that amount of salt. Course, I haven't done the math. But it would seem to me that the back in == salt taken out. We'd only be changing the net salinity by the amount of water subtracted. Without having done the…
The problem with brine from desalination is that it kind of behaves like a heavier liquid, sinking to the bottom. That causes it to stay together, taking longer to mix with the regular ocean; and the coastal seafloor there is a lot of life that doesn't appreciate water with double the salinity of regular ocean.
To solve that you can just dilute it more, either mixing with some other waste water stream or by releasing it over a larger area rather than a single outlet.