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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#391

Post by lobocinza »

singleshot_ wrote: post #228
londons_explore wrote: post #218

Earlier 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.

Surely the salt falling on the farmland would have some kind of deleterious effects, right? I imagine you’re not trying to grow sea cucumbers in this scenario, and I’m not sure most land plants would be crazy about a ton of salt. Just ask the Carthaginians… Guessing that you know something I don’t here, though.

This is what happened with Aral Sea.

Re: Why is desalination so difficult?

#392

Post by enjoylife »

ktta wrote: post #366
Nifty3929 wrote: post #343

Earlier quoted context omitted.

Residential water use is nothing. In California, where we have persistent water shortages, residential, commercial and industrial water use ( including all landscaping, golf courses, etc) all put together still only amount to 10-20% of overall water use, depending on rainfall

Is the only remaining use left agricultural?

Looks like on average the data shows 42% agricultural. https://water.ca.gov/Programs/California-Water-Plan/Water-Po...

Re: Why is desalination so difficult?

#393

Post by niemandhier »

superq wrote: post #328
lisper wrote: post #88

Earlier quoted context omitted.

You've pretty much nailed it except for one minor nit: > It takes less energy but produces a concentrated liquid waste stream that must be disposed of. This implies that creating a concentrated waste stream is a problem unique to reverse osmosis. It isn't. No matter what you do you're going to end up with a bunch of salt that you have to get rid of somehow.

The Israelis ran into this problem with their mineral extraction in the Dead Sea, so they're bulldozing the dry salt waste to build a physical wall for border security (more than 10 meters high) that's apparently hard to climb: https://www.youtube.com/watch?v=tgTheUjeDlg

That will make the region hostile to life for 1000s of years even if climate patterns were to change.

Re: Why is desalination so difficult?

#394

Post by gabereiser »

theresistor wrote: post #257
gabereiser wrote: post #254

Earlier quoted context omitted.

It’s important to note that the energy needed for RO to work is due to the high pressures needed to ram that little H2O molecule through that virgin nanohole. 700-900wh for a trickle of 14gal/h. At least that what I’m getting on my sailboat.

That's quite inefficient if you're using a Clark pump. I'm currently spending about 1000W to making ~40G/h with a Schenker Zen150.

Blame SeaWaterPro for the underpowered HPP.

Re: Why is desalination so difficult?

#395

Post by [deleted] »

lagolinguini wrote: post #157
cyberax wrote: post #31

Earlier quoted context omitted.

The thing is, it doesn't have to be. The energy released by mixing salt into water is small, around 3.9kJ/mol. Molar mass of salt is 58g/mol, and the average sea water salinity is around 3.6% So a cubic meter of sea water will have 1000*0.036/0.058=620 moles of salt, and it'll require 2.4MJ of energy to remove the salt in a perfect desalinator. In more common units, 2.4MJ is about 0.75 kWh. Around here electricity is…

Correct me if I am wrong as I am not a physicist. I see a point that is important to consider, that you have potentially overlooked. First, you assume that dissolution of salt is a completely reversible thermodynamic process, which is fine. But considering it a reversible process, in order to reverse the process we need to do a certain amount of work which you have calculated. In order to do work we need an engine. T…

[deleted]

Re: Why is desalination so difficult?

#396

Post by im3w1l »

codedokode wrote: post #312

The author have mentioned that water with more than 1-2 promille of salt is considered not drinkable. But I have a bottle of carbonated mineral water from the mountains and the label says it contains 6-9 g of salts per 1 liter. So I guess it depends on which salt it is. Also, the author mentions that people in US use 1100 liters per day (which is too much in my opinion), but not all this water needs to be drinkable,…

Interesting. Maybe we could use an ion-exchanger to swap the salt for another one that the body is more tolerant of.

Re: Why is desalination so difficult?

#397

Post by refurb »

niemandhier wrote: post #393
superq wrote: post #328

Earlier quoted context omitted.

The Israelis ran into this problem with their mineral extraction in the Dead Sea, so they're bulldozing the dry salt waste to build a physical wall for border security (more than 10 meters high) that's apparently hard to climb: https://www.youtube.com/watch?v=tgTheUjeDlg

That will make the region hostile to life for 1000s of years even if climate patterns were to change.

It’s literally meters away from the Dead Sea that has salt crystallizing on the beaches.

Ain’t changing anything in the environment.

Re: Why is desalination so difficult?

#398

Post by refurb »

wongarsu wrote: post #270
daveslash wrote: post #216

Earlier quoted context omitted.

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…

Double the salinity? Other comments suggest it’s only slightly more salty than sea water.

Only a very small fraction of water is isolated from a very amount of sea water.

Re: Why is desalination so difficult?

#399

Post by singleshot_ »

londons_explore wrote: post #235
singleshot_ wrote: post #228

Earlier quoted context omitted.

Surely the salt falling on the farmland would have some kind of deleterious effects, right? I imagine you’re not trying to grow sea cucumbers in this scenario, and I’m not sure most land plants would be crazy about a ton of salt. Just ask the Carthaginians… Guessing that you know something I don’t here, though.

I believe that when every spec of salt creates a far larger water droplet - and the overall concentration in rain ends up tiny. If anything, doing lots of this might reduce the salt concentration in soils, due to increased rainfall.

I guess what I’m forgetting is that the water has a chance to wash salt away back to the ocean. But how much salt does one drip of water absorb? What if the spec of salt that starts in the water droplet is as much as the droplet can wash away?

I suppose it must be slowly becoming clear that I have no idea what I’m talking about (save for a nickel worth of Roman history).

Re: Why is desalination so difficult?

#400

Post by mcv »

SoftTalker wrote: post #346
rs999gti wrote: post #278

Earlier quoted context omitted.

Can't the salt just be dumped in the desert?

Just pump it back into the ocean. The little bit of concentrated brine from a desalination plant will be quickly diluted; the ocean contains a LOT of water.

Bury it deep below the surface. In old salt mines, perhaps.
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