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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#121

Post by emptybits »

tibbon wrote: post #78
emptybits wrote: post #74

"Turning sea water into clean, drinkable water costs $2 to $5 for 1000 gallons. Less than half a penny per gallon is obviously absurdly cheap." - Elon Musk, 2023-05-07 https://twitter.com/elonmusk/status/1655262008898383872?ref_...

I'm looking at this and unclear what he's talking about. Essentially, citation needed Elon?

Musk made the same "absurdly cheap" claim in a recent interview with Bill Maher.

https://www.youtube.com/watch?v=oO8w6XcXJUs [skip to 14:45]

I don't think he gave any substantiation to the claim then either. (He dismissed delivery of clean water to the world as a problem not interesting enough for him to spend time on.)

Re: Why is desalination so difficult?

#122

Post by jbm »

ramesh31 wrote: post #102
jbm wrote: post #99

Earlier quoted context omitted.

Is there a practical reason why it would be difficult to do this with solar power? Is this a process that does not adapt well to intermittent power sources?

>Is there a practical reason why it would be difficult to do this with solar power? Same reason it's hard to do anything with solar. Grid scale storage is an unsolved problem.

I wasn't too clear about what I meant, but is storage even necessary here? For example, would there be an issue if the desalination process was left in an intermediate state for X hours / days while power is intermittent?

I wonder if there are more energy-expensive desalinization processes that are better to use with intermittent power sources, like solar.

Re: Why is desalination so difficult?

#123

Post by grogenaut »

tivert wrote: post #119
dfxm12 wrote: post #89

Earlier quoted context omitted.

It takes less energy but produces a concentrated liquid waste stream that must be disposed of. I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere…

> I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere? Too much salt can kill stuff (e.g. people, plants), so I suppose that makes it "toxic." May…

Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are.

What's wrong with pumping 10% CO2 into your office constantly from a compressed gas plant extracting oxygen and argon next door?

Re: Why is desalination so difficult?

#124

Post by [deleted] »

cyberax wrote: post #31
Animats wrote: post #3

It's not difficult. It's just energetically uphill. Undoing entropy costs energy. Second Law of Thermodynamics.

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…

[deleted]

Re: Why is desalination so difficult?

#125

Post by ericlewis »

lisper wrote: post #88
akiselev wrote: post #47

Is 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…

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.

good thing people need salt!

Re: Why is desalination so difficult?

#126

Post by jerf »

tivert wrote: post #119
dfxm12 wrote: post #89

Earlier quoted context omitted.

It takes less energy but produces a concentrated liquid waste stream that must be disposed of. I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere…

> I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere? Too much salt can kill stuff (e.g. people, plants), so I suppose that makes it "toxic." May…

There are several practical problems dumping the brine back in the sea.

If you dump it on a living ecosystem you tend to kill it. Living ecosystems are, unfortunately, concentrated right where we are desalinating and is cheap to dump.

Compounding this problem is that water mixes much more slowly than your intuition suggests. It can stay a coherent mass of high-salt water way longer than you'd think, killing as it goes. This is one of the more surprising things I've learned in the past few years, honestly. Your kitchen-scale-based intuition of how long it takes for liquids of different characteristics to blend together turns out to be way off.

Trying to pipe it away to somewhere where it is less of a problem is expensive.

In the long term just dropping it back into the ocean is not a big deal, but that short term is surprisingly destructive. You'd think you could just drop it in and maybe a few hundred feet from the outlet it would be all dissipated and harmless, but unfortunately the physics don't work out that way.

Re: Why is desalination so difficult?

#127

Post by mcpackieh »

ramesh31 wrote: post #114
mcpackieh wrote: post #106

Earlier quoted context omitted.

> It's not even remotely economical without huge government subsidies. Why shouldn't drinking water of all things receive subsidies? Why must drinking water be a for-profit enterprise?

>Why must drinking water be a for-profit enterprise? It's about sustainability, not profit. Of course wealthy nations can (literally) burn enough money turning fossil fuels into water to make their population comfortable and happy. But most can't, and the externalized cost is unimaginable at a global scale.

Clean energy exists, it would require more subsidies but so what?

Re: Why is desalination so difficult?

#128

Post by stouset »

ericlewis wrote: post #125
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.

good thing people need salt!

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.

Re: Why is desalination so difficult?

#129

Post by janalsncm »

ramesh31 wrote: post #102
jbm wrote: post #99

Earlier quoted context omitted.

Is there a practical reason why it would be difficult to do this with solar power? Is this a process that does not adapt well to intermittent power sources?

>Is there a practical reason why it would be difficult to do this with solar power? Same reason it's hard to do anything with solar. Grid scale storage is an unsolved problem.

This doesn’t make sense to me. You don’t need to store energy to desalinate water. You can store the final product. And water storage is a solved problem. At times where supply exceeds demand, use excess energy to desalinate more water. When energy demand is high, desalinate less.

Re: Why is desalination so difficult?

#130

Post by jjk166 »

iudqnolq wrote: post #113
bufferoverflow wrote: post #108

Earlier quoted context omitted.

Yes, but you pay for that when you purchase the products. These costs are baked in.

But we're talking about increasing the per-gallon cost of everything. So the baked in cost would increase.

Or moving water intensive operations away from areas where water is expensive. No one is going to be doing desalination in Michigan.
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