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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#181

Post by js2 »

aeonsky wrote: post #172
dstainer wrote: post #23

Slightly off topic, however, the post references the Carlsbad desalination facility. If you find yourself in San Diego and like oysters, I would highly recommend you checkout the Carlsbad Aquafarm. Take the tour and pick up some oysters. What's really interesting and relevant to the topic is that the oyster farm serves as a pre-filter to the desalination plant and there's an symbiotic relationship between the plant a…

Pick up some oysters, for eating? If these oysters serve as a pre-filter for the plant, would you not want to eat them as these oysters would contain all sorts of pollution?

They clean the oysters before selling them:

> This was the first oyster farm to feature an inventive “depuration and purification” process, which involves immersing the oysters in triple-filtered seawater once they reach full size. This ensures that the oysters are a completely safe, top-quality delicious shellfish product.

https://www.usatoday.com/story/travel/10best/2022/08/04/how-...

https://en.wikipedia.org/wiki/Depuration

TIL.

Re: Why is desalination so difficult?

#182

Post by Loquebantur »

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!

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.

Re: Why is desalination so difficult?

#183

Post by DayDollar »

lisper wrote: post #154
darkclouds wrote: post #142

Earlier 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.

It would rust and crusted with chalk and beneath that layer untouched.

Re: Why is desalination so difficult?

#184

Post by m463 »

lisper wrote: post #154
darkclouds wrote: post #142

Earlier 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.

People who want to recover old coins such as encrusted roman coins will soak them in distilled water to dissolve the minerals in a pretty short time. The metal usually is not affected.

Re: Why is desalination so difficult?

#185

Post by stOneskull »

jononomo wrote: post #43
zokier wrote: post #9

Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?

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?

#186

Post by Zetice »

gamegoblin wrote: post #77

I was surprised how cheap it is. Desalinated water costs ~50 cents per 1000 liters [1]. That's about the same amount of water as a typical American household uses per day. 50 cents per day for a fully desalinated water supply is... incredibly cheap. If you're interested in water policy and water management / engineering, I cannot recommend enough reading the book "Let There Be Water: Israel's Solution for a Water-Sta…

To be fair to the author, difficult here could just mean “more complex than it seems” which he does a good job of illustrating, specifically around the additional concerns that go in beyond the actual processing of the water.

He says it’s viable for many applications.

Re: Why is desalination so difficult?

#187

Post by cyberax »

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…

You are correct. Although technically, dissolution is not a reversible process. That's why you need to input energy to reverse it.

Carnot cycle, technically, doesn't apply to all energy sources directly.

For example, solar panels have their "hot side" at around 6000K, so Carnot efficiency would be close to 100%. Real solar panels have other limiting factors, and I believe the absolute achievable theoretical maximum is around 80%.

On the other side of the spectrum, wind turbines have very lousy Carnot efficiency because they're exploiting a temperature difference of just a few degrees. However, the "Carnot tax" is not paid by us directly, so we don't really care about it.

Re: Why is desalination so difficult?

#188

Post by weaksauce »

gamegoblin wrote: post #77

I was surprised how cheap it is. Desalinated water costs ~50 cents per 1000 liters [1]. That's about the same amount of water as a typical American household uses per day. 50 cents per day for a fully desalinated water supply is... incredibly cheap. If you're interested in water policy and water management / engineering, I cannot recommend enough reading the book "Let There Be Water: Israel's Solution for a Water-Sta…

That's not too bad. the normal baseline consumer costs are actually more expensive than that. normal base use in irvine ca is 1.78 per 748 gallons which is almost 3000 liters. (2831.488 liters)

I assume that's cost to make and not total cost to consumer post treatment plant distribution and maintenance so it would be more expensive than that but still in the ballpark of reasonable.

Re: Why is desalination so difficult?

#189

Post by aidenn0 »

eutectic wrote: post #66
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…

I think it's more a problem of entropy; You're taking a high-entropy mixture and trying to extract a pure substance. Think sorting red and blue lego.

The comment is still useful; if the red legos and the blue legos are different sizes it's pretty easy to sort them mechanically.

Re: Why is desalination so difficult?

#190

Post by margalabargala »

jerf wrote: post #126
tivert wrote: post #119

Earlier quoted context omitted.

> 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 goe…

> water mixes much more slowly than your intuition suggests

As a visceral example of this in the other direction, the freshwater plume of the Amazon River extends more than 60km into the ocean. [0]

I would love to see these plants placed in areas where there's a nearby dry below-sea-level basin, into which the brine may be discharged. The Salton Sea in CA is one example, there's another similar location in Egypt I'm aware of. The advantage of such locations is they are usually extremely hot and arid, which means there's generally not much of a local ecosystem or human population, and there is ample solar power availability.

[0] https://eos.org/science-updates/the-amazon-rivers-ecosystem-...

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