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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#101

Post by tguvot »

ALittleLight wrote: post #93
ramesh31 wrote: post #70

Earlier quoted context omitted.

>In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. Israel does this by burning massive quantities of fossil fuels: https://en.wikipedia.org/wiki/Energy_in_Israel#/media/File:E... It's not even remotely economical without huge government subsidies. Completely untenable with current technology for poorer countries, or anyone that cares at all about carbon e…

Couldn't they just replace fossil fuels with nuclear and solve the emissions issue?

Not sure that largish, easily targetable, nuclear facility on a tiny spot of land is a good idea.

Re: Why is desalination so difficult?

#102

Post by ramesh31 »

jbm wrote: post #99
ramesh31 wrote: post #70

Earlier quoted context omitted.

>In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. Israel does this by burning massive quantities of fossil fuels: https://en.wikipedia.org/wiki/Energy_in_Israel#/media/File:E... It's not even remotely economical without huge government subsidies. Completely untenable with current technology for poorer countries, or anyone that cares at all about carbon e…

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.

Re: Why is desalination so difficult?

#103

Post by cyberax »

Animats wrote: post #49
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…

> 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. > A thousand liters takes about 3kwh. So, if those numbers are right, desalination is currently at about 25% of theoretical energy efficiency. Is that correct?

I made a small arithmetic mistake at the end, 2.4MJ is about 0.66kWh

But otherwise it's correct, we're at about 20% of the theoretical maximum. The best RO systems are right now working towards 2kWh per cubic meter: https://uh.edu/uh-energy/educational-programs/tieep/content/...

Re: Why is desalination so difficult?

#104

Post by vladraz »

at_a_remove wrote: post #71
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 only have a BS in Physics but you're basically correct. But to make it even more simple and divorced from method: 1) There's a large difference in energy and entropy between seawater and drinkable "fresh" water. This represents a bare minimum expenditure, below which you can never go, lest you attempt to create a perpetual motion machine. 2) No matter how you do it: Well, now you have a bunch of previously dissolve…

A sensible thing to do is to turn the waste brine water into a resource. Since it's already been pumped up, pour it out into an evaporation pond to increase humidity in an area that could benefit from it, and then scoop up the salt to extract valuable minerals.

Re: Why is desalination so difficult?

#105

Post by cratermoon »

LordDragonfang wrote: post #76
cratermoon wrote: post #26

Earlier quoted context omitted.

A desert is a living ecosystem. Evaporating salt water would leave behind substantial salt solids. Think Great Salt Lake.

I mean, that's actually not too much of a downside, since salt is a fairly valuable economic good - Indian salt farmers do exactly that to harvest the salt crystals.

Destroying existing ecosystems is kind of part of how we got where we are now.

Re: Why is desalination so difficult?

#106

Post by mcpackieh »

ramesh31 wrote: post #70
nerbert wrote: post #34

Earlier quoted context omitted.

In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. If there is a real need, and a will to address it, we have everything we need to to it. https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...

>In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. Israel does this by burning massive quantities of fossil fuels: https://en.wikipedia.org/wiki/Energy_in_Israel#/media/File:E... It's not even remotely economical without huge government subsidies. Completely untenable with current technology for poorer countries, or anyone that cares at all about carbon e…

> 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?

Re: Why is desalination so difficult?

#107

Post by Timshel »

function_seven wrote: post #100
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…

It’s toxic only because of concentration. AKAIK, there aren’t any compounds in the brine that weren’t present in the seawater to begin with. The solution is dilution, but I’m sure it’s easier for me to type that than it is to achieve in a desalination plant. But, why not really-long-pipe-with-small-holes-along-the-length? That seems to me like a simple mechanism to send the brine back into the ocean without causing a…

I believe I had read just dilute it until concentration is ok then release. But might be more tricky than that ^^ : https://www.sciencedirect.com/science/article/abs/pii/S00489...

Re: Why is desalination so difficult?

#108

Post by bufferoverflow »

iudqnolq wrote: post #82
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…

*typical American household uses directly. Don't forget food, industrial, etc

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

Re: Why is desalination so difficult?

#109

Post by tguvot »

jbm wrote: post #99
ramesh31 wrote: post #70

Earlier quoted context omitted.

>In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. Israel does this by burning massive quantities of fossil fuels: https://en.wikipedia.org/wiki/Energy_in_Israel#/media/File:E... It's not even remotely economical without huge government subsidies. Completely untenable with current technology for poorer countries, or anyone that cares at all about carbon e…

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?

One of desalination facilities I think is actually solar powered. It mix of evaporative desalination with power generation (giant tower that a bunch of mirrors focus light one). Not sure if it's in production now.

But in general in Israel solar has a couple of problems: very dusty (sand storms) and local electrical company which tends to create problems

Re: Why is desalination so difficult?

#110

Post by xenadu02 »

dfxm12 wrote: post #89
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…

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…

No, fresh water just enters the water cycle. It will eventually evaporate or end up in a river and back into the ocean where it will be reunited with the salt. The overall salt concentration of the ocean would not be changed unless we sequestered the fresh water permanently. Even then it would take a tremendous effort to make even the tiniest difference in salinity.
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