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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#201

Post by coryrc »

mcpackieh wrote: post #106
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…

> 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?

Because the US has plenty of free water that's growing alfalfa in the desert. Subsidies are unnecessary.

Re: Why is desalination so difficult?

#202

Post by mr-ron »

agnosticmantis wrote: post #196

Given the countless environmental challenges we are facing (and causing), we should more seriously and openly consider putting a stop to exponential population growth as an (at least short-term) solution. It’s astonishing how some people preach blind faith in our ability to just find solutions for problems caused and exacerbated by never-ending population growth without identifying it as the root cause. Why is it a g…

Its not exponential. In fact its estimated to level off over the next century: https://ourworldindata.org/world-population-growth

Re: Why is desalination so difficult?

#203

Post by eutectic »

aidenn0 wrote: post #189
eutectic wrote: post #66

Earlier quoted context omitted.

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.

Yes, but there will always be some necessary energy input.

Re: Why is desalination so difficult?

#204

Post by chiefalchemist »

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.

You have that problem only if you decide to remove all the salt. You could decide to remove say 20% of the water and make that only H2O; effectively getting H20 and a higher concentration of salt water as outcomes.

I say this having recently read about the desalination plant in Dubai.

Re: Why is desalination so difficult?

#205

Post by soperj »

jakear wrote: post #98
luxuryballs wrote: post #62

Earlier quoted context omitted.

yeah but I usually feel quite refreshed after coming out of the waves, but keep in mind we don’t need clean feelings to survive, we do need fresh drinking water

Salt overexposure is liable to dry up your skin and cause rashes/cracking leading to vectors for infection.

Is it? My parents have a salt water pool and swim in it daily through the summer.

Re: Why is desalination so difficult?

#206

Post by sophacles »

dylan604 wrote: post #149
jjk166 wrote: post #130

Earlier quoted context omitted.

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

why not? we're growing heavy water using crops in places with little water. logic is not always the deciding factor if involved at all in a lot of modern things

There are a few very big salt mines below the great lakes. We could totally get a desalinization plant in Michigan by pulling water from the lakes, salt from the Detroit salt mine, and combining them in the input stream to the plant!

Re: Why is desalination so difficult?

#207

Post by AnimalMuppet »

margalabargala wrote: post #190
jerf wrote: post #126

Earlier quoted context omitted.

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…

The Salton Sea is over a hundred miles from the ocean, though, with at least one mountain range in the way. It might be a good dumping ground (or maybe not - there at least were fish in it, if there still are, this would almost certainly kill them). But it would definitely take a lot of energy to pump the water there.

Re: Why is desalination so difficult?

#208

Post by omoikane »

luxuryballs wrote: post #35

Can I shower in salt water? Then all I need to do is desalinate drinking water. “Distributed” home desalination for drinking water seems like the best approach in my mind, then people can pay as much or as little they need, but I have no real data to back this up.

Related, it seems inconvenient that we haven't evolved to be able to just drink salt water, like cats can.

Re: Why is desalination so difficult?

#209

Post by sophacles »

coryrc wrote: post #201
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?

Because the US has plenty of free water that's growing alfalfa in the desert. Subsidies are unnecessary.

Are you sure? The alfalfa itself is subsidized. (I mean subsidies besides giving all the water to the farmers thing).

Re: Why is desalination so difficult?

#210

Post by emptybits »

jjk166 wrote: post #143
iudqnolq wrote: post #91

Earlier quoted context omitted.

It's very easy to forget that we use orders of magnitude more water than comes out of our tap. A pound of beef is about 2,000 gallons, so that would be a nontrivial price increase.

Probably shouldn't be raising cattle in areas that would need desalination to produce those 2000 gallons.

Or maybe such water-intensive food production should require consumption of the planet's nearly unlimited seawater. Pass the greater expense to the cattle producers and/or consumers. Leave the planet's relatively limited freshwater sources for direct human consumption, or food production that's more sustainable, or other urgent and necessary activities. The beef lobby may not agree.
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