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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#161

Post by scythe »

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…

The "clever" version I've seen in some papers is to use reverse electrodialysis to recover energy as you dilute the waste brine with seawater. AFAIK, this has not been incorporated into any existing installations.

Re: Why is desalination so difficult?

#162

Post by tiku »

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.

salt battery's.

Re: Why is desalination so difficult?

#163

Post by [deleted] »

suddenclarity wrote: post #145
stouset wrote: post #128

Earlier quoted context omitted.

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.

No reason to keep the 1:1 ratio. Use the salt to replace our current salt mines/outtakes and then use the water as an addition to our current freshwater usage.

[deleted]

Re: Why is desalination so difficult?

#164

Post by iudqnolq »

Veserv wrote: post #132
iudqnolq wrote: post #120

Earlier quoted context omitted.

That equates to around 4,000 liters per person per day. Freshwater withdrawals is a very broad category, it also includes water released to turn hydropower turbines. But it's also unfair to compare across countries without taking into account water sent between countries in the form of produce and products.

You were quibbling about how 1,000 liters per day does not adequately account for all usage. Freshwater withdrawals is on average going to be a overestimate and the US is one of the worst outliers with significant agricultural exports which are one of the largest contributors to differential water withdrawals. Despite this, it would only be $2.00 per day for a fully desalinated water supply even if we safely overesti…

I agree with everything you say (with the assumption the person I replied to has the correct price, which I know nothing about)

Re: Why is desalination so difficult?

#165

Post by bsder »

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…

Agreed. Desalinization is cheap. It works just fine for providing water for coastal people. As California is proving.

What it does not do is provide freshwater for argibusinesses. As California is also proving in the Central Valley. :(

Re: Why is desalination so difficult?

#166

Post by bsder »

dylan604 wrote: post #156
mcpackieh wrote: post #127

Earlier quoted context omitted.

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

Wind powered desalination seems like a perfect combo seeing as it's pretty much always windy on the coast. California's May gray and June gloom makes me thing would keep solar from my first option.

HVDC links to El Centro and Arizona would make solar just fine for Southern California.

Re: Why is desalination so difficult?

#168

Post by rtkwe »

xenadu02 wrote: post #110
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…

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.

It will change the salinity on the short term though at the release location and the amount a large plant will be discharging is enough to alter the local salinity so long as the plant remains operational which will negatively affect sea life in that area.

Re: Why is desalination so difficult?

#169

Post by marcosdumay »

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…

> As it turns out, doing that takes a lot of energy

The change in entropy between a batch of saline water and a batch of fresh water and enough saline water that its concentration don't change is about the same as letting that same fresh water fall for 200m and converting the resulting energy into heat (at 300K).

What means that desalination will take a lot of energy whatever method you use. There are distillation procedures close to perfect efficiency that wouldn't take much more energy than reverse osmosis; and of course, electrical separation is that one method with lots of promise but that stops due to material related problems every time it's tried. It just so happen that we know how to scale reverse osmosis up cheaply and reliably; but this looks like a feature of our technology and not anything intrinsic.

Re: Why is desalination so difficult?

#170

Post by NegativeK »

profsummergig wrote: post #60

While on the issue of water: Some countries have floods in one part, and drought in the other. The floods are so bad that large numbers of people die. Here's a great challenge that should be worked on: how to capture the flood water and use it to mitigate the droughts. Could something like Elon Musk's Boring company concept fix this?

People are constantly talking about using pipes to solve the US desert southwest's water crisis, as if building a water pipeline for thousands of miles isn't some gigantic logistical and engineering nightmare -- as well as a bureaucratic nightmare, since most places with fresh water don't want people in the desert literally sucking them dry.
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