Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why is desalination so difficult?

practical.engineering

Post reply on HN
Print view View original

361–370 of 644 posts

Re: Why is desalination so difficult?

#361

Post by dylan604 »

6510 wrote: post #358
dylan604 wrote: post #330

Earlier quoted context omitted.

you're desalinating water to have an abundant source of fresh water. why in the world does taking fresh water and adding salt being mined from the ground together to make salt water to then desalinate make any sort of sense?

We already have abundant fresh water. We use it to transport sewage, fertilizers and other crap to the ocean. It would be silly to try to clean it afterwards - but here we are? Adding salt is a great idea, it pushes our collective stupidity to a noteworthy level of nonsensicalness. It makes me wonder what other hard to remove poisons we could add to challenge ourselves. Perhaps design a new disease?

>We already have abundant fresh water.

Clearly, you're not having a rational conversation with comments like this. If there was an abundance of fresh water, the western half of the US would not have been in severe drought conditions for the past however many years. This is where I leave you as you are just making things up like and not even having a good faith conversation

Re: Why is desalination so difficult?

#362

Post by TheDudeMan »

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…

Grady mentioned a maximum theoretical efficiency. How close are we to that?

Re: Why is desalination so difficult?

#363

Post by 101011 »

nardi wrote: post #331
aerio wrote: post #321

Earlier quoted context omitted.

I don't think the article dodges the question at all. Did you properly read it or just skim it? It goes over multiple ways of desalinating water, distillation and osmosis - as you also cover. The most relevant paragraph to the question and a bit of a conclusion seems to be: >And that’s the problem with desalination. It’s kind of like the nuclear power of water supply. It seems so simple on the surface, but when you a…

The article talks about why it's maybe politically or economically difficult, but not why it's physically difficult.

Subtracting economics from the equation, aren't there a very limited number of physically difficult problems?

Re: Why is desalination so difficult?

#364

Post by at_a_remove »

nephanth wrote: post #352
at_a_remove wrote: post #71

Earlier quoted context omitted.

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…

> There's a large difference in energy and entropy between seawater and drinkable "fresh" water I agree with the entropy part, but isn't the energy practically the same for similar quantities/ temperature?

Really good question! The answer is: it's complicated.

When you dissolve a "salt" (the whole class of them, rather than just NaCl), there is a lattice energy (you are tearing these crystals apart) and a hydration energy, which are a little give and take from an energy standpoint. Most salts dissolving are slightly exothermic. NaCl dissolving is very slightly endothermic.

Seawater? Well, remember, there's a lot of dissolved solids in there, not just salts. So you have a summation of dissolving a whole menagerie of different things into your water. Last I heard, and it's been many years since I went near anything like that, yes, there's both an entropy and an energy cost, although I would personally dread trying to do calorimeter measurements to verify it experimentally.

Re: Why is desalination so difficult?

#365

Post by salawat »

Fatnino wrote: post #355
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.

Need for uses other than eating. Like de-icing roads.

Whoch chews the hell out of cars, requiring additional stock to be made... Salt is hell.

Re: Why is desalination so difficult?

#366

Post by ktta »

Nifty3929 wrote: post #343
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…

Residential water use is nothing. In California, where we have persistent water shortages, residential, commercial and industrial water use ( including all landscaping, golf courses, etc) all put together still only amount to 10-20% of overall water use, depending on rainfall

Is the only remaining use left agricultural?

Re: Why is desalination so difficult?

#367

Post by 6510 »

dylan604 wrote: post #361
6510 wrote: post #358

Earlier quoted context omitted.

We already have abundant fresh water. We use it to transport sewage, fertilizers and other crap to the ocean. It would be silly to try to clean it afterwards - but here we are? Adding salt is a great idea, it pushes our collective stupidity to a noteworthy level of nonsensicalness. It makes me wonder what other hard to remove poisons we could add to challenge ourselves. Perhaps design a new disease?

>We already have abundant fresh water. Clearly, you're not having a rational conversation with comments like this. If there was an abundance of fresh water, the western half of the US would not have been in severe drought conditions for the past however many years. This is where I leave you as you are just making things up like and not even having a good faith conversation

I'm just watering the lawn.

Re: Why is desalination so difficult?

#368

Post by margalabargala »

AnimalMuppet wrote: post #207
margalabargala wrote: post #190

Earlier quoted context omitted.

> 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.

It would take one mildly expensive tunnel. As a bonus, you might be able to put hydro turbines on it, if they can handle the salt.

The Salton Sea's salinity is already well over that of the ocean (44g per liter compared to 35), and is getting saltier. Considering that the very existence of the Salton Sea is already an ecological disaster, I don't really see dumping salt into it to be another one.

The Salton Sea is 70 miles from the Pacific Ocean. If this were built, it would be only the third longest water tunnel in the world, and second in the US.

Re: Why is desalination so difficult?

#369

Post by gcanyon »

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…

So the obvious question to me is: is there no other physical property that can be used to separate the hydration shell + ion molecule combinations from the just-water molecules?

Different magnetic charge?

Different mass?

Different chemical reactivity?

¯\_(ツ)_/¯

I know the answer must be 'no' since if there were a better answer, none of what I'm thinking of requires more than high-school-level chemistry to discover/exploit.

Re: Why is desalination so difficult?

#370

Post by mvkel »

Fatnino wrote: post #355
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.

Need for uses other than eating. Like de-icing roads.

Salt doesn't work below -15C; most snowmelt is actually other solutions these days
Post reply on HN