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

351–360 of 644 posts

Re: Why is desalination so difficult?

#351

Post by iudqnolq »

codedokode wrote: post #316
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.

4,000 liters per day is too much. It is 10 full bathtubs. Do Americans leave the tap open or do they take a bath 10 times per day?

Maybe my comment wasn't clear. That figure isn't great - we just divided the total amount of freshwater diverted to some kind of use by the population. That even includes letting water flow past hydropower turbines. It's obviously too big.

But not counting the water used to put food on your table is too small. So this establishes some bounds.

Re: Why is desalination so difficult?

#352

Post by nephanth »

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…

> 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?

Re: Why is desalination so difficult?

#353

Post by tguvot »

janalsncm wrote: post #347
tguvot wrote: post #317

Earlier quoted context omitted.

desalination plants don't serve one city. they serve country. also, in general, it's not economical to build plants that do not work 24/7 unless you are country that can spend x3 to overbuild and keep equipment idle Israel btw supplies desalinated water to Jordan and PA.

I think we’re talking past one another. Solar is definitely economical in certain circumstances, and every country overbuilds to some extent. Whether building excess solar makes sense is a question of cost of a marginal unit of energy, since the water doesn’t care what powered its desalination. The energy isn’t wasted since it will eventually need to come from somewhere. Another consideration is political sovereignty…

i was talking about the fact, that if suggested to desalinate water only during sunny hours it requires overbuilding desalination plants. not solar

Re: Why is desalination so difficult?

#354

Post by acidburnNSA »

no_wizard wrote: post #283
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…

Nuclear energy has entered the chat Why not use nuclear power plants to power desalination plants? I even wonder if some of the salt from the brine could be fed into certain types of nuclear reactors (Molten Salt Reactors and the like, possibly) making it an even more symbiotic relationship

Massive-scale desalination with nuclear has been proposed for decades by many nations. Here's a proposal (originally pushed by JFK) for using them to transform the Middle East into a luscious mecca, thereby solving any arable land scarcity issues (from 1967)

https://babel.hathitrust.org/cgi/pt?id=uc1.$b643596&view=1up...

More modern take here: https://www.iaea.org/topics/non-electric-applications/nuclea...

Re: Why is desalination so difficult?

#355

Post by Fatnino »

stouset wrote: post #128
ericlewis wrote: post #125

Earlier quoted context omitted.

good thing people need salt!

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.

Re: Why is desalination so difficult?

#356

Post by Fatnino »

fsckboy wrote: post #293
mschuster91 wrote: post #252

Earlier quoted context omitted.

The problem is that salt brine from seawater contains a lot of side stuff - you still need purification at a scale you don't need mining rock salt.

is this true? I can understand why it would be true, but "sea salt" is a real thing that's been traditionally made in seaside communities through to today, and I find it hard to believe they're cleaning it at anything finer than a gross scale. https://en.wikipedia.org/wiki/Fleur_de_sel

Still made all around the south bay.

Re: Why is desalination so difficult?

#357

Post by rtkwe »

Robotbeat wrote: post #199
rtkwe wrote: post #168

Earlier quoted context omitted.

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.

Only if the discharge is not diluted sufficiently. There is nothing about reverse osmosis that is fundamentally more toxic or harmful than the typical evaporation that takes place naturally in the ocean. And it’s pretty dishonest to claim otherwise. If there’s a problem with too high salinity of discharge, that’s an engineering problem that should be fixed with greater dilution.

Of course but read the comment I was replying to, to my reading they're saying the regular water cycle would take care of the excess salinity which is drastically wrong in the human scale and a complete misread of the issue. The problem isn't that we'll over salinate the whole ocean but that locally the released brine will kill some, potentially large, area of underwater life before it diffuses back down to the rough background levels.

Re: Why is desalination so difficult?

#358

Post by 6510 »

dylan604 wrote: post #330
6510 wrote: post #327

Earlier quoted context omitted.

I think it sounds quite sensible. How is it different from using all fresh water as an all you can dump free-for-all public sewage system before wondering where the drink is coming from? In India they poop in the water then take a bath in it and cook their food in it. In the west we do our pooping upstream because it looks so much cleaner that way. NO! I fail to see how adding salt before desalination is any less sen…

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?

Re: Why is desalination so difficult?

#359

Post by mianos »

SoftTalker wrote: post #346
rs999gti wrote: post #278

Earlier quoted context omitted.

Can't the salt just be dumped in the desert?

Just pump it back into the ocean. The little bit of concentrated brine from a desalination plant will be quickly diluted; the ocean contains a LOT of water.

Particularly if they distribute it over a large area, as they do in practice and shown in the vid.

Re: Why is desalination so difficult?

#360

Post by rtkwe »

Robotbeat wrote: post #340
8n4vidtmkvmk wrote: post #247

Earlier quoted context omitted.

What would you dilute it with? Surely not the water we just extracted?

You dilute it with more seawater. The salinity of the ocean changes significantly just due to evaporation. The band of variability is roughly 30 to 40 grams of salts per liter for sea surface salinity. So if your discharge is within that band, it's not going to be a significant ecological problem (especially if the discharge is at the right location). It's no more a "mass ecological problem" than surface evaporation…

Mass dilution is going to be fairly expensive because you have to move a significant multiple of the water you actually process. The brine waste water is roughly double the salinity of normal sea water at 70 ppt so to get back to the 33-37 ppt that's the real average band you're looking at moving a lot of sea water to dilute the output. If you run the numbers you're looking at 15:1 of volume to get 70ppt to 37ppt using 35ppt water all while ensuring you're at a place with enough natural mixing that your inputs and outputs don't start feeding back into each other.
Post reply on HN