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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#171

Post by rtkwe »

tivert wrote: post #131
grogenaut wrote: post #123

Earlier quoted context omitted.

Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are. What's wrong with pumping 10% CO2 into your office constantly from a compressed gas plant extracting oxygen and argon next door?

> Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are. How large would those brackish areas near the outlets be? It seems to be that would be a big problem in an enclosed bay, but much less so on a shore facing open ocean. Could they run pipes out a few kilometers with small, regular holes (maybe some modifica…

The pipe with many holes is one of the solutions used today it's just imperfect and requires a lot of pipe to make sure the waste output isn't too concentrated in a single area.

Re: Why is desalination so difficult?

#172

Post by aeonsky »

dstainer wrote: post #23

Slightly off topic, however, the post references the Carlsbad desalination facility. If you find yourself in San Diego and like oysters, I would highly recommend you checkout the Carlsbad Aquafarm. Take the tour and pick up some oysters. What's really interesting and relevant to the topic is that the oyster farm serves as a pre-filter to the desalination plant and there's an symbiotic relationship between the plant a…

Pick up some oysters, for eating? If these oysters serve as a pre-filter for the plant, would you not want to eat them as these oysters would contain all sorts of pollution?

Re: Why is desalination so difficult?

#173

Post by eichin »

samstave wrote: post #39
zokier wrote: post #9

Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?

You know this is how the Phoenecians became the dominant culture, its also where the term "salary" comes from and "worth his weight in salt" -- as salt was the only known preservative of the massive amounts of Tuna the phoenecians were catching and shipping throughout the mediterrainian - and made them a super-power - they had control of the preservance of food over shipping distances... Salt was used as money. EDIT:…

Probably (not me, I don't have downvote bits) because it turns out there's a lot of "game of telephone"/"folk etymology" about this legend which apparently started in the 1800s in english; it's not actually historical as you've expressed it.

Re: Why is desalination so difficult?

#174

Post by Gibbon1 »

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?

I feel like I never get enough of the operational details to know. But intermittently running a capital intensive thing has bad economics. If the capital cost per m3 is $0.50 when the plant runs 24/7. It'll be $2.0/m3 if you cut it back to 6 hours a day.

However the details are important. You'd need to do a deep operations analysis to get an answer. That also would include energy storage as well.

Re: Why is desalination so difficult?

#175

Post by cyberax »

olejorgenb wrote: post #147
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…

According to the paper [1] he talks about in the video, the theoretical limit is a function of %salt removed and %waste-water. For 90% salt removal with 50% waste-water, they say the limit is 1.09kWh per cubic meter (3.924 MJ) NB: It is not 100% clear to me if the result is independent of the type of technology, but they do claim: > We first derive the general expression of the thermodynamic minimum energy of separat…

Yes, their result will approach mine if the amount of rejected water approaches 100%.

Their result is independent of technology, it's derived from fundamental thermodynamic principles.

Re: Why is desalination so difficult?

#176

Post by toast0 »

dylan604 wrote: post #137
Timshel wrote: post #107

Earlier quoted context omitted.

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...

dilute it with what? the clean water you just removed the salt from?

Sewage treatment outflows maybe? Wouldn't be enough in a community where all the freshwater is coming from desalination, because not all of the freshwater goes into the sewage system, but it might be workable in communities where desalination is augmenting other sources of freshwater.

Re: Why is desalination so difficult?

#177

Post by 11101010001100 »

tshaddox wrote: post #22
11101010001100 wrote: post #8

Earlier quoted context omitted.

You need to bring the coins uphill in the first place.

Right, but that’s easy. If desalination only required lifting each unit of water a few inches we probably wouldn’t say it’s particularly difficult.

Easy as in moving mass against a gravitational potential? Didn't know building a rocket was easy.

Re: Why is desalination so difficult?

#178

Post by jtriangle »

caseyohara wrote: post #92
qorrect wrote: post #51

Earlier quoted context omitted.

[flagged]

Deserts are far more alive and biodiverse than people think. > The Sonoran Desert encompasses 120,000 square miles of southwestern Arizona, southeastern California, and in Mexico, northwestern Sonora and most of the Baja Peninsula. With nearly 3500 species of plants, 500 species of birds, and 1,000 species of bees, the Sonoran is the most biodiverse desert on earth.

You do have to consider though, how important are those ecosystems?

Because I see this often used as a reason we can't do something, but they never qualify it with reasons why one should care.

I also pose this as a completely honest question, as I don't really know if you wiped out every desert ecosystem with solar/desal/etc if it'd actually affect anything else.

Re: Why is desalination so difficult?

#179

Post by lambdasquirrel »

zokier wrote: post #9

Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?

I think the key difficulty is condensing a very large quantity of water out of an even larger quantity of air, in the desert. The thermodynamic equilibrium of water vapor vs water-in-condensed-form isn’t going to work well for you here, even after nightfall. The very reason that it was possible to evaporate the water out (i.e. the air is very dry) cuts back the other way.

Re: Why is desalination so difficult?

#180

Post by the_sleaze9 »

aeonsky wrote: post #172
dstainer wrote: post #23

Slightly off topic, however, the post references the Carlsbad desalination facility. If you find yourself in San Diego and like oysters, I would highly recommend you checkout the Carlsbad Aquafarm. Take the tour and pick up some oysters. What's really interesting and relevant to the topic is that the oyster farm serves as a pre-filter to the desalination plant and there's an symbiotic relationship between the plant a…

Pick up some oysters, for eating? If these oysters serve as a pre-filter for the plant, would you not want to eat them as these oysters would contain all sorts of pollution?

A strong case for not eating any oysters at all -- but don't forget they're totally aphrodisiacs
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