[ajb]

There is a fundamental minimum amount of energy needed to desalinate: you can't take less energy to do it,than you could gain back (from osmotic pressure) if you allowed the desalinated water to expand a cylinder containing the residual brine. This is large. This paper is a thermal method, so it doesn't have an electricity input, but to justify their efficiency claim, they should really compare against what you could do by using the same surface area for solar panels, driving a conventional setup. My (limited) understanding is that conventional reverse osmosis is not far from the theoretical optimum, energy-wise, the main difficulties being operational (the membranes need declogging). And of course RO is more expensive than rain.

This paper is interesting, however, in directly producing crystalline salt, which is lower volume than brine and easier to dispose of, maybe even valuable.

[patates]

I always thought that if separating water and salt were easy, our bodies would have evolved to do it so that we'd be able to drink sea water and be fine. It must have been so expensive that searching for fresh water was worth it or there were plenty of fresh water that it was never a evolutionary pressure. Evolving kidneys capable of concentrating urine beyond 3 something percent concentration (sea water) perhaps required a massive restructuring of our internal organs and a huge constant energy expenditure, so we kept seeking fresh water.

ps. I have no clue what I'm talking about

[Tagbert]

It’s mostly that it takes energy. If fresh water is we drink that. There aren’t a lot of places where only salt water is available so, for most animals, it isn’t worth it to have evolved a way to extract water from salt water.

Animals in the ocean of course do live without fresh water. Some of them just live off of water extracted directly from their food or from metabolizing that food, which produces water. Some animals have specialized cells that excrete salt so that can take in salt water and separate out the salt.

[blackoil]

Salt water fish can process sea water, no point in evolving for saltier brine if you have oceans of 3% water.

[scythe]

>I always thought that if separating water and salt were easy, our bodies would have evolved to do it so that we'd be able to drink sea water and be fine.

Unfortunately for terrestrial animals, it's just not that simple. Seawater contains a lot of microbial life, some of which can be infectious or toxic. Going to the coastline to drink is potentially hazardous, because it usually means descending a hill on a predictable route which will be attractive to predators. And you need to get pretty far into the water, usually, because of nasty stagnant runoff, which can come from decaying matter that washes ashore, and sand in the surf. That means you risk drowning. Plus, you don't just need the energy for desalination, but the infrastructure (similar problem to real life!), which means more and larger juxtamedullary nephrons in the kidney, which is already a major weak point on the back due to the high blood flow in the kidney. Meanwhile, most of your food contains a lot of water, especially if you're one of the 99.99999% of animal species that doesn't cook it.

[otterdude]

Thermal methods require energy, it seems like this substrate is effective at maintaining its solar-thermal absorbing properties better than a material that will attract salts

> Testing their solar-thermal desalination technique using samples of water from the Pacific, Atlantic, and Indian Oceans, Guo and his team were able to make the surface self-cleaning. In other words, it extracted freshwater and directed the remaining salts to the passive region where they could be later collected without reducing the panel’s efficiency.

This is not "large" this is a moderate improvement. Albedo is likely only marginally affected, and the solar power input over area is the same.

Depending on this cost of this process it could very likely be a wash in terms of NPV

[westurner]

ScholarlyArticle: "Extreme salt-resisting multistage solar distillation with thermohaline convection" (2023) https://www.cell.com/joule/fulltext/S2542-4351(23)00360-4 .. https://scholar.google.com/scholar?cites=7551078272963689346...

"Desalination system could produce freshwater that is cheaper than tap water" (2023) https://www.eurekalert.org/news-releases/1002811

ScholarlyArticle: "Highly efficient and salt rejecting solar evaporation via a wick-free confined water layer" (2022) https://www.nature.com/articles/s41467-022-28457-8

"Solar-powered system offers a route to inexpensive desalination" (2022) https://news.mit.edu/2022/solar-desalination-system-inexpens...

[xhkkffbf]

I remember the MIT press release. I wonder if they've found any commercial success.

[pfdietz]

MIT had a spin-out company some years ago doing HDH (Humidification-Dehumidification) desalination.

In thermal cycles, the problem has been in the condensation step. If there is a carrier gas present this inhibits heat/mass transfer at the condenser surfaces. The usual way of getting around this has been to operate the system with no carrier gas, but that requires pressures below atmospheric pressure, requiring strong walls to withstand external atmospheric pressure.

The MIT invention was a bubble tray contactor, where air is bubbled up through trays of progressively cooler water. The water/air bubble interface provides a large surface area at low cost. One of the markets for this was cleaning up brine from fracked wells.

The company, Gradiant, is still around but has evolved to involve a wider range of water treatment technologies (which is very sensible from a business viewpoint, as customers buy complete solutions, not individual technologies). https://www.gradiant.com/

[CuriouslyC]

If this can be applied to mine effluent, you could replace the maybe with most certainly. Sulfuric acid effluent lakes leech all sorts of valuable metals out of the ground.

[cornholio]

Focusing on pure energy efficiency might be missing the point of economic efficiency.

An RO desalination plant needs electric energy to drive the pumps, which might be generated by panels which are 15-20% efficient. So, if you can have cheap thermal desalination panels, they come out ahead even if 6x less energy eficient, you avoid the whole expensive and fragile desalination plant and you gain a low skill, distributed setup.

[ajb]

This is valid for some use cases, but then it needs to be compared with other solar distillation methods, of which there are already a variety at different levels of energy efficiency, complexity, and land use.

[cyberax]

> My (limited) understanding is that conventional reverse osmosis is not far from the theoretical optimum, energy-wise, the main difficulties being operational (the membranes need declogging). And of course RO is more expensive than rain.

RO is about 2-4x the theoretical minimum, depending on how much water you're willing to reject.

[rtpg]

This is a weird angle I think? Desalination brine is a real problem, so if you can eliminate that then efficiency is less of an issue (especially given that desalination plants are often in places with a lot of sunlight!).

You don't want to be super duper inefficient but "no waste that has to be dumped back out" feels really big to me

[xyzzyz]

Brine is very easy to dispose of: you just pump it back to where it came from. Solid crystalline salt, on the other hand, is a hassle.

[ceejayoz]

> Brine is very easy to dispose of: you just pump it back to where it came from.

Easy, but not necessarily good for the spot you're pumping concentrated salt back into.

[ashdksnndck]

If you use fat pipes that go a decent distance from shore, diluting your brine with ocean water, you’ll have a negligible impact on the ocean. The problem is if you dump lots of brine in shallow waters. Old designs did have that flaw, but it’s not that difficult to design around this constraint now that we know about it.

IMO this is an issue where NIMBYs are using environmental concerns as a smokescreen to block new desal plants from ruining the vibe at their beachfront property. Rhymes with the opposition against offshore wind farms.

[iamjs]

The city of Corpus Christi, TX is currently considering options for desalination plants—all of which pump their brine into the shallow water inside the bay or the ship channel.

[senordevnyc]

Sounds on brand for Texas.

[Someone]

> The problem is if you dump lots of brine in shallow waters. Old designs did have that flaw, but it’s not that difficult to design around this constraint now that we know about it.

I think that problem was known (and discarded as not important) when the first serious water desalination plants were built.

[xp84]

I can probably be convinced pretty easily with some evidence of that, but you’ll never convince the contingent who is convinced it’ll kill sea life at any concentration or location, so, being able to shut them up by saying “we have no wastewater, we load rail cars with crunchy salt and use it for stuff” still has value.

[__MatrixMan__]

I wish we could reimagine carbon credits to that degree of stringency. You offset a kg of carbon emissions? Let's see that kg.

[kortilla]

The goalposts will just shift to attack that excess salt instead. It’s like all of the FUD about datacenter water usage while people shove almonds in their mouths.

[nkmnz]

In Germany, it's the water usage of a Tesla plant vs. the neighboring asparagus farm.

[FartyMcFarter]

Yeah. Worrying about salt in the sea is like worrying about oxygen in the air. Can too much oxygen in the air sometimes be a problem? Yeah, in some corner cases. Is it a major problem that we can't solve? Not at all.

[0xFF0123]

Isn't it more akin in this case to worrying about too much carbon dioxide in the air?

[FartyMcFarter]

Why is it akin to that? Doesn't the salt come from the sea in the first place?

[mejutoco]

That makes sense to me. At the same time I know the mediterranean sea is heating up more because it cannot move heat out quick enough. I dont know of any mediterranean air, so I believe more closed water zones would behave different than, lets say, the atlantic ocean.

[SoftTalker]

The brine came from the ocean. So just dilute it back to close to ambient salinity using municipal waste water that you are discharging anyway.

[ceejayoz]

> The brine came from the ocean.

Sure, and enriched uranium comes from the ground, but that doesn't mean it's safe to dump it back in after the enrichment process!

> So just dilute it back to close to ambient salinity using municipal waste water…

Wouldn't it generally be easier to process that municipal waste water, as is already fairly common?

[Joker_vD]

> Sure, and enriched uranium comes from the ground

Uranium can also come from the ocean water (there is, apparently, quite a lot of it in there, relatively speaking). Japan experimented with the technology in the nineties, but it really was much cheaper to just mine it from the ground, so they abandoned it.

[somenameforme]

It's about 3 parts per billion. Uranium is about $85/pound, so you'd need to be able to completely process/extract about 40 million gallons of saltwater for $85 to break even. The real cost there is orders of magnitude higher. It's one reason the claim about the Earth having vast amounts of uranium is quite disingenuous. The amount of cost efficient accessible uranium is only enough to last ~1 century at current consumption rates. If nuclear energy scaled up significantly, we'd run out in a matter of decades if not less, or we send the price of uranium skyrocketing and the price arguments would need to be significantly adjusted.

[numpad0]

Japan is also barred from doing own enrichment, being a non-nuclear state. Though, there nevertheless is a dormant set of requisite facilities.

[SoftTalker]

The analogy would be if you "un-enrich" it. Then it's safe. Or at least no worse than when you took it out of the ground.

[ceejayoz]

> The analogy would be if you "un-enrich" it.

But you're doing that with the same water you're trying to make in the first place!

[turkeyboi]

Enriched uranium is perfectly safe to dump but it would be stupid to do so. Fission products are nasty but uranium itself is not, comparatively.

[Enginerrrd]

Municipal waste water is a much cheaper way to get desalinated water in the first place though.

[lazide]

except for the pharmaceuticals anyway

[Enginerrrd]

That’s been a solved problem, engineering-wise, for a while.

The advanced treatment stages take care of it. Between UV, ozone, and nanofiltration, etc. we can remove the pharmaceuticals.

Actually the problem is the water comes out too pure out of a well designed water reuse system, to the point where the mineral content can be too low and you need to add some back in.

[shermantanktop]

Hey, it's free viagra, prozac, progesterone and multivitamin supplements, all in a glass.

[analog31]

Someone tell me why this is stupid, which it probably is: Put the desalination plant on a tanker ship and let it do its duty out in the middle of the ocean, then cruise back to port and dispense the water.

[threwrfaway]

Actually it's easy and ok. Just mix it with the treated sewage right before it returns. Simple mass action implies the salinity hasn't changed.

But wait! There's water mass loss due to leaky pipes and outdoor pools!

Mixing salt water and brine is perfectly ok. Just use a phase diagram.

[xyzzyz]

Maybe, but dumping crystalline salt is even worse to the spot you’re dumping it on.

[asdff]

It doesn't need to be crystalline salt. Just mix the brine with seawater at a really high ratio of sea water to brine then dump that out. 100:1 ratio should be fine I would guess. Quick search suggests seawater salinity variance is already like 10%-15% or so. Even better if you pipe it offshore where currents will take it and not somewhere that doesn't circulate.

[xyzzyz]

Yes, that's my point: if you're next to the ocean, disposing of brine is extremely easy.

[pajko]

You could put it back into old salt mines.

[kijin]

Even better, just package it up and eat it instead of digging underground and creating more pollution.

It's not every day that industrial waste happens to be not only edible but also tasty. Too tasty, in fact. Salt is addictive.

[xp84]

It’s not going to be pure NaCl though; making Morton salt with it would make sense only if it wouldn’t cost more to process it (net of its resale value) than just disposing of it somewhere not particularly sensitive. I’d propose the Utah salt flats or indeed, kinda love the idea of just sticking them in a salt mine that is all tapped out. If it used to be chock full of salt it seems pretty environmentally fair to make it salty again.

[card_zero]

I wonder. It would have to dissolve, a big block of salt would take a while, kind of like the erosion of cliffs where the salt comes from in the first place. Eh, I guess you're right though, the fish wouldn't like that at all.

[iberator]

that's 200% bullshits. Countries that invested into desalination plants are known to create death zones right where brine is sent back - even if miles from the coast

[qurren]

Why? Just build mountains out of it and maybe even open a salt-ski park in the tropics for people who don't have snow.

[asdff]

There are salt mountains lining most midwestern freeways as it is for winter.

[ssl-3]

Assuming my constants (35g/kg of salt in seawater, 650k tons of salt dumped by the state of ohio every year, 81 gallons per day of individual domestic water usage) are correct and my napkin math isn't completely buggered, and if we look at the salt as a primary product instead of just waste:

Ohio DOT's use of road salt would allow for fresh water to be provided for somewhere in the neighborhood of 160,000 people.

On one hand, that's nowhere near enough people; it's a small drop in a giant thirsty bucket of water consumption. So we'll still need salt mountains, salt re-distribution vessels, and/or other ways to deal with excess salt.

On the other hand, 160k is a lot of humans. So perhaps we should look into doing things like this anyway.

(But we probably won't. Ohio gets road salt primarily from a mine under Lake Erie that has a very conveniently-located terminus near downtown Cleveland. The mine directly loads trucks, freight trains, and ships...and it's near the point of use already. It's pretty efficient.)

[xp84]

I just realized that future archaeologists will be tracing our roads using the salt residue!

[Ekaros]

Actually I have been thinking about this. Surprisingly straight and long cuts in rock formations might be a real thing to track. In at least some places at least some rock blasting is preferred to get aggregate for road foundations. And these tends to be rather straight and rather steep.

[make3]

or just you know.. asphalt residue

[lightedman]

"Solid crystalline salt, on the other hand, is a hassle."

Just make prettier-than-Himalayan salt lamps out of it and sell it to hippies. Easy solution.

[rapnie]

That only shifts the problem. Now we need an increased supply of hippies that are hard to come by in a low hippie-tolerant environment.

[galaxyLogic]

I think I read somewhere that salt can be used as energy storage medium? So we could get both water and batteries for renewal energy.

[xyzzyz]

It’s about thermal storage, you don’t use table/sea salt for that, and you don’t need a lot of salt, because the salt is in a closed loop; it’s not being consumed.

[galaxyLogic]

But more thermal storage you want more salt you want, and it's gotta cost something, right?

https://en.wikipedia.org/wiki/Molten-salt_battery

[xyzzyz]

If you read the article you sent me, you'll learn that, just as I said, you don't use sodium chloride, aka table salt, aka sea salt, for these purposes.

[aeonfox]

A better example are sodium ion batteries, which are about to take off in a big way

https://www.catl.com/en/news/6812.html

[RobotToaster]

> Solid crystalline salt, on the other hand, is a hassle.

Just put it on your fries.

[nkrisc]

In an ideal world that crystalline salt by product could be used to offset any imported or mined salt, further reducing the environmental impact of those operations.

[darksnart]

Oh no, the hassle of managing the raw input for several key industrial processes that is created for free as a side product of MAKING WATER DRINKABLE WITH FREE ENERGY FROM THE SUN is TOO MUCH OF A PROBLEM! Especially considering we could instead murder millions of fish - which we then can’t eat- in the process! This entire technology is doomed!

Come on guys please at least attempt to think what you’re about to type, please, I beg you.

[cyanydeez]

yeah, if you like to kill everything in a few 100 feet radius and kill some more in the zone of reliance.

this is delusional ecological

[xp84]

So, we could just dump it on the salt flats in Utah? Plenty of places are already super salty, so nothing lives there (unless it’s able to handle that).

[xyzzyz]

Brine might be bad to the place you dump into, but crystalline salt is even worse.

Overall though, it’s just such a tiny concern. Ocean is huge. If we kill everything in a 100 foot radius, that’s 0.0000000008% of the ocean being destroyed. Less than a drop in a bucket.