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.

• today at 5:31 PM

This appears to be the same New Rochester article as 4 days ago with 20 comments.

https://news.ycombinator.com/item?id=48349507

They are talking about lithium recovery, but there is a less exotic byproduct I'm interested in. One tonne (≈ 1 m^3) of seawater contains about 1.3 kilograms of magnesium, equivalent to about 4 kg of magnesite ore. Typical desal prices are on the order of $1 per tonne. Magnesite ore goes for about $100 per tonne, so the crude magnesium in a tonne of seawater is worth about $0.40, which could account for a substantial fraction of the desalination cost. (These numbers are very rough.)

Now you ask: why don't we just recover magnesium from brines if it's so great? Magnesium recovery from seawater isn't that easy: typically you have to treat it with some kind of alkali (often Ca(OH)2), so the cost is dominated by the extraction process (your alkali is consumed!), and you're competing with a pretty cheap ore. But if you have a solid byproduct, instead of a liquid, the options for magnesium recovery might be a lot more efficient, potentially offsetting the cost.

The fourth-most-prevalent ion, sulfate, might also be interesting, at least in a hypothetical post-petroleum future where sulfur as a byproduct of fossil fuel extraction is no longer "free". Sulfate is also annoying to extract from seawater, but again if we have a solid, the rules change.

As for the "table" salt itself, I think we'd quickly saturate (!) the market.

Awesome, love seeing stuff out of Rochester - RIT or UofR or any of the nearby schools.

Totally underrated area for academic pursuits.

• 0x59

  today at 6:09 PM

  Agree! Shout out to the Laboratory for Laser Energetics

> without waste

...except for the huge piles of salt.

If the salt was not waste, surely people would already be extracting it from the brine and the existing methods would also be "without waste".

• eimrine

  today at 5:51 PM

  Persian Gulf has 20% more salt in water because of the humans which are throwing the oversalinated waste back into the sea. Dehidrated salt may be a big deal for some areas because of no waste into input.

  • Jblx2

    today at 6:06 PM

    >Persian Gulf has 20% more salt in water because of the humans

    I would like to read more about this from an authoritative source.

    • tdb7893

      today at 6:28 PM

      Through the magic of Googling "Persian Gulf salinity" it seems like it's more that it's a shallow Gulf in a dry area so it has significant evaporation. Desalination does effect it but it's only a few percent of the total evaporation (which is still surprisingly big) and doesn't sound like the main driving factor.

      https://www.frontiersin.org/journals/marine-science/articles...

      https://www.sciencedirect.com/science/article/abs/pii/S14635...

What about removing oil from water, have we conquered that yet?

"in mice". No, wait, that auto-reply is for cancer breakthrough.

Let me check, is that a wonderful battery ? Nope.. A promising fusion ? Neither...

Ok, so this must be the fourth kind of pseudo-wonder discovery that will maybe make it out of the lab in 20 years, if the research team managed to get scraps of funding while VC pick the next way to waste pensioners money.

Anyway, whenever they have desalinated enough water to get each researcher a pint, the round is on me.

This reads like hyperbole:

> The brine byproduct wreaks havoc on sea life when it’s deposited back into the ocean by raising the salt level and lowering oxygen in the water.

Managing return of concentrated brine should be entirely tractable in the literal ocean.

• rconti

  today at 5:23 PM

  Sure, but typically desalination plants are located in a single physical place, so a discharge pipe dumping brine 24x7 is bad for all of the things around it, as the local concentration is extremely high.

  • joshred

    today at 5:29 PM

    Seems like you could run a long perforated tube to diminish that effect.

    • dieselgate

      today at 5:39 PM

      I wonder what the linear diffusion gradient would look like for that. Like the perforated garden hoses or whatever for soaking soil. Aquatic organisms grow so quick though very curious on the constraints for something like this.

    • dylan604

      today at 5:33 PM

      I liked the idea of loading it up on a ship that sails out releasing as it goes out and back. Make it solar powered or even go old school with literal sails.

      • sgc

        today at 5:46 PM

        I thought they tend to pipe far out and discharge as far below the surface as possible, since there is a lot of surface life and it is less damaging this way.

        Ships (with long submerged pipes) would be prone to weather events and generally less reliable than an installed pipe. Perforation would be prone to clogging from build up so a nonstarter I would expect. Adding flex tubing and a relocation robot would be a maintenance headache as well. Not sure there is an easy optimization.

      • scythe

        today at 6:05 PM

        If you want to be really clever about it, maybe the ship is powered by the brine.

        https://en.wikipedia.org/wiki/Osmotic_power

    • 01100011

      today at 5:31 PM

      And it doesn't even need to be a rigid pipe. A flexible pipe made out of, say, waterproof fabric, could be cheaply made to extend miles while remaining open due to the pressure of the water pumped into it.

      • dylan604

        today at 5:34 PM

        Things left underwater tend to collect things on it which would make this much less porous over time.

• gausswho

  today at 6:26 PM

  It kinda depends where it's deposited, right? The expected AMOC collapse is fundamentally about salt imbalance.

• wolfi1

  today at 5:25 PM

  depends of course, how easy does the brine dissolve, how long does it take that it is so diluted that it can't do any harm, without that information it's not easy to tell

  • dylan604

    today at 5:31 PM

    These are often built near shallower parts along the coast where changes are more pronounced.

• boxed

  today at 6:04 PM

  I mean.. we really want to permanently desalinate the ocean somewhat too so putting the brine back seems kinda stupid. Put it on land, let it dry, sell some as table salt and dump the rest into abandoned mines.