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Comment by squarefoot | Hacker News Reader

FTA: "Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers."

The capabilities are freaking interesting, but let's say someone builds a big enough settlement on a coastline or island in which every building has one or more of these devices on its rooftop. Would this release back enough salt so that the surrounding seawater becomes hostile to its previous lifeforms? Also, since then the water used in the process would contain more salt, how much would that render the device less efficient?

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This chunk from the article is, unfortunately, an outright lie.

By definition, removing salt from seawater leaves you with water and salt. The article handwaves the salt away by supposing that the desalinator will float on top of the ocean.

In actual production configurations other than towing it behind your sailboat, you will end up with a brine pool that needs to be disposed of, or a concentration in the wicking material that prevents low-concentration seawater from entering, or a pumping system.

This is not just a common negative externality, but a casual lie about it.

I don't think they are talking about the impact on the environment. I think they are talking about the procedures for operating the equipment.

The way I read it, they mean whoever operates this device does not have to periodically go empty some bin/tank of salt or brine.

An analogy would be to a frost-free refrigerator. When you say the freezer compartment of your fridge is frost-free, you don't mean that it never generates frost. You mean it does generate frost, but it also automatically removes it. It's "free" of frost in the sense that you are free of doing a chore that you have to do with a freezer that lacks this feature.

If you operate it directly on the open ocean, towing it behind your sailboat, you don't have to do anything special.

If you operate it in a commercial, protected context where you pump water through it, you will generate brine.

If you operate it en-masse in the ocean, the things that are insignificant at the scale of 1 and 10 square meters may become significant at the scale of 1 and 10 square kilometers.

All of these things are true for other desalinization systems, too. Claiming "you don't have to worry about brine" because it floats on the water surface is misleading at best. You can put any system you want on the water and make it float -- we have concrete and steel hulls, no problem -- but that doesn't solve the concentration problem.

It sounds like they're targeting small deployments in the developing world, at the scale of 1 to 10 square meters, not large commercial systems.

From the article:

> In production, they think a system built to serve the needs of a family might be built for around $100... The hope is that it could ultimately play a role in alleviating water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.

The problem with brine from desalination plants is that it's released at high concentration in one spot. This would not suffer from that problem.

Water in open ocean is not still - there are currents. I suppose if you had a large enough operation you could create a gradient of sufficient size to be self-sustaining without further mechanical agitation (such as your example of towing behind a ship). It would be an interesting exercise to see a large collection of these in operation. Heck even if you had to pump ocean water on land and back out to dilute the brine this process would still use significantly less energy overall than current desalinization plants.

Speaking of current plants, a recently constructed one in Carlsbad CA leverages the cooling outflow of a power plant to handle the dilution. The power plant was already there and so was the outflow - these things can be planned to leverage overlapping uses to further increase efficiency.

> supposing that the desalinator will float on top of the ocean.

I think that as its solar, you only have 50% duty cycle (more or less) so the idea is that you "just" slow down the water supply to remove the salt over night.

I wonder if the fresh water used in such a way wouldn’t eventually be released back into the ocean thus somewhat mitigating the effect.

There are two counterpoints to the issue of excess salt being an environmental hazard.

A) brine can be introduced back into the ocean combined with waste water. In the Cape Town drought of 2017 some small desalination plants were brought into service very quickly, and the brine was expelled in the same pipe as the outflow from a (treated waste) sewerage plant.

B) the ocean is big - very big - and at least by our coast seldom "calm". So outflow of anything would disperse very quickly. Hot-water outflow from a nuclear power station dissipates very quickly for example - typically within tens of metres of the outflow.

C) the natural salinity of the ocean varies a fair bit at the very local level - think river mouths - storm water - evaporation etc. Outside of specifc closed bodies of water (Dead Sea etc) we'd need desalination on a massive scale to even measure the impact.

Why release the salt back into sea water? This is salt production alongside desalination.

We already have a massive oversupply of salt from existing desalination. The cost of table salt is basically all transportation at this point.

> The cost of table salt is basically all transportation at this point.

Using the salt on site would save the transport, though?

Can be buried also.

In large enough systems some kind of “salt removal” mechanism might be added - I think it wasn't added to a PoC just to make it as simple and cheap as possible.

Salt release is already a huge environmental concern with exiting desalinization systems. Energy consumption and installation costs aren't the sole (and probably not the main) problems. Desalinization is inherently a pollution.

Why not release the salt back to the sea? Presumably it would not increase the salt content of the ocean very much considering the vastness of the ocean and the sheer quantity of fresh water that is being added to it by melting glaciers, etc? What am I missing?

The salt you release isn't instantly diluted in the whole ocean and we're speaking, for a plant, of millions cubic meters of salt enriched water per day. The release area is just dying.

Can you provide a source for that? I dove near a desalination plant exit pipe in Saudi, the impact on the marine live did not seem to big in the area. But it was a small plant for a few thousand people and we did not do a proper assessment.

I don't doubt there is an effect, but am not convinced that the "release area just dying" is correct.

https://www.keiken-engineering.com/en/brine-disposal-how-eff...

Brine disposal is indeed a complex issue. Should be noted the above URL is from a desalinization industry proponent so I would say it's probably a little more optimistically biased but it was one of the better summaries I found so it still has value from that perspective. If you do more digging around each of the solutions they discuss you can find more pro's/con's for each and you will quickly find out that it is a pretty significant issue.

The link does not really show any evidence that marine ecology at brine outlets is disturbed. It does however suggest that pouring the brine on land or roads (for de-icing) are alternative solutions, which I doubt ;).

I was more thinking of proper studies, such as reviewed in [0], which says in the abstract: "Ecological monitoring studies have found variable effects ranging from no significant impacts to benthic communities, through to widespread alterations to community structure in seagrass, coral reef and soft-sediment ecosystems when discharges are released to poorly flushed environments. In most other cases environmental effects appear to be limited to within 10s of meters of outfalls."

So, impact yes, but "release area is just dying", probably not.

[0] https://www.sciencedirect.com/science/article/abs/pii/S00431...

This makes sense to me. I wonder how feasible it would be to release the saltier water over a broader area to reduce the salinity in any one area? Also, there are at least some ecological benefits to increasing the amount of fresh water. An extreme example is the lake that appeared in the middle of a desert in the United Arab Emirates. I have no idea how we weigh one against another.

Now you’re paying for salt transportation again.

Not if you have many small desalination plants distributed over a larger area. This would work well for less-densely populated areas.

That is acceptable

Why not just collect the salt, and sell it as... salt? Even if its not ok for cooking, it should be fine for roads? I am sure there are other industrial uses for salt.

Salt from roads is seriously contaminating our environment and significantly impacting farming in more and more areas.

Ever hear of the Romans salting the earth of people the conquered? It was so they couldn't grow crops - yet we routinely do this to ourselves - pretty daft.

We also farm in a way that is almost optimized for erosion of topsoil. We’ve already lost a third of it and it only renews on geological timescales. Daft is too polite.

Actual salt is rarely used on the roads in the USA anymore. Majority use magnesium chloride now. Don’t know about the rest of the world.

Lots of reasons, pros and cons. I work in automotive and hate it.

A small part of it can be avoided by combining this with a salt evaporation pond (https://en.wikipedia.org/wiki/Salt_evaporation_pond)

Problem will be that we’ll get way too much salt (a few grams of salt per liter of water), so it won’t help much.

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