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Comment by fnordpiglet | Hacker News Reader

My understanding the big drawback of desalination is the residual substances. They trivialize it in this article as salt, but it’s actually a slurry of heavy metals and other toxic stuff, with lots of salt.

This system (better described here [0]) simply washes the brine out as it makes it into the ocean, so the hypertoxic concentrate isn’t an issue. (Original paper here [1])

“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.”

0 https://news.mit.edu/2020/passive-solar-powered-water-desali...

1 https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...

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What's the actual difference between not taking the non-water out of the ocean vs putting the non-water back into the ocean?

The sea becomes more salty over time when just the salt is added back.

Not the sea, but the localized area might have a brine lake, which is a deadzone for marine life there. If diffused over a large area, it will not have an impact. The only problem is cost.

The minuscule amount of salts being added into the ocean does nothing to the whole ocean. One river will have washed out more salt.

Salinas or salt marshes are highly productive for native species adapted to high salt levels. Brine shrimp feed Flamingoes (if you are lucky) or other aquatic bird species.

No, you can't just assume this salt burden is fine, and that we get good salt marshes from it, but it's also not entirely true that high salt level brine is going to "destroy" the ecology. It's going to alter it, for sure. That isn't always very good, it depends what else has also destroyed the ecology, and what upsides there are to making a salt marsh.

https://www.sciencedirect.com/science/article/pii/S034181622...

Except this article is about irrigating with sea water to mix with fresh water. A salt water marsh is not a particularly concentrated salt water, and is diluted by rain and water flow above and under ground. The brine from desalination is extremely concentrated and toxic. In theory you could dilute it with large amount of fresh water to make salt marshes, but the entire point is to make fresh water from salt water. For a salt marsh you’re better doing as in the article you linked and pumping sea water from the ocean.

Yes, but after the fresh water is used, it will flow downwards and rejoin the ocean.

It's called the water cycle for a reason. Water doesn't disappear after being used.

In any event, the Greenland ice sheet is on track to melt over the next few centuries and add 2,850,000 cubic kilometers of water to the ocean. We're not in danger of making it too salty with desalination.

Sure, but it's about locality/concentration - it's the difference between walking through rain without a hat, and getting a bucket of water to the face. It takes time for the salinity to even out, and in the meanwhile things in the area are exposed to way too-concentrated salt.

Or perhaps it's like drowning - sure, people can swim for a total period of hours a day, but they're never underwater for a continuous duration of more than a few minutes in a row.

Considering the effect of evaporation, I think it would be a non-issue. When oceans evaporate they increase salinity, but then rivers will bring that water back.

The Mediterranean Sea once completely evaporated, leaving a large salt deposit. Later, the Gibraltar barrier broke, allowing water to flood in with new salts, forming a saltier-than-average sea: 3.8 g/100g vs 3.4 in the World Ocean. But that is no problem for marine fauna and vegetation.

> What's the actual difference between not taking the non-water out of the ocean

Hold on, still parsing…

The brine is likely highly valuable. e.g. lithium processing, etc.

Very likely with cheap or negative prices for brine we will solve many other resource scarcity issues.

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