As a visceral example of this in the other direction, the freshwater plume of the Amazon River extends more than 60km into the ocean. [0]
I would love to see these plants placed in areas where there's a nearby dry below-sea-level basin, into which the brine may be discharged. The Salton Sea in CA is one example, there's another similar location in Egypt I'm aware of. The advantage of such locations is they are usually extremely hot and arid, which means there's generally not much of a local ecosystem or human population, and there is ample solar power availability.
[0] https://eos.org/science-updates/the-amazon-rivers-ecosystem-...
The Salton Sea's salinity is already well over that of the ocean (44g per liter compared to 35), and is getting saltier. Considering that the very existence of the Salton Sea is already an ecological disaster, I don't really see dumping salt into it to be another one.
The Salton Sea is 70 miles from the Pacific Ocean. If this were built, it would be only the third longest water tunnel in the world, and second in the US.
It really doesn't, or perhaps only in the most pedantic sense.
That "ocean" is still the mouth of the Amazon itself, and most of it's less than 10 meters deep. It's really the estuary of the river itself. See: https://www.frontiersin.org/articles/10.3389/fmars.2017.0002...
http://essay.utwente.nl/79579/1/Dols%2C%20F.J.%201862227%20_...
This is false and somewhat dishonest. This is simply a choice of not diluting it enough. There is nothing about the discharge from reverse osmosis that is any more fundamentally toxic than the natural process of evaporation.
Proper dilution is essential, and treating the discharge as fundamentally toxic actually undermines the engineering to do this proper dilution because people will figure “oh well, I guess there’s nothing we can do as it’s going to tend to kill no matter what.”
People need to stop misleading about discharge toxicity.
If that choice were up to engineers, it's fine.
In practice that choice is actually made by MBA types, a field where harmful short-termism is almost a religion.
You can't just wave a word at it like dilution and think you're solving an engineering problem.
If you got 1 gallon out of sea water, what do you think you need to dilute it to be safe? Typically, it's 99%∆.
It'll kill however, plenty of things.
If you dump it on a living ecosystem you tend to kill it. Living ecosystems are, unfortunately, concentrated right where we are desalinating and is cheap to dump.
Compounding this problem is that water mixes much more slowly than your intuition suggests. It can stay a coherent mass of high-salt water way longer than you'd think, killing as it goes. This is one of the more surprising things I've learned in the past few years, honestly. Your kitchen-scale-based intuition of how long it takes for liquids of different characteristics to blend together turns out to be way off.
Trying to pipe it away to somewhere where it is less of a problem is expensive.
In the long term just dropping it back into the ocean is not a big deal, but that short term is surprisingly destructive. You'd think you could just drop it in and maybe a few hundred feet from the outlet it would be all dissipated and harmless, but unfortunately the physics don't work out that way.