How many have? You seem to know a lot about it

The water wars prediction is especially interesting because when i was young they always pointed to a few locations where it would happen first, and top of the list was almost always Golan Heights where there were already water related tensions and conflict - however what actually happened is Israel invested in a huge desalination project which now supplies all their tap water, they also supply water to their neighbour Jordan who pays in solar power that's used to run the system.

Saudi Arabia likewise has eased tensions within it's borders through huge desalination investment, annalists predicted that competition for water resources would tear the country apart but again what actually happened is they invested in desalination which currently supplies around half their tap water and it's helped add perceived legitimacy and gained support for the ruling family. Not people i like seeing gaining support but a far better outcome than the chaos and human suffering which was predicted.

Remineralization is a well known and solved problem in existing processes (RO and distillation). I personally built and ran a high performance RO that produced < 5ppm TDS water on my farm, and experienced the ill effects of drinking demineralized water. Excess thirst, urination and muscle cramps.

The WHO published an excellent study on this issue. The issue is not an actual lack of minerals, as we get very little of our minerals from water, but the fact that your body is not designed to absorb water with zero solutes. Effectively the sensors in your gut go out of span and you end up dumping solutes from your bloodstream instead.

The solution is very simple and cheap, that is to add any salt so that your body can detect it. Sodium chloride is fine, being "normal salt" of course, but I add potassium chloride as most of us get too much sodium as is. This is available as "salt substitute".

A pinch in a glass or a tablespoon in a 5 gallon jug is enough to completely eliminate any ill effects. Commercially as mentioned you can blend back some of the brine stream. It's not really a problem as long as you don't effectively deliver DI water to your customers by accident.

yes they do that in some facilities, it's called sewer discharge and can be quiet effective in a well monitored and designed system, surface water discharge uses a similar method of dribbling brine into the water as part of a system that uses ocean currents and tides to disperse the brine back into the ocean.

While brine return is a complicated and important step it's really not some major ecosystem destroying problem in any of the modern installs - it's just important to model and monitor the system, the same way sewage systems find a location where currents carry stuff away and allow it to disperse brine return systems do, with brine it's just stuff that belongs in the ocean anyway so it all mixes back in fairly quickly.

A lot of people seem to like to learn the difficulties involved in a new tech and then just use negative thinking to exaggerate it into a reason the tech will never be useful even after decades of improvement and investment. There are huge projects around the world which have done really positive things for local ecosystems, they're even refilling the sea of galilee after decades of over extraction and allowing groundwater levels to restore.

Are you worried it's not going to make its way back to the ocean?

Definitely something needing consideration. Currently reverse osmosis plants are pumping super saline water out. I haven't looked too closely at how much damage they're doing. I've heard it's a problem they're trying to address

I also know ice cap melting is reducing salinity in other areas, which effects tempurature absorption and currents.