I'm not sure dumping it in the sea would be a problem. At first it seems it would have to be because it would seem to lead to ever increasing brine levels which surely cannot be good.
But I'm not sure the brine levels would actually be ever increasing.
Consider the water that we take out. Short term it goes somewhere on land to be used for some purpose humans deem useful to have land water for, but after that I'd guess that much of it eventually returns to the sea.
At some point wouldn't we reach a steady state where the rate we are taking water out to desalinate equals the rate that water from past desalination is returning?
At that point brine levels would be higher than before we started widespread desalination, but they would not longer be rising. As long as that new steady state higher brine level is not too high it might be OK.
"Wastewater brine can pose a significant environmental hazard, both due to corrosive and sediment-forming effects of salts and toxicity of other chemicals diluted in it."
Unpolluted brine from desalination plants and cooling towers can be returned to the ocean. To limit the environmental impact, it can be diluted with another stream of water, such as the outfall of a wastewater treatment or power plant. Since brine is heavier than seawater and would accumulate on the ocean bottom, it requires methods to ensure proper diffusion, such as installing underwater diffusers in the sewerage.[13] Other methods include drying in evaporation ponds, injecting to deep wells, and storing and reusing the brine for irrigation, de-icing or dust control purposes.[12]
Seems like a well known problem with some solutions already in place
But I'm not sure the brine levels would actually be ever increasing.
Consider the water that we take out. Short term it goes somewhere on land to be used for some purpose humans deem useful to have land water for, but after that I'd guess that much of it eventually returns to the sea.
At some point wouldn't we reach a steady state where the rate we are taking water out to desalinate equals the rate that water from past desalination is returning?
At that point brine levels would be higher than before we started widespread desalination, but they would not longer be rising. As long as that new steady state higher brine level is not too high it might be OK.