Technology

Let me put it in a way you might understand:

• Ethical problem - Potentially no correct solution, tradeoffs likely.

• Engineering problem - Smart people do maths until problem is solved.

The verdict on brine impact is not out yet AFAIK. It impact species differently.

The good news is that they found no significant changes in the organisms living on the seafloor and other biological indicators. The researchers attributed this to the long history of industrial activities in the area, including cooling water discharge from a power plant adjacent to the desalination plant, which had already perturbed the natural setting.

Study of brine discharge from desalination plant finds good news and bad news (2019)

They are developing technologies to deal with brine like solar powered brine crystallization or mixing it with sea water before reintroducing it. More are discussed quickly in this review: Characteristics of Desalination Brine and Its Impacts on Marine Chemistry and Health, With Emphasis on the Persian/Arabian Gulf: A Review. That one is actually good and very readable to the layman (i.e me):

The conclusion puts it well

The physicochemical qualities of desalination brine are influenced by a number of factors, including the quality of the input water, desalination processes, and discharge methods. These features determine how it affects the chemistry and health of the ocean. The tolerance of marine species to the impacts of salt varies. Several brine management solutions have been developed lately, with increasing water recovery to eventually achieve zero-liquid discharge being the most promising strategy for reducing the harmful impact on marine health.

Much better explanation than my off the cuff attempt, great added detail.

I believe most places pump in extra seawater to heavily dilute the brine, then release the extra-salty water over a wide area back onto the sea.

The byproduct is a sort of like salt water, but a lot more concentrated. It’s mostly NaCl, but there’s also various other anions and cations such as Al, Ca, Mg, K, SO4 etc. Those metals came from the ocean, so you might be inclined to think that you can dump them back into the ocean. The problem comes when you dump a lot of that stuff and you get very high concentrations locally. When the concentration of those compounds is within the normal range, sea life can handle it. Once it’s above the limit, you can expect things to struggle or die. Eventually, it will get diluted in the ocean, but before that the concentrations will be high enough to cause damage to most living things.