The energy cost of desalination is a function of the salt concentration (saltier water => more power per liter of fresh water). So when doing commercial desali

The energy cost of desalination is a function of the salt concentration (saltier water => more power per liter of fresh water). So when doing commercial desalination you don't want to produce very high salinity output (because it means that at some point in the process you were getting fresh water from very salty water ==> more power). Of course, this has to offset against the cost of pumping et al, so there's an optimal cost output salinity. The TL;DR for most modern desalination plants is that the output salinity is not that high. Typically under 7% dissolved solids (versus the ocean at about 3.5%). Meaning that pumping it back into the ocean is fine. To put it a different way: If the overall salinity of the body of water being desalinated rose appreciably, it would probably render the desalination plant uneconomic.