I'm more interested in what you possibly looked up that made it hard to find...
> Furthermore, chemicals such as biocides, surface−active agents, anti-scale additives, and solid residues from filter backflushing may be present in the effluent discharge on a continuous or periodic basis, posing a risk to the environment
> Brine effluents from RO desalination plants not only have a high salt content but typically also contain compounds from the desalination process, such as phosphonate-based antiscalants and ferric (or alum) sulphate-based coagulants
[1] https://www.frontiersin.org/articles/10.3389/fmars.2022.8451...
[2] https://www.sciencedirect.com/science/article/abs/pii/S00431...
> However, concern also exists regarding the use and release of toxic anti-foulants and anti-scalants to maintain plant infrastructure
[3] https://www.sciencedirect.com/science/article/abs/pii/S00431...
> The super-salty substance is made even more toxic by the chemicals used in the desalination process, researchers reported in the journal Science of the Total Environment.
> Copper and chlorine, for example, are both commonly used.
[4] https://phys.org/news/2019-01-brine-highlights-toxic-problem...