I was curious too. The article mentions that Dubai’s old reverse osmosis systems rely on flash distillation which adds a lot of heat to the system and thus the brine being returned to the gulf waters.

“Unlike reverse osmosis, which removes salt and other contaminants by pushing water through a semipermeable membrane, multistage flash distillation relies on heat. Decades ago, when the U.A.E. began exploring desalination, the technology could better handle the Gulf’s high salinity, though reverse osmosis can now do the same. And although both technologies create brine, the byproduct of multistage flash distillation is far hotter, further disrupting the ecosystem.“