Why would their be brine? Run the brine through another filtration system, and keep running through another and another until you eventually get salt
Osmotic pressure rises with brine concentration—diminishing returns issue.
Cost. Yes, you can eventually get just water and salt. But every process you use to extra additional water costs more money. And clean water doesn’t sell for but so much.
This is why desalination has never taken off: every part of it has a steep cost/benefit from energy costs to disposal costs to environmental impacts (e.g. killing every living thing at the brine dumping point). But salt water is relatively abundant, particularly for places that need a lot of water. So the search continues for a cheaper, more effective, less damaging way to do it. That’s the whole purpose of this article. Sadly, it seems that breakthrough has yet to be found.