system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it so difficult to turn sea water in to drinkable water?
gpt
It is not difficult, reverse osmosis works exceptionally well.

But.... it takes a lot of energy to run the pumps, and it takes water to back-flush system. 

Energy = $$$$. Most desalination plants are conveniently located near power plants for this reason.