You don't get all the water out that you put in. Currently, very efficient systems output about half the input as pure water, and half as doubly-impure. Less ef

You don't get all the water out that you put in. Currently, very efficient systems output about half the input as pure water, and half as doubly-impure. Less efficient systems are more like 1:2 or worse. I'm not an expert and can't explain it much beyond that. This summarizes the math better than I probably could: https://en.wikipedia.org/wiki/Reverse_osmosis#Disadvantages https://en.wikipedia.org/wiki/Reverse_osmosis#Disadvantages