[quote=Hoppocrates;2473442]FYI, someone in my local brew club sent our local supermarket RO water to lab for analysis. Nearly came out distilled.
It depends on the nature of the feed water and the rejections of the individual ions. If you have feed water with TDS of 100 and get 98% rejection then your permeate should run TDS 2. If your feed TDS is 1000 then you should have permeat at 40. Things vary somewhat depending on the particular ion content (TDS is sort of an equivalent sodium chloride). My feed water (softened well water) usually runs TDS 165 or so and I can get permeate TDS ranging from 1 - 5. Typically it's 2 or 3.