Originally Posted by mabrungard
RO water from a properly operating unit should have a TDS of less than 10 ppm.
I agree in part, but that's not completely true. We would all like to have our RO put out lower than 10ppmTDS, but most don't. I try to explain to customer's why I can get their RO to produce twice as good as they can all the time. Some listen, but sadly most don't.
The rule we generally go by is percentage. We want the RO rejection to be 85% or better(90% for rental customers.) Usually, you can get a quality RO to run at 90+% for 10+years. A lot of times the lower end "box store" units will give you 90+% for only a couple of years.
In short, if your supply water to the RO is 400ppmTDS we strive for a maximum of 40-50ppmTDS from the RO. A little higher isn't a big deal, but still the lower the better.
In closing; my supply to my RO is 360-500ppmTDS. My RO has ran at 10ppmTDS or lower for over 13 years. (Just showing that you CAN get a quality RO to produce great RO water for much longer than most people think or say.)