I have the Duo 60 watermaer and I'm getting 480ppm from the kitchen faucet. That is just within the EPA limit of 500ppm. It has always been high, starting at about 380ppm when I commissioned the new membranes. I wash the prefilter every two weeks, or more often in turbid water.
I've read that many people are experiencing half that number. The membranes were new in 11/2016, and we cleaned two of the three compartents in the water tank (we decided to leave the one under the freezer for another time).
Anyone have an idea why I might be getting such high numbers?