My tap water is at 54 TDS. My RODI system from BRS also has a TDS meter built in and at the beginning of the process when I start up my RODI system and have the water "back flushing" (or whatever the correct term is) the TDS usually reads around 20ish depending on how long the filters have been used. After a few minutes of flow both in and out read zero TDS, not only my meter but also on the BRS built in TDS meter.The waste line goes to drain and will have a slightly increase TDS from you supply water (try measuring both). You should have a “T” valve which you open each time you run the unit until the TDS creep is discarded. This T valve is situated between the RO membrane and your DI resin. You then close this valve to send the water from the RO membrane through the DI resin, producing the cleanest water, usually a TDS of 1 or 2 maximum.