Quick take
Compare feed TDS and RO TDS under steady-state flow. A rising rejection percentage problem is more informative than the absolute TDS number alone.
Membrane aging or fouling
RO membranes lose performance over time and can foul from hardness, iron or other feed-water conditions.
Seal or bypass leakage
An incorrectly seated membrane, damaged O-ring or internal bypass can mix feed water into the permeate stream.
Pressure and temperature matter
Low pressure and cold water reduce membrane production and can change apparent performance. Compare measurements under similar conditions.
Do not use TDS as a safety verdict
TDS is useful for membrane trending but does not identify lead, PFAS, arsenic or other individual contaminants.