Is zero tds possible with RO only?

[sphelps]

Yeah it's possible the DI actually does very little, will usually drop the TDS by around 10ppm at most. I've setup RO systems in Calgary before and got 0 without the use of DI.

280 does seem awfully high for Calgary though, but it could just be your location. It likely depends on what kind of solids you're dealing with, perhaps the pre-filters are doing a lot of the work prior to it hitting the RO membrane. It would be interesting to compare TDS before and after the pre-filterers.