I think you had a malfunctioning system then. There really is no way anything can get past the RO membrane. RO systems typically have two particle filters to keep junk from clogging the RO membrane because it is SO fine. This is why keeping these particle filters changed out regularly extends the life of your (expensive) RO membrane. If you had junk in your storage tank you either had a busted ROfrom lack of maintenanc, or the drain back from the RO leaked into your clean water tank.
I am sure you can do a little search and find out for yourself. The sediment filter removes sand, soil, silt, rust and other things that come thru the pipe itself. The carbon can absolve chlorine, pesticides, industrial solvents ...
Unless you can get 0 TDS all the time, there will be some impurities in that storage tank. If your tap water starts out with TDI of ~200, your filter can reduce that to 0-3. Some parts of the city of Houston start with TDI of 300-400, you can get that down to 5 maybe and that is when all stages of your filter are new. In 3 months, 6 months, a year, they do a little worse and every day, that 2, 3% of "waste" accumulates at the bottom of your storage tank.
Let me amend that, if you use DI resin you can get your TDS down to 0-3 but you don't want nor need that 100% pure water for drinking. That is meant for salt water aquarium.
