in an ro/di unit there are several stages to the filter. the Di is the most exensive to replace. If you run pure tap through just the DI cartridge it wont last long for one. but in the RO each stage takes something out including chlorine which is devastating to DI resins. I wouldnt use just a DI cartidge.
lets say your water has a TDS of 100 the ro will take that down to 5-10% OF THE ORIGINAL so that when the DI filter starts its work it can actually take it to 0 or close to it and thats what we look for.
here is a normal stage outlook on RO/DI units
Stage 1 A high capacity, durable and long lasting sediment filter.
Stage 2 An acid washed* Gem coconut carbon block filter that removes up to 99% of chlorine.
Stage 3 Another acid washed* Gem coconut carbon block filter that removes the remainder of chemicals and chlorine, rated for 7,500 gallons of water. Together, the two carbon filters assure that chemicals and chlorine have been removed providing high quality water and protecting the most expensive part of the unit (the FilmTec membrane**) and all of our carbon filters are phosphate free to assure water purity.
Stage 4 A 50GPD FilmTec membrane that removes the majority of TDS (Total Dissolved Solids) such as bacteria, viruses, arsenic and mercury.
Stage 5 RO Side -- A 10" in-line carbon filter for further purity.
Stage 6 DI Side -- A DI filter that removes whatever remaining TDS may have been left behind by the RO membrane, thus resulting in the purest water possible
just useing the DI bypasses all the other stages and makes for a short life of an expensive filter cartridge.
HTH
Mike