As mentioned before, the sediment and carbon filters are only there to remove particulate and chemical matter before it reaches and fouls the RO membrane. There are a few different kinds of carbon - Carbon block, GAC (Granular Activated Carbon), catalytic and PAC (Powder Activated Carbon). We normally use two carbon block filters for their superior chlorine removal and its ability to remove some chloramine. The catalytic carbon is used mainly for chloramine removal. The RO membrane is there for removal of bacteria, viruses, pathogens, endotoxins, metals, salts and other TDS. Keep in mind that this process still leaves some TDS, but it's within safe levels. The DI is normally used as a post filter for the membrane. It removes the remaining TDS by the ion exchange process using anion and cation resin beds. Its main function is to remove the rest of the salt and metal ions from the water. For drinking water units, a "Taste and Odor" filter is installed after the RO membrane. This does not remove any TDS, but it cleans up any taste or odor that may be left after the RO process.