DI is Demineralization by Ionization. A DI filter removes the few mineral ions that make it through the RO membrane; this makes sure that the water is as pure as possible. The DI filter also removes nitrates that are not removed by the membrane. The DI filter is a cartridge filled with an ion exchange resin in the form of small beads. When the water passes by the beads, the minerals and other ions leave the water and attach to the beads.