Pre-filter is a combination for the first three stages of purification in RO purification, stage 1 is sediment filter which also known as Polypropylene Sediment Cartridge Filters. second and third, carbon stages. which is normally named as GAC (Granular Activated carbon) and CTO (Chlorine, Taste, and Odor) filterPP-Sediment Removes Rust, Dirt, Sand, leaves, and other sediments.