Greatwell F2ROG400 High Capacity Pre-Filter Set includes one PAC Composite Filter and one Activated Carbon Filter that work as Stage 1 & 3 of the ROG400 high capacity tankless RO system. The first stage- PAC composite filter #FPAC1 is a 3-in-1 carbon wrap filter that works as a combination of two polypropylene sediment filters and a carbon filter. It removes dirt, rust, sand, and other particles and polishes water to ensure better taste and odorless high quality H2O. The third stage- Activated Carbon Filter #FAC1 can absorb chlorine, halide, organic contaminants, odors, and other impurities that contribute to bad tastes and odors. It is recommended that you replace these two filters every 6- 12 months.