Pentek GAC20-BB Granular Carbon Filter Cartridge 4.5" X 20" 20 MicronGAC Series cartridges effectively reduce unwanted tastes, odor and chlorine taste & odor from your drinking water.* They are designed to allow maximum contact between the water and carbon, ensuring maximum adsorption. The construction of the 20 micron filter cartridge allows water to enter at one end and pass through the entire length of the carbon bed before exiting the other end of the cartridge, while an internal expansion pad minimizes channeling