Pentek WS-20BB Softening Resin Cartridge 4.5" X 20"Pentair Pentek WS Series of Water Softening Filter Cartridges are designed to allow optimal contact between water and resin surfaces, ensuring maximum ion exchange. The internal expansion pad minimizes channeling, while the post filter eliminates any potential for bypass. Designed for low water usage, these units will easily deliver softened product water through the use of a sodium based, non solvent rinsed softening resin. The cartridge style design