WS-20BB Pentek Whole House Filter Replacement CartridgeThe PENTEK WS 20BB cartridge is 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. Under optimal operating conditions the WS 20BB can effectively treat water in applications desiring low hardness and scale deposits with a capacity of 4,500 grains.