Pentek WP5BB20P Polypropylene String-Wound Cartridge 4.5" X 20" 5 MicronWP BB Series Cartridges are manufactured in 10" and 20" lengths and in 4. 5" diameters from a durable polypropylene string that is wound around a rigid polypropylene core. The WP BB Cartridges are an economical solution to reduce fine sediment including rust, sand and scale particles. The material in the WP BB Cartridges is a fibrous polypropylene media wound in a precise pattern around a polypropylene core, providing greater surface area to