Pentek WP25BB20P Polypropylene String-Wound Cartridge 4.5" X 20" 25 Micron (Full Case)SOLD IN CASE QUANTITY (6 in a case) WP 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 25 micron string wound filter cartridge is 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