The SWC-45-2020 string wound filters are a cost-effective solution for the reduction of sand, silt, rust, dirt and scale particles. Hydronix string wound filter cartridges are made from 100% polypropylene cord. The Hydronix SWC-45-2020 filter cartridges are suitable for residential as well as commercial applications. This Hydronix polypropylene string wound filter fits most 20-inch Big Blue Filter Housings. The SWC series filters are suitable for both well and municipal water. The Hydronix string wound water filter should be changed when a decrease in pressure is noticed. Not intended for use with water that is microbiologically unsafe.