The 20in x 4.5in pre-filter replacement cartridge prevents silt, dirt, rust and other sediment particles as small as 1 micron from entering the main Optim H2O whole house filter. This pre-filter should be replaced every 6 months depending on water quality and sediment levels. Compare to Pelican Water, Express Water, Spring Well Water Filtration Systems, Ne Aqua Systems, Bluonics, Hardless NG, Culligan, APEC Water Systems, i Filters, & More!