This high quality kit includes (3) KleenWater KW4520SW string wound water filter cartridges and (1) o-ring compatible with Pentair and Pentek "Big Blue" water filters. The cartridges are made in the USA of FDA grade polypropylene and will trap sediment down to 5 microns. To provide prospective to the 5 micron sediment entrapment rating, it is important to note the naked human eye can see down to 20 microns and a human hair is 100 microns in diameter. <br> <br>The cartridge should be changed every 6 months or upon substantial pressure reduction. The KW4520SW is a compatible replacement to Watts SF5-20-425, Pentek WP5BB20P, Shelco MS20FP545, Shelco MS20PP545 and Liquidtec SWF-45-2005. <br> <br>The o-ring will accommodate filter housings which require large diameter cartridges. These cartridges have an approximate diameter of 4.5 inches. The o-ring itself actually measures approximately 5.465 inches (inside diameter). We recommend consumers keep a spare o ring on hand to avoid urgent situations. This o-ring is compatible with Part # 151122. <br> <br>An o-ring facilitates a water tight seal between the water filter sump and the water filter head. The most common o-rings used in water filtration are made from Buna N material. Buna N is also commonly referred to as nitrile. This is the ideal choice for cold water filtration because they are inexpensive and have superior performance characteristics. The o-ring and cartridge are rated for a maximum temperature of 180 degrees.