7100449 - 20 In Full Flowcarbon Block 5 Micron Nominal 40,000 Gallon FilterWatts 7100449 20" Full Flow Carbon Block 5 Micron Nominal 40,000 Gallon Filter The 20 inch Full Flow 5 nominal micron carbon block uses top of the line coconut shell housing. It is NSF standard 42 certified. This low cost carbon block filter outperforms granular activated carbon cartridges in chlorine reduction, lower pressure drop, lower cost and offers no water channeling. It offers a long life of up to 40,000 gallons at 4 gallon per minute flow