The EQ-304 is a replacement cartridge for the 10 inch sediment pre-filter of the Rhino Whole House Filter System. This five-micron pre-filter removes rust, dirt and microscopic particles from your water before filtration. To maintain optimum filtration, you should replace these filters every three months or 25,000 gallons - or as needed. You"ll know the pre-filter needs changing when the entire surface has changed color.