The 3M Aqua-Pure Under Sink Reverse Osmosis Replacement Water Filter Cartridge AP5527 is a multi-stage reverse osmosis replacement water filter set of two cartridges that is for use with the NSF certified AP-RO5500 reverse osmosis system. This set of cartridges, as part of the system, is the pre-filter and post filter which reduces particulate, chlorine taste and odor.