The 3M Aqua-Pure AP5527 Under Sink Reverse Osmosis Filter Cartridge 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. The 3M Aqua-Pure AP5527 Under Sink Reverse Osmosis Filter Cartridge is also known as 5598101 and 55981-01. The AP-5527 is used in the water filter system 3M Aqua-Pure AP-RO5500.