The Pelican Water Reverse Osmosis Drinking Water System Filter Replacement Membrane is a 4-stage system. These 4-stages include a 20 micron sediment pre-filter, a granular activated carbon (GAC) pre-filter, reverse osmosis membrane and a granular activated carbon post-filter. The membrane design reduces the amount of water down the drain so it fills up the reserve tank quicker. For best results replace every 18 months.