Tier1 Comparable TFM-18-A Reverse Osmosis Membrane Reverse Osmosis drinking water is an affordable way to have excellent drinking water right from the tap. A replacement membrance offering 18 gallons of high quality drinking water per day. It is important to match the correct flow rate of the membrane with the flow rate of your RO system, for peak efficiency Great return on your investment: replace the TFM-18-A membrane approximately every 2 years, depending on water quality and your usage