This is the check valve fitting for all Reverse Osmosis Systems that Aquasafe offers. The check valve is the elbow located on the reverse osmosis membrane housing to which the blue tube is connected. Its purpose is to prevent back pressure caused by filling the holding tank from ever reach the R.O Membrane. This greatly helps to improve the output production function of the reverse osmosis membrane during the last stages of filling the holding tank.For the convenience of our clients, the check valve serves the same purpose as would a permeate pump.