the salt particles in the seawater are rejected from passing through the membrane to the freshwater side and remain behind on the concentrated salt water side the reverse osmosis membrane can be thought of as a number of sealed envelopes connected at their open ends to a tube there are spacers between each envelope which allow water to flow across the membranes the membrane envelopes and spacers are then wound around the tube like a roll of paper towels the reverse osmosis membranes are then closed in a fiberglass shell the membranes are connected and to end usually six to seven membranes together and housed in vessels that are built to withstand pressures up to twelve hundred psi as the pressure icy water enters the pressure vessel and flows across the membrane surface the water molecules are forced into and through the membrane envelopes leaving the salt molecules behind the d salted water passes through the membrane and emerges at low pressure where it is collected in a tube and directed to one end of the pressure vessel the concentrated salt stream that is rejected from flowing through the membrane continues to pass across the membrane surface where it is collected separately.