Final answer:
A membrane-type humidifier separates water from the gas stream using a semipermeable membrane and the process of osmosis.
Step-by-step explanation:
A membrane-type humidifier separates the water from the gas stream by means of a semipermeable membrane. This membrane allows the passage of water molecules but prevents the passage of solute molecules. When a dilute solution and a concentrated solution are separated by a semipermeable membrane, water molecules move from the dilute solution to the concentrated solution through the process of osmosis. The membrane in the humidifier must be able to tolerate high pressure to prevent the solute molecules from passing through.