A membrane is a selective layer that allows certain components to pass while rejecting others. Membrane technology uses these selective layers for efficient separation without the need for phase transitions. Membrane technology is used in a wide variety of processes, such as desalination, water purification, beer clarification, gas separation, and hemodialysis. Membranes are manufactured in various geometries: flat sheets, spiral wound, capillary, and tubular and various pore sizes: micro-, ultra- and nanofiltration. Separation by means of membranes requires pressure difference as the driving force.