What is a membrane?
Membranes are thin films of porous material which can be used for a number of chemical separations. Although many membranes are made from polymer films, membranes can be formed from ceramics, carbon fiber, and porous metal substrates. The pores can range from atomic dimensions (< 10 angstroms) to 100+ microns.
How are membranes used?
The small pores of the membranes can serve as a physical barrier, preventing passage of certain materials such as salt, bacteria and viruses while allowing the free passage of water and air. The desalination of water using reverse osmosis is a well known use of membranes as a filter.
Recently, recovery of water from sewage and recovery of whey protein from waste streams during cheese making have been carried out with ultrafiltration and microfiltation membranes which require much less pressure than reverse osmosis. While pressure is used to drive filtration, electrical current, osmotic pressure, and temperature can also be used to preferentially allow one component in a mixture to pass freely through the membrane while retaining the rest. The membrane structure and chemistry can also serve to carry out other separations.
Membranes provide a high surface area material where chemical reactions or diffusion can take place. For example, bundles of hollow fiber membranes (membranes in a thin tubular form) are used in dialysis to purify the blood by removing certain toxins. Membranes can also be used to carry out solvent extraction and catalysis while also serving to separate the reactants.
Hydrophobic membranes can be used to prevent passage of liquid water but allow vapor to pass (like Goretex). This property has been exploited in membrane distillation where brackish water is heated using solar power and the pure water vapor passes through the membrane and condensed to produce very high quality water. This uses less energy than boiling and utilizes bountiful but low value energy in remote areas.