Semipermeable membranes sometimes meet this ideal by having pores through which one substance can pass while others cannot.
Synthetic membranes are available with a wide range of pore sizes.
Very minute pores allow only molecules of gases or water to pass, intermediate sizes of pores discriminate between molecules such as different proteins, and larger pores may be just fine enough to prevent passage of particles and cells such as microorganisms. In other words, as material passes through the membrane and reaches the product compartment, the concentration difference between compartments declines. At some point the concentration driving force becomes too small to keep transfer rate in a practical range. This means that there will be a product stream and a waste stream that may present disposal problems. Convert saline water to potable water Sterilize air or water by filtering out microorganisms Concentration of solutions by expelling water Recovery of industrial products that do not pass the membrane Collection of materials for analysis, e.g., using a membrane filter instead of the MPN test for estimating coliforms in water.