An ideal method for separation would be a barrier that allows the desired substance to pass while restraining other materials. 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. This next link duplicates term project above.