Several terms, each with the prefix “nano-”, are now in use in the fields of filtration and its related separation processes, and it is important to be able to differentiate among their nuances of meaning. Firstly, there is nanometer itself, from which the others derive, and which is precise, measuring one thousandth of a micrometer or one millionth of a millimeter. It is thus a very small dimension – smaller than most viruses, smoke particles and protein molecules.
This reference to molecular dimensions is an important one for the first specific filtration usage. The development of membrane processes in the early 1960s led initially to reverse osmosis, and then to ultrafiltration. By the mid-1980s it was apparent that there were process application needs for a membrane a little looser than the RO types, but not going as far as ultrafiltration, i.e. allowing passage of some small dissolved ions, and the term nanofiltration was adopted for this class of membranes. Nanofiltration is a diffusion process, defined by its process separation capability, rather than by any actual dimension of the membrane.
True filtration processes are continually seeking smaller cut points so as to produce cleaner filtrates. This calls for finer filter media components – particles or fibres – given that finer components achieve finer degrees of separation. Hence the term nanoparticle as the basis for media made by compressing and sintering a mass of exceedingly fine particles. A degree of licence has to be accepted in such usage, since a lot of material described as made from nanoparticles in fact uses particle diameters of a few hundred nanometers. Nevertheless, ceramic membranes are now being made by the sintering of particles that are under 100 nm in size.
For many filtration applications, fibrous media are more useful than sintered granular materials, however fine the latter might be. The relatively recent ability to produce nanofibres by extrusion and drawing from the molten state, by methods such as electrospinning, has provided a welcome new range of media materials, with fibre diameters of consistently less than 100 nm. These materials are, of course, very delicate, and need careful support on strong substrates, but can now be made from inorganic as well as organic materials.
The term “nanotechnology” is now widely used, referring to a whole range of scientific, engineering and manufacturing activities involving very small things. The ability of very small particles to create strong flavours or odours is one reason for their success, but, unfortunately, the term has entered the public consciousness with a component of “fear of the unknown” attached to it. This does not concern filtration processes as such, but it does concern nanofibre and nanoparticle production and use, and the makers and users of these materials will have to take care not to magnify the concern.