In the past, air filters were columns that approached diameters of one-fourth of the fermenter diameter. The packing was slag wool that lumped up with repeated use, fiberglass that broke down because of repeated thermal expansion and contraction, or beads of carbon that sometime underwent spontaneous combustion and melted the column. Carbon packing works fairly well but is too bulky. Currently, there is a pronounced trend to use of membrane filters in a cartridge configuration for air sterilization to obtain excellent performance with units of relatively small size. Moisture is bad for all methods of air sterilization and may help microorganisms to pass. A membrane pore size of 0.2 to 0.3 micrometers is recommended. Hydrophillic membranes should not be used because moisture held tightly in the pores is not dislodged unless there is quite high pressure drop across the membrane. Moisture tends to drain from hydrophobic membranes and collect in a sump. The units are modular and housed in a shell with a manifold. Sizing is based on the number of cartridges needed.
Air leaving a vessel in which pathogenic organisms are cultured is sterilized by heating. Air in a room for culturing microorganisms may be exposed to ultraviolet light to reduce the number of potential contaminants. Ultraviolet light penetrates poorly through glass, so organisms in shake flasks are not killed. Usually, a single light switch turns on white light before a person enters, and the u.v. goes on when the person flips the switch on leaving.
There are also ultraviolet lights mounted in flow devices for water sterilization, but quartz bulbs or enclosures are needed to circumvent the attenuation of u.v. wavelengths. Such devices are also plagued by turbidity in the water and by dirt forming on the transparent surfaces.
There have been some attempts to commercialize enzymatic sterilization of air. The basic concept is to bring microorganisms and viruses into contact with enzymes that attack nucleic acids. Viruses are destroyed by passage through a labyrinth of surfaces coated with deoxyribonuclease enzymes.