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.