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.