Sterilization of air
• Aerobic fermentations require huge volumes of air.
• At 0.1 to 1 volume of gas per volume of liquid per minute.
• Volume of air must be absolutely sterile.
• Fed-batch or continuous culture systems place even more
demands on air sterilization, since air filters cannot be
replaced during the run period.
• An air supply of this magnitude requires sizable air
compressors.
• Adiabatic compression of air can increase the air
temperature (typically 150° to 220°C).
• Dry heat effective in killing organisms than moist heat.
• To kill spores, a temperature typically of 220C for 30 s is
required.
• Consequently, the adiabatic compression of process air
certainly aids air sterilization.
• Since the exit air cools rapidly and the pipes connecting
the compressor to the fermenter are difficult to maintain
as absolutely sterile, an air-filtration step is always used.
• Filtration is the most common method for sterilizing air in
large-scale bioprocesses; heat sterilization of gases is
economically impractical
• The filtration of gases can be accomplished using either
depth filters or surface filters.
• Depth filters consisting of compacted beds or pads of
fibrous material such as glass wool have been used widely
in the fermentation industry.
• Distances between the fibres in depth filters are typically
2-10 pm, about 10 times greater than the dimensions of
the bacteria and spores to be removed.
• Air-borne particles penetrate the bed to various depths
before their passage through the filter is arrested; the
depth of the filter medium required to produce air of
sufficient quality depends on the operating flow rate and
the incoming level of contamination.
• Cells are collected in depth filters by a combination of
impaction, interception, electrostatic effects, and, for
particles smaller than about 1.0 pm, diffusion to the fibres.
• Depth filters do not perform well if there are large
fluctuations in flow rate or if the air is wet; liquid
condensing in the filter increases the pressure drop,
causes channelling of the gas flow, and provides a pathway
for organisms to grow through the bed.
• Depth filters using glass wool were used, but depth filters
have been almost totally replaced with membrane
cartridge filters, which are surface filters.
• Depth filters using glass wool can show shrinkage and
hardening upon steam sterilization. In such cases,
channeling can occur, and the filter becomes far less
effective than would be predicted.
• More recent advances in the design of fiberglass filter
cartridges have overcome much of this disadvantage.
• Another serious problem with such fibrous filters is
wetting. If a filter wets, an easy path becomes available
for a contaminant to penetrate through it.
• A wet filter also greatly increases pressure drop. Thus, any
condensation within such a filter must be avoided.
• Surface filters (membrane cartridges) work using another
mechanism for particle removal, i.e sieving effect.
• Membranes with uniformly small pores prevent the
passage of particles with a radius larger than the pore
radius.
• Such filters can be steam-sterilized many times. Also, any
condensate formed on the nonsterile side cannot pass into
the sterile side.
• With both depth filters and membrane cartridges,
pressure drop is critical.
• The energy input for compressed air for a commercial-
scale process is significant.
• Thus, the design engineer has to balance the assurance of
sterility against pressure drop.
• These filters use steam-sterilisable polymeric membranes
which act as surface filters trapping contaminants as on a
sieve.
• Membrane filter cartridges typically contain a pleated,
hydrophobic filter with small and uniformly-sized pores
0.45 pm or less in diameter.
• The hydrophobic nature of the surface minimises
problems with filter wetting while the pleated
configuration allows a high filtration area to be packed
into a small cartridge volume.
• Pre-filters built into the cartridge or up-stream reduce
fouling of the membrane by removing large particles, oil,
water droplets and foam from the incoming gas.
• Filters are also used to sterilise effluent gases leaving
fermenters. In this application, the objective is to prevent
release into the atmosphere of any microorganisms
entrained in aerosols in the headspace of the reactor.
• The concentration of cells in fermenter off-gas is several
times greater than in air. Containment is particularly
important when organisms used in fermentation are
potentially harmful to plant personnel or the environment;
companies operating fermentations with pathogenic or
recombinant strains are required by regulatory authorities
to prevent escape of the cells.
•
• Generally, the bioprocess engineer is not involved in the
design of membrane cartridge units , but the testing of
such units for effectiveness is important.
• A filter should be evaluated with several different types of
tests.
• Because of the high costs associated with the loss of a
batch due to contamination, the choice of air filter to give
dependable protection for a fermentation while
minimizing pressure drop is critical.
• So far we have considered only inlet gas streams. With
fermentations involving pathogens (disease-causing
organisms) or recombinant DNA, all organisms must be
removed from the exhaust gas.
• The concentration of microbes in the exit gas is far higher
than in the inlet gas.
• Catalytically aided combustion (incineration) of the exit
gas is an effective, but expensive, solution. Consequently,
filtration of the exit air is of increasing importance
