The problems associated with producing large volumes of sterile air for aerobic fermentations are unique (see Chapter 5). Although sterilization by heating is technically possible, it has generally been regarded as too costly for full-scale operation (Cherry et al., 1963), although it might be used in the treatment of exhaust gases (Walker et al., 1987).
Absolute fixed-pore membrane systems using pleated membranes of PTFE are now widely used in the fermentation industry (Chapter 5) and have proved to be very reliable. This is very important when considering the costs associated with loss of fermentation batches due to contamination and production downtime due to filter failure. Banks (1979) has reported that a contamination probability of 1 in 1000 is economically acceptable for microbial batch fermentations, while in large scale animal-cell culture processes contamination rates as low as 2% are now achievable (Spier, 1988).
Operating costs will be based on the estimated life of the filters. Factors to consider include the cost of replacement filters or filter materials, servicing and labour. Even if the filters could be cleaned there must be an allowance for depreciation due to normal wear and tear. Savings may also be made by introducing series filtration whereby the major part of the foreign matter from the air stream is taken out by varying degrees of coarse filtration, thus reducing renewals of the more expensive high-efficiency filter media such as membrane filters.
The treatment of fermenter exhaust gases to satisfy containment requirements is also important (see also Chapters 5 and 7). Treatment is normally by filtration with 0.2-/um hydrofilters, but in-line incinerators may be an alternative approach. Filtration is usually cheaper but it may be necessary to supplement filtration with incineration depending on the process and scale (Walker et al., 1987).
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