Ley, Ronald John; (2000) The effect of containment measures applied to the exhaust gas of a fermenter. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Many industrial fermentations have the potential to release micro-organisms either accidentally or incidentally during the course of their operation. The agitation and aeration of microbial broths can produce foams and aerosols that must be contained to meet biosafety regulations. The Turbosep foam separator, which is widely used, is a static device designed to separate the foams emerging from highly aerated fermentation processes. The foam is recycled to the fermenter, allowing the exhaust gases to pass out to the local environment often through a High Efficiency Particulate Air (HEPA) filter. The influence of the operating parameters of fermentation on the release of microorganisms into the exhaust gas was investigated. The release of process cells was monitored using a cyclone-quantitative polymerase chain reaction methodology. It was found that microbial release rates increased with increasing agitation and aeration rates. However, micro-organisms were released into the exhaust gas in only very low numbers. At an agitation rate of 500 rpm and an aeration rate of 1.5L min-1, approximately 1.5x105 E.coli cells were released per minute from a 2L fermenter. Based on this result it can be estimated that 5x107 cells will be released over the course of a 5.5 hour fermentation. This is equivalent to the loss of 3.3μL of broth at harvest The installation of the Turbosep onto a fermenter was found to reduce the microbial burden of the exhaust gas by approximately 6 orders of magnitude whilst recirculating foam. In the absence of foam the microbial burden of the exhaust gas was reduced by approximately 4 orders of magnitude. Exhaust gas containment can, in light of the data presented here, utilise alternative methods of prefiltering to minimise the challenge to the HEPA filter. However as this challenge is minimal, containment systems should be designed to reduce the accidental release rather than the incidental. A novel control strategy for the addition of anti-foam into a fermenter fitted with a Turbosep has been developed. Differential pressure measurements across the Turbosep were linked to the provision and regulation of antifoam addition. Using the Turbosep in conjunction with the differential pressure control strategy allowed the addition of antifoam to be totally automated and dependent on the requirements of the fermentation at any time. The Turbosep was determined to reduce the antifoam requirements of a 6000L E.coli fermentation by approximately 66%, leading to a 16% increase in productivity without changing any of the process operating parameters Computational fluid dynamics (CFD) has been used to model the performance of the Turbosep in terms of particle collection efficiency. Performance curves were produced with approximately the same shape and d50 as those obtained by experiment. Furthermore the predicted pressure drops were in excellent agreement with the measured data. The CFD model was able to predict the salient features of the Turbosep flow field, thus providing a better understanding of the fluid dynamics of the device. The CFD model also provided a reliable and relatively inexpensive method of redesigning the Turbosep to increase performance. The predicted particle collection efficiency of the new design was at least 30% higher than the original design.

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