Testing and evaluation of off-gas filters for bioreactors by a new bacterial aerosol challenge test method (TBAC)

A TNO bacterial aerosol challenge (TBAC) filter test rig was developed for direct assessment of the effectiveness of bioreactor off-gas filters as an alternative to routinely applied indirect wet integrity testing (IT). This TBAC test rig is based on bacterial aerosol challenging with Pseudomonas diminuta and dual monitoring by laser particle counting (LPC) and Andersen microbial sampling (AMS) of viable cells. The TBAC filter test rig is able to reproduce the various conditions encountered in fermentation processes. In experiments with several filters from one class, it was demonstrated that some filters were actually penetrated by up to 3,000 viable cells per test, despite their approval by commercially available IT test equipment. Repetitive filter use, prolonged use, and autoclaving of filters resulted in an increase in pressure drop over the filter but improved the performance of leaking/deviant filters due to building up of a filter cake (this phenomenon was identified by electron microscopy). The integrity tests used were found inadequate for accurate assessment of filter quality. Certification of filter lots by random tests of commercially available off-gas filters using sensitive direct methods such as those presented here might be advisable, as not all filters purchased were of appropriate quality.