Dispersion of Respirable Aerosols in a Fermenter and their Removal in an Exhaust System

Abstract

When strains of bacteria and fungi are changed through genetic engineering and are used in industrial processes to bring about improved product yields, these new strains may expose humans to new types of RNA or DNA. Some industrial fermentation processes involve materials that are potential health hazards if aerosolized, released to the ambient environment, and inhaled by workers nearby. To study the potential release of aerosol from a fementer, a measurement system was developed to explore respirable aerosol formation characteristics and controllability. The system, which provides real-time information on the concentrations and size distributions of 0.1 to 3.0-μm-diameter residues of aerosolized liquid droplets, was used to investigate the influence of aeration rate, agitation rate, bacterial growth, and the addition of antifoaming agent on aerosol concentration in the head space and two locations in the exhaust system of an industrial pilot scale seed fermenter equipped with a mechanical foam breaker.

The experimental data obtained indicated that the majority of effluent aerosol particles released were in the submicrometer size range. This includes bacteria and growth products. Particle concentration counts in the exhaust system increased by a factor of 100 during the fermentation process (the microbial growth period). This dramatic increase was shown to correlate with bacterial growth, foaming, and subsequent breaking of the uppermost foam layer into small droplets by the mechanical foam breaker. The ceramic filter in the exhaust system of the fermenter removed over 98 percent of the aerosol particles by count larger than 0.7 μm diameter, but up to 50 percent by count of the 0.1 to 0.3-μm particles were released into the ambient air. Thus, while virtually 100 percent of particle mass is removed in the exhaust, extremely small bacteria may escape to the surrounding environment.