ASABE Technical Library - Abstract

Abstract. Off-gas analysis is a common method for assessing the performance of many bioprocesses, but often little regard is given to the dissolved gas concentrations to which the microbes are actually exposed. In this work, a membrane-inlet mass spectrometer (MIMS) was used to quantitatively assess dissolved gas (H₂ and CO₂) concentrations during thermophilic fermentation of 2 g L^-1 cellobiose with Clostridium thermocellum in a N₂-sparged bioreactor. During baseline operating conditions (100 rpm mixing and 10 mL min^-1 N₂ sparging) H_2(aq), and to a lesser extent CO_2(aq), were detected in levels higher than would be predicted using gas-phase measurements and the assumption of equilibrium with the liquid phase. Under these conditions, H₂ supersaturation ratios (R_H2) as high as 22.8 were observed, and CO₂ supersaturation ratios were measured to be as high as 1.6. In an effort to improve liquid-to-gas mass transfer and reduce supersaturation, a second condition was evaluated in which stirring was increased to 200 rpm, N₂ sparging was increased to 100 mL min^-1, and surface mixing was applied. Under this condition, H₂ supersaturation was reduced (maximum R_H2 = 9.6), while CO_2(aq) measurements showed no significant supersaturation. Accounting for the elevated [H₂]_(aq) in the measured with the MIMS led to an improved electron balance, and in a similar way the additional CO_2(aq) (when supersaturated) led to improved carbon recovery. This work shows how liquid-to-gas mass transfer can influence carbon flux in microbial metabolism, which is an important process engineering parameter in scale-up.