Effect of pH control on the anaerobic H‐B‐E fermentation of syngas in bioreactors

Abstract

BACKGROUND Syngas and some waste gases are composed mainly of carbon monoxide, carbon dioxide and hydrogen, which can be used by some acetogenic bacteria to produce ethanol, butanol or hexanol, and represents an attractive alternative to the conventional ABE (acetone–butanol–ethanol) fermentation. Experiments were carried out in bioreactors under different conditions (pH 5.75 and 4.75) with continuous supply of a mixture of CO/CO2/H2/N2 converted by Clostridium carboxidivorans into hexanol, butanol and ethanol (H‐B‐E fermentation). RESULTS Applying different pH control strategies will affect the syngas fermentation pattern, among others in terms of bioconversion rates as well as final concentrations of acids and alcohols. The highest concentrations of alcohols were obtained at pH 5.75, i.e. 2.7 g L−1 ethanol, 1.9 g L−1 butanol and 0.85 g L−1 hexanol, whereas the maximum production rates were observed at pH 4.75, reaching 0.048 g‐ethanol h−1 g−1‐biomass, 0.037 g‐butanol h−1 g−1‐biomass, and 0.026 g‐hexanol h−1 g−1‐biomass. However, a low pH negatively affects growth and acids production in the first metabolic step, with lower growth and acids production at pH 4.75 than 5.75. Growth rates reached 0.0057 h−1 and 0.072 h−1, respectively, at pH 4.75 and 5.75. CONCLUSIONS Maintaining initially a higher pH of 5.75 allows accumulating higher concentrations of acids than when natural acidification takes place. Such higher concentrations of acids allow the production of higher amounts of alcohols as end metabolites, showing the importance of pH on bioconversion and biomass growth. © 2017 Society of Chemical Industry

Effect of salinity on C1-gas fermentation by Clostridium carboxidivorans producing acids and alcohols

“…1, as this does also not affect the main goal and conclusions of the study, focusing on estimating salt inhibition. In any case, it can be stated that the production of such compound was similar as in other related studies (Fernández-Naveira et al 2017b, 2019). During the low pH conditions, CO consumption started decreasing down to 74–76% (Fig.…”

“…The medium used in the continuous gas-fed bioreactor experiment described here, and in similar experiments with C. carboxidivorans and syngas as a carbon source described in recent literature and the initial sodium chloride concentration (Fernández-Naveira et al 2016a, 2017b) correspond to the composition commonly recommended in the standard culture medium (2 g/L sodium chloride). This corresponds to an initial conductivity around 14 mS/cm.…”

Evaluation of Media Components and Process Parameters in a Sensitive and Robust Fed-Batch Syngas Fermentation System with Clostridium ljungdahlii

“…A further important point of focus of this study was the evaluation of changes on the gas consumption profile of the culture. Not many studies can be found on the topic of syngas which show a system with the ability to track changes in the consumption of the gas substrates with on-line analytics, even if it is considered to be very relevant information [17]. In many cases, studies focusing on the effect of medium components or gas composition are performed in batch, with serum bottles, with the limitations that this implies (difficult pH control, no continuous gas feed and no possibility of out-gas analysis).…”

Experimental Design to Improve Cell Growth and Ethanol Production in Syngas Fermentation by Clostridium carboxidivorans

“…At the beginning of fermentation (4 to 5 h), a decrease in pH values before glucose depletion is detected, and this is followed by an increase in pH levels on both media. These phases are described in literature as acidogenesis and solventogenesis [27].…”