A kinetic model for quantitative evaluation of the effect of hydrogen and osmolarity on hydrogen production by Caldicellulosiruptor saccharolyticus

Abstract

Background: Caldicellulosiruptor saccharolyticus has attracted increased interest as an industrial hydrogen (H2) producer. The aim of the present study was to develop a kinetic growth model for this extreme thermophile. The model is based on Monod kinetics supplemented with the inhibitory effects of H2and osmotic pressure, as well as the liquid-to-gas mass transfer of H2. Results: Mathematical expressions were developed to enable the simulation of microbial growth, substrate consumption and product formation. The model parameters were determined by fitting them to experimental data. The derived model corresponded well with experimental data from batch fermentations in which the stripping rates and substrate concentrations were varied. The model was used to simulate the inhibition of growth by H2and solute concentrations, giving a critical dissolved H2concentration of 2.2 mmol/L and an osmolarity of 0.27 to 29 mol/L. The inhibition by H2, being a function of the dissolved H2concentration, was demonstrated to be mainly dependent on H2productivity and mass transfer rate. The latter can be improved by increasing the stripping rate, thereby allowing higher H 2productivity. The experimentally determined degree of oversaturation of dissolved H2was 12 to 34 times the equilibrium concentration and was comparable to the values given by the model. Conclusions: The derived model is the first mechanistically based model for fermentative H 2production and provides useful information to improve the understanding of the growth behavior of C. saccharolyticus. The model can be used to determine optimal operating conditions for H2production regarding the substrate concentration and the stripping rate. © 2011 Ljunggren et al; licensee BioMed Central Ltd.