| Summary: | In this paper, the inhibitory effects of added hydrogen in reactor headspace on fermentative hydrogen production from acidogenesis of glucose by a bacterium, <i>Clostridium acetobutylicum</i>, was investigated experimentally in a batch reactor. It was observed that hydrogen itself became an acute inhibitor of hydrogen production if it accumulated excessively in the reactor headspace. A mathematical model to simulate and predict biological hydrogen production process was developed. The Monod model, which is a simple growth model, was modified to take inhibition kinetics on microbial growth into account. The modified model was then used to investigate the effect of hydrogen concentration on microbial growth and production rate of hydrogen. The inhibition was moderate as hydrogen concentration increased from 10% to 30% (<i>v/v</i>). However, a strong inhibition in microbial growth and hydrogen production rate was observed as the addition of H<sub>2</sub> increased from 30% to 40% (<i>v/v</i>). Practically, an extended lag in microbial growth and considerably low hydrogen production rate were detected when 50% (<i>v/v</i>) of the reactor headspace was filled with hydrogen. The maximum specific growth rate (µ<sub>max</sub>), substrate saturation constant (ks), a critical hydrogen concentration at which microbial growth ceased (H<sub>2</sub>*) and degree of inhibition were found to be 0.976 h<sup>−1</sup>, 0.63 ± 0.01 gL, 24.74 mM, and 0.4786, respectively.
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