Biohydrogen production from xylose at extreme thermophilic temperatures (70 °C) by mixed culture fermentation

Biohydrogen production from xylose at extreme thermophilic temperatures (70 degrees C) was investigated in batch and continuous-mode operation. Biohydrogen was successfully produced from xylose by repeated batch cultivations with mixed culture received from a biohydrogen reactor treating household solid wastes at 70 degrees C. The highest hydrogen yield of 1.62 +/- 0.02 mol-H-2/Mol-xylose(consumed) was obtained at initial xylose concentration of 0.5 g/L with synthetic medium amended with I g/L of yeast extract. Lower hydrogen yield was achieved at initial xylose concentration higher than 2 g/L. Addition of yeast extract in the cultivation medium resulted in significant improvement of hydrogen yield. The main metabolic products during xylose fermentation were acetate, ethanol, and lactate. The specific growth rates were able to fit the experimental points relatively well with Haldane equation assuming substrate inhibition, and the following kinetic parameters were obtained: the maximum specific growth rate (mu(max)) was 0.17h(-1), the half-saturation constant (K-s) was 0.75 g/L, and inhibition constant (K) was 3.72 g/L of xylose. Intermittent N-2 sparging could enhance hydrogen production when high hydrogen partial pressure (>0.14 atm) was present in the headspace of the batch reactors. Biohydrogen could be successfully produced in continuously stirred reactor (CSTR) operated at 72-h hydraulic retention time (HRT) with 1 g/L of xylose as substrate at 70 degrees C. The hydrogen production yield achieved in the CSTR was 1.36 +/- 0.03 mol-H-2/Mol-xylose(consumed),, and the production rate was 62 +/- 2 ml/d.L-reactor. The hydrogen content in the methane-free mixed gas was approximately 31 +/- 1%, and the rest was carbon dioxide. The main intermediate by-products from the effluent were acetate, formate, and ethanol at 4.25 +/- 0.10, 3.01 +/- 0.11, and 2.59 +/- 0.16 mM, respectively. (C) 2008 Elsevier Ltd. All rights reserved.