Biohydrogen Production by Dark Fermentation of Standard Xylose in a Semi-continuous Reactor

The continuous production of biohydrogen by dark fermentation was evaluated in a bench reactor with thermally pretreated anaerobic sludge and synthetic solution of xylose supplemented with nutrients. The reactor operated at 35 degrees C in semi-continuous mode, which consisted of 16 h feeding cycles (for a hydraulic retention time of 48 h), mixing until completing 19 h of fermentation, and stopping the mixture for 5 h for sedimentation of sludge, followed by the withdrawal of supernatant. The reactor was operated with standard xylose as substrate for 70 days under different chemical oxygen demand (COD) loads and mixing modes during fermentation (intermittent or continuous). A yield of 1.4 mmol H-2/mmol xylose was obtained under an organic loading rate of 2.5 g COD/L.d. The consumption of xylose (76.5%) and the H-2 yield, which was approximately 41% of the maximum theoretical yield (3.33 mmol H-2/mmol xylose with acetic acid as a byproduct), indicates that further studies are necessary to optimize the biohydrogen production in semi-continuous reactors. However, the following parameters led to greater stability of H-2 production: adaptation of microorganisms to xylose through prolonged operation in a semi-continuous regime as well as the constant mixing of the reactor contents to reduce gradients of pH, organic acids, and local inhibition of microorganisms. Additionally, an increase in influent alkalinity maintained the pH at levels suitable for H-2 producers. Graphic Abstract

Automated summary

The study evaluated the continuous production of biohydrogen in a bench reactor with thermally pretreated anaerobic sludge and synthetic xylose solution supplemented with nutrients. It found that further studies are necessary to optimize biohydrogen production in semi-continuous reactors, with key parameters for stability including microbial adaptation to xylose and constant mixing of reactor contents.