Biohydrogen production from xylose by fresh and digested activated sludge at 37, 55 and 70 °C

Abstract

<p>Two heat–treated inocula, fresh and digested activated sludge from the same municipal wastewater treatment plant, were compared for their H<sub>2</sub> production via dark fermentation at mesophilic (37 °C), thermophilic (55 °C) and hyperthermophilic (70 °C) conditions using xylose as the substrate. At both 37 and 55 °C, the fresh activated sludge yielded more H<sub>2</sub> than the digested sludge, whereas at 70 °C, neither of the inocula produced H<sub>2</sub> effectively. A maximum yield of 1.85 mol H<sub>2</sub> per mol of xylose consumed was obtained at 55 °C. H<sub>2</sub> production was linked to acetate and butyrate production, and there was a linear correlation (R<sup>2</sup> = 0.96) between the butyrate and H<sub>2</sub> yield for the fresh activated sludge inoculum at 55 °C. Approximately 2.4 mol H<sub>2</sub> per mol of butyrate produced were obtained against a theoretical maximum of 2.0, suggesting that H<sub>2</sub> was produced via the acetate pathway prior to switching to the butyrate pathway due to the increased H<sub>2</sub> partial pressure. Clostridia sp. were the prevalent species at both 37 and 55 °C, irrespectively of the inoculum type. Although the two inocula originated from the same plant, different thermophilic microorganisms were detected at 55 °C. Thermoanaerobacter sp., detected only in the fresh activated sludge cultures, may have contributed to the high H<sub>2</sub> yield obtained with such an inoculum.</p>