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

Two heat-treated inocula, fresh and digested activated sludge from the same municipal wastewater treatment plant, were compared for their H2 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 H2 than the digested sludge, whereas at 70 °C, neither of the inocula produced H2 effectively. A maximum yield of 1.85 mol H2 per mol of xylose consumed was obtained at 55 °C. H2 production was linked to acetate and butyrate production, and there was a linear correlation (R2 = 0.96) between the butyrate and H2 yield for the fresh activated sludge inoculum at 55 °C. Approximately 2.4 mol H2 per mol of butyrate produced were obtained against a theoretical maximum of 2.0, suggesting that H2 was produced via the acetate pathway prior to switching to the butyrate pathway due to the increased H2 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 H2 yield obtained with such an inoculum.