Thermophilic fermentative hydrogen production from various carbon sources by anaerobic mixed cultures

In the present work, various carbon sources, xylose, glucose, galactose, sucrose, cellobiose, and starch were tested for thermophilic (60 °C) fermentative hydrogen production (FHP) by using the anaerobic mixed culture. An inoculum was obtained from a continuously-stirred tank reactor (CSTR) operated at pH 5.5 and HRT 12 h, and fed with tofu processing waste. The dominant species in the CSTR were found to be Thermoanaerobacterium thermosaccharolyticum and Clostridium thermosaccharolyticum, which are well known thermophilic H2-producers in anaerobic-state, and have the ability to utilize a wide range of carbohydrates. When initial pH was adjusted to 6.8 ± 0.1 but not controlled during fermentation, vigorous pH drop began within 5 h, and finally reached 4.0–4.5 in all carbon sources. Although over 90% of substrate removal was achieved for all carbon sources except cellobiose (71.7%), the fermentation performances were profoundly different with each other. Glucose, galactose, and sucrose exhibited relatively higher H2 yields whereas lower H2 yields were observed for xylose, cellobiose, and starch. On the other hand, when pH was controlled (pH ≥ 5.5), the fermentation performance was enhanced in all carbon sources but to a different extent. A substantial increase in H2 production was observed for cellobiose, a 1.9-fold increase of H2 yield along with a substrate removal increase to 93.8%, but a negligible increase for xylose. H2 production capabilities of all carbon sources tested were as follows: sucrose > galactose > glucose > cellobiose > starch > xylose. The maximum H2 yield of 3.17 mol H2/mol hexoseadded achieved from sucrose is equivalent to a 26.5% conversion of energy content in sucrose to H2. Acetic and butyric acids were the main liquid-state metabolites of all carbon sources while lactic acid was detected only in cellobiose, starch and xylose exhibiting relatively lower H2 yields.