Optimizing hydrogen production from a switchgrass steam exploded liquor using a mixed anaerobic culture in an upflow anaerobic sludge blanket reactor

In this study, the operation of an upflow anaerobic sludge blanket reactor (UASBR) producing hydrogen (H2) from a steam-exploded switchgrass (SWG) liquor was statistically optimized. The factors consider included pH, hydraulic retention time (HRT) and linoleic acid (LA) concentration. Under optimal operational conditions (pH 5.0, 10 h HRT and 1.75 g L−1 LA), which were close to the predicted conditions using the D-optimality index, the maximum H2 and methane yield observed were 99.86 ± 5.6 mL g−1 TVS and 0.5 ± 0.1 mL g−1 TVS, respectively. Under maximum H2-producing conditions, high levels of acetate plus butyrate were observed with low levels of ethanol and lactate. A principal component analysis revealed that clustering of the samples was based on the operating conditions and fermentation metabolites. The microbial profiles revealed that by lowering the HRT from 16 to 8 h or decreasing the pH from 7.0 to 5.0 in the controls caused a 50% reduction in the relative abundance of the terminal restriction fragments belonging to the methanogenic population (Methanobacteria, Methanomicrobia, Methanococci). With LA treatment, H2 producers (Ruminococcaceae and Clostridiaceae) were dominant and methanogens were inhibited and/or washed-out from the UASBR.