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This study assessed the effect of hydraulic retention time (HRT) ranging from 24 to 3 h on continuous dark-fermentative H 2 production in four bioreactors operated at pH 5.0, 5.5, 6.0 and 6.5. A mixture of cellobiose, xylose and arabinose was used as the substrate. The highest hydrogen production rate between HRTs of 24 and 12 h was observed at pH 6.5, while at HRT below 12 h at pH 6.0. At a HRT of 3 h it reached 11.4 L H 2 /L-d. Thus, the optimum pH for H 2 production depends on the HRT. The highest sugar utilization was obtained at pH 6.0 and 6.5 and decreased in the following order: cellobiose > xylose > arabinose. The pH conditions, in contrast to HRT, were found to have a significant influence on the bacterial composition. Low diversity in bacterial culture dominated by the Clostridium genus allows for stable and high H 2 production performance., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The objective of the study was to valorize waste stream for the co-production of hydrogen and caproate via open culture fermentation (OCF). Batch studies confirmed that the use of sugar (lactose) together with carboxylates (lactate and acetate) may allow mutual coexistence of chain elongation and dark fermentation processes. During the continuous test in an upflow anaerobic sludge blanket reactor (UASB), acid whey was used as a model feedstock due to a high concentration of lactose and lactate. Shortening hydraulic retention time (HRT) to 2.5 days allowed the co-production of hydrogen and caproate with almost complete methanogenesis inhibition. During the 50 days period, the average hydrogen and caproate production were 1.78 ± 0.75 L H2 /L/d and 133.4 ± 17.9 mmol C/L/d, respectively., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Dark fermentation is a promising method to produce hydrogen from lignocellulosic biomass. This study assessed the influence of temperature, phosphate buffer concentration and substrate concentration on direct hydrogen production form cellulose using response surface methodology. Mixed bacterial culture was successfully enriched on cellulose and used as an inoculum for hydrogen production. The model indicated that the highest cumulative hydrogen production (CHP) of 2.14 L H 2 /L medium could be obtained at 13.5 g cellulose /L, 79.5 mM buffer and 32.6 °C. However, hydrogen yield is then only 0.58 mol H 2 /mol hexose due to low substrate conversion efficiency (SCE). Simultaneous optimization of CHP and SCE with desirability function approach resulted in the H 2 yield of 2.71 ± 0.1 mol H 2 /mol hexose and 93.8 ± 1.8% SCE at 3.35 g cellulose /L, 69 mM buffer and 32.9 °C. Phosphate concentration above 80 mM decreased H 2 production, but had positive effect on cellulose consumption. The bacterial community analysis showed that Ruminiclostridium papyrosolvens was responsible for cellulose hydrolysis. Lachnoclostridium sp. was positively correlated with ethanol production at high phosphate buffer concentration, while Caproiciproducens sp. with caproate production at low buffer concentration. The obtained results opens the possibility of simultaneous hydrogen and caproate production from cellulosic substrates. [Display omitted] • RSM was applied for dark-fermentative H 2 production from raw cellulose. • 2.14 L H 2 /L could be obtained at 13.5 g cellulose /L, 79.5 mM buffer and 32.6 °C. • Cellulose and buffer concentration had significant interactive effect on H 2 amount. • H 2 yield of 2.71 mol H 2 /mol hexose , with 94% cellulose utilization was obtained. • 20 mM buffer was positively correlated with caproate and Caproiciproducens sp. [ABSTRACT FROM AUTHOR]