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This study assessed the effect of hydraulic retention time (HRT) on biohydrogen production by dark fermentation using an anaerobic fluidized bed reactor fed with cheese whey synthetic wastewater. The reactor of 1.2 L was operated with an approximate carbohydrate concentration of 2800 mg L -1 , and HRTs of 6, 4, 2, 1 and 0.5 h. Acetic and butyric acids were the main metabolites produced in the reactor. The hydrogen production increased from 0.039 to 1.43 L-H 2 h -1 L -1 -reactor as the HRT decreased from 6 to 0.5 h. The best hydrogen yield (HY) (2.73 mol-H 2 mol-carbohydrate -1 ) and carbohydrate consumption (81.02%) were achieved at a HRT of 6 h (12.34 kg-COD m -3 d -1 ) followed by the HY of 2.43 mol-H 2 mol-carbohydrate -1 at a HRT of 0.5h (163.02 kg-COD m -3 .d - 1 ). An increase in the influent pH in the operational phase 05 (HRT of 0.5 h) favored hydrogen production despite the reduced carbohydrate conversion compared to phase 01 (HRT of 6 h). In both conditions, the reactor presented the highest amount of acetic acid, indicating that the acetate route favored the hydrogen yield production. The HRT reduction led to an increase in microbial diversity, as evidenced by the Shannon-Wiener coefficient of 2.586, which corresponds to the operational phase with a HRT of 2 h.

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By-products from biofuels production, such as crude glycerol and sugarcane molasses, are interesting carbon sources for anaerobic biological production of hydrogen and metabolites. The objective of this work was to promote the co-fermentation of those two substrates in an anaerobic fluidized bed reactor for obtaining biofuels and value-added products. The total and useful volumes of the reactor were 1.2 and 0.945 L, respectively. Sludge from a sewage treatment plant UASB reactor was used as the source of microorganisms. The crushed tire was applied as a support material to guarantee microbial adhesion. The molasses concentration was kept constant (4 g/L) and the crude glycerol varied (phase 1 = 1 g/L; phase 2 = 2 g/L, phase 3 = 3 g/L and phase 4 = 0 g/L) for a constant hydraulic retention time of 4 h. The highest average hydrogen yield (3.77 mol-H 2 /mol glucose) and the highest average hydrogen production rate (0.34 L-H 2 /h L reactor) occurred in the fourth phase of operation (without crude glycerol). The main soluble metabolites produced were 1,3-propanediol (14.4-38.8%), propionate (12.1-37.5%) and acetate (13.2-26.9%).