Simultaneous production of biohydrogen and bioethanol with fluidized-bed and packed-bed bioreactors containing immobilized anaerobic sludge

Hydrogen and ethanol are promising biofuels and of great potential to become alternatives to fossil fuels. In this work, two bioreactor systems, namely fluidized-bed (FBR) and packed-bed (PBR), were developed to produce H2 and ethanol simultaneously from dark fermentation of carbohydrate substrates using polyethylene–octane elastomer immobilized anaerobic sludge as the biocatalyst. The H2 and ethanol production in FBR essentially increased with increasing upflow velocity (vupvup), as sucrose and fructose was better substrate for the yield of H2 and ethanol, respectively. With FBR operated at vup=0.91 cm s−1vup=0.91 cm s−1, sucrose gave the highest H2 production rate (59 mmol h−1 l−1) among the three sugar substrates (sucrose, glucose, and fructose) tested, but the best H2 yield (1.04 mol mol hexose−1) was obtained with glucose at vup=0.55 cm s−1vup=0.55 cm s−1. For ethanol production in FBR, fructose was the favorable substrate, resulting in maximum ethanol production rate and yield of 378 mmol h−1 l−1 and 0.65 mol mol hexose−1, respectively, when operating at vup=0.91 cm s−1vup=0.91 cm s−1. At a hydraulic retention time of 4 h, the PBR system produced H2 and ethanol at a slower rate of 16 and 6 mmol h−1 l−1, respectively, by using glucose. However, the yields of H2 and ethanol were comparable to those for FBR. The soluble metabolites were dominated by ethanol, accounting for 43–65% of total soluble microbial products. The production of acetate and butyrate was less significant when compared to cultures optimized for H2 production. Comparison of the yield of H2 and ethanol shows that production of H2 and ethanol was reversely correlated. The total energy generation based on the heat values of H2 and ethanol was calculated to assess the overall efficiency of energy production. In FBR, the energy generation rate was higher when a faster upflow velocity was used. The best energy generation rate and yield was 526 kJ h−1 l−1 and 1048 kJ mol hexose−1, respectively, both occurred with fructose-feeding FBR operated at vup=0.91 cm s−1vup=0.91 cm s−1. The PBR system displayed a lower energy generation rate, whereas the energy yield was comparable or even higher than those for FBR.