Fermentative biohydrogen production in sugarcane biorefineries: Advances, challenges and prospects

In a context of growing importance of alternative carbon-low hydrogen production methods, this review addresses aspects of fermentative biohydrogen (bioH2) production from juice, molasses and vinasse, the primary substrates available in sugarcane biorefineries, following three primary fronts. The characterization of the main bioreactors utilized and the most frequent environmental and operating conditions are initially detailed. Performance results in terms of bioH2 production, substrate conversion, soluble phase metabolites distribution and microbial community characterization are addressed in the sequence. Finally, some challenges and prospects are discussed. Thermophilic (55 degrees C) fixed-film and fluidized-bed reactors were identified as the primary systems used in bioH2 production, with most common applied organic loading rates (OLR) varying within 50-90 and 30-120 kg COD m- 3 d-1, respectively. Overall, hydrogen production rates of up to 10,000 NmL H2 L-1 d-1 prevail, regardless of the substrate and reactor, whilst the hydrogen yield systematically decreases as the OLR is increased. Metabolic pathways involving lactic acid production and consumption seem to determine whether or not bioH2 (almost always together with butyric acid) will be produced (mainly by Clostridium and Thermoanaerobacterium genera). Exploiting substrate co-fermentation, finding low-cost pH control approaches and better characterizing microbial metabolic functions were identified as key points to be addressed in future research. Identifying uses that energetically and economically justify producing fermentative bioH2 from sugarcanederived substrates also needs to be critically defined. Once these major challenges are solved, the scale-up of bioH2-producing systems has potential to be rationally implemented in sugarcane biorefineries.