Fixed bed in dark fermentative reactors: is it imperative for enhanced biomass retention, biohydrogen evolution and substrate conversion?

Recent studies have revealed gaps related to the bed purpose in fixed-film reactors and its influence on biohydrogen (bioH2) evolution via dark fermentation, given the pivotal role of the basal portion (i.e., feeding chamber) in converting fresh substrate and establishing bioH2 precursor metabolic pathways, which is opposed to that observed in the bed zone. This study aimed to assess the influence of different cell immobilization arrangements (as well as its absence) on biomass retention and washout and their effects on bioH2 evolution and metabolic profiles of an anaerobic tubular reactor (AnTR, which does not include support material), an anaerobic structured-bed reactor (AnSTBR, which includes support material laid out in an orderly pattern) and an anaerobic packed-bed reactor (AnPBR, which considers support material laid out in a disorderly pattern) operated under equivalent operating conditions in the feeding chamber. Higher bioH2 evolution was achieved in the AnSTBR (743.25 mL-H2 L-1 d-1) compared to the AnPBR (662.04 mL-H2 L-1 d-1) and AnTR (586.22 mL-H2 L-1 d-1). For the former, a massive carbohydrate-uptake occurred in the feeding chamber and at the beginning of the bed zone, both comprising an intense metabolic activity versus the upper regions. Conversely, this pattern was restricted to the AnPBR feeding chamber, while an alternative source of lactic activity occurred in the bed zone. Overall, the presence of media in the bed zone proved to be imperative to ensure suitable biomass retention in quanti-qualitative terms in opposition to no-bed, influencing biomass distribution, specific substrate conversion, bioH2 evolution and metabolic pathways.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.