Biohydrogen production and metabolic pathways in dark fermentation related to the composition of organic solid waste

This study aims to investigate the effect of solid substrates composition on hydrogen production performances, metabolic pathways and microbial community changes in batch reactor and their dynamics in continuous reactors (CSTR). Hydrogen is an ideal energy carrier which has gained scientific interest over the past decade. Biological H₂, so-called biohydrogen, can especially be produced by dark fermentation processes concomitantly with value-added molecules (i.e. metabolic end-products), while organic waste is treated. However, the effect of solid organic waste composition on biohydrogen production in dark fermentation has not yet been clearly elucidated. In this study, a bibliographic review was made on hydrogen production from agricultural waste. This survey on literature showed that diverse performances were reported on hydrogen production due to the variability in substrate compositions and experimental conditions. After having optimized a protocol of biohydrogen potential test (BHP), a wide variety of organic solid substrates aiming to covering a large range of solid waste was tested to provide a comparable data analysis. The results of a PLS regression showed that only soluble carbohydrates or easily available carbohydrates correlated with hydrogen production. Furthermore, hydrogen yields correlated as well with butyrate H₂-producing pathway which is consistent with the literature knowledge. A predictive model of hydrogen yield according to carbohydrate content was proposed. Then, experiments were carried out in CSTR with Jerusalem artichoke tubers as a case study. It was shown that low organic loading rate favored continuous hydrogen production while higher organic loading introduced hydrogen competition pathways and decreased the overall hydrogen yields. Moreover, 16S rRNA gene based CE-SSCP profiles showed that increasing OLR had a significant effect on the microbial diversity by favoring the implementation of microorganisms not producing hydrogen, i.e. lactic acid bacteria. (author)