The biological production of hydrogen (H2) is an interesting alternative by the possibility of using renewable biomass without generating polluting products. The coffee processing residues are attractive due to its abundance and low use in fermentation processes. However, low H2 yields are usually observed as a result of its consumption by bacteria or methanogenic archaea. Among them, hydrogen producing bacteria can change their metabolism to homoacetogenesis using the H2 as substrate to generate acetic acid. The mechanisms that lead to this metabolic change are still unclear. The objective of this study will be evaluate the effect of physical-chemical and microbiological parameters in H2 production and homoacetogenesis. To achieve this objective, two types of coffee processing waste will be tested: solid waste (husk) pretreated in hydrothermally reactor and wastewater from the wet coffee processing (including washing, depulping and degumming of the coffee fruits). The conditions of the hydrothermal pretreatment (temperature and time) will be selected by comparing highs and lows severities in the H2 production. Variables will be evaluated are pH (4,0, 5,5 and 7,0) and substrate (husk and wastewater) and substrate concentration (5, 15 and 25 g COD L -1 for the wastewater and 2, 4 and 6 g L-1 for the husk), by performing fermentation with mixtures of husk and wastewater. Reactors will be of 500 ml with 50 % headspace and 50 % of the reaction volume and temperature of 35°C. Fermentation will be natural, in order to encourage the development of microbiota from the wastes. Physical-chemical and chromatographic analyzes will be performed, such as chemical oxygen demand, solids, phenols, biogas composition, H2 production, sugars, organic acids and alcohols. The condition with highest H2 production and homoacetogenesis will be selected to perform phylogenetic characterization by sequencing the 16S rRNA (bacteria) and 18S (fungi) on a large scale (next generation).
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