Spontaneous dark fermentation in a pre-seeded liquid-gas membrane bioreactor: Impact of wine and coffee biowaste microflora on continuous biohydrogen production

Abstract

One of the challenges of continuous bioproduction of hydrogen by dark fermentation is to extract the gases produced and efficiently maintain the active bacterial consortium in the bioreactor. In this study, the process of hydrogen production was intensified by coupling dark fermentation to in situ gas extraction using a hollow fiber membrane module working as liquid-gas contactor. The value of this continuous liquid-gas membrane bioreactor (L/G MBR) was previously proved with a model substrate and the stabilization of a bacterial consortium deposited on the fibers. This work in an original way attempts to answer the question of the impact of feeding the bioreactor with unhygienized biomass having its own bacterial microflora capable of producing hydrogen. A new L/G MBR was inoculated with digestate from endogeneous dark fermentation of grape must deposits, mainly composed of hydrogen-producing bacteria. It was subsequently operated as a continuous liquid-gas membrane bioreactor (L/G MBR) without additional bacterial seeding, using different biomasses. The feasibility of spontaneous biohydrogen production in the L/G MBR was demonstrated for several organic byproducts originated from agriculture or the food industry (grape must deposits (same biomass for feeding), grape pomace, and coffee silverskin), chosen for their potential to ensure year-round availability. Production performances of 0.9–4.1 L H2/L/d and 18–86 LH2/kgCOD were obtained from the different waste liquid fractions. The bacterial and metabolic profiles of biomass digestates, produced by endogenous fermentation in a semi-batch bioreactor or by spontaneous fermentation in the L/G MBR were compared and showed that the bacterial consortium found in the digestate was linked either to the biomass’ indigenous flora or to the L/G MBR seeding. Thus, H2 production from coffee silverskin (a biomass that did not impose its own microflora) could be enhanced by seeding the L/G MBR with selected bacteria, implanted in the bioreactor. This offers exciting perspectives for the further development of efficient low-tech processes for producing biohydrogen from biomass.