Coupling Mixed Culture Fermentation and Photo fermentation for Bio H2 Recovery: Preliminary Assessment of the Fermentation Yields and PNSB Growth on Fermentative Broth

Abstract

Hydrogen has been widely recognized as a promising tool to address global warming problems, due to its high energy density and the sustainability of its utilization. Among the possible methods for its production, the biological processes have gained a lot of interest in recent years, as they would allow for an alternative and environment-friendly production. In this preliminary study, a two-stage co-fermentation/photo-fermentation process has been investigated. The first stage focused on the production of a medium enriched in volatile fatty acids (VFAs) to be used as a substrate for Purple Non-Sulphur Bacteria (PNSB). The co-fermentation of an organic waste mixture and sewage has been investigated, evaluating the VFAs production yields at different organic loadings (0, 5, 10, 15, 20, 25 kgTVS(mixture) m-3). The highest VFA production yields (410.0 and 381.1 gVFA(COD) kgTVS-1) were obtained at 10 and 15 kgTVS(mixture) m-3, respectively. Two PNSB strains, Rhodopseudomonas palustris and Rhodospirillum rubrum, were fed with the fermentation broth, and compared in terms of biomass production. Despite no significant hydrogen production being observed, both strains showed a consistent boost in growth yields when fed with the fermentative broth rather than with the standard growth medium (a two-fold higher biomass production for Rps. palustris, and eight fold higher for Rsp. rubrum). This suggests how the use of fermentative effluents as a culture medium for PNSB cultivation is a viable option to foster their growth. However, the limited hydrogen production in all the tested conditions is in disagreement with the literature evidence and certainly requires further investigation to detect any possible cause of inhibition.