Photo-fermentation to unlock a locally available hydrogen source for biogas upgrading within two-phases anaerobic digestion processes

Biological biogas upgrade can be achieved by supplying hydrogen in-situ or ex-situ during the anaerobic digestion process. Among the possible methods for hydrogen 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 study, the VFAs enriched fermentate produced as an intermediate in a two-phases anaerobic digestion process was used as a growth medium for Purple Non-Sulfur Bacteria (PNSB), to unlock a locally available hydrogen source within the process. Two PNSB strains, Rhodopseudomonas palustris and Rhodospirillum rubrum, were tested on suspended and immobilized cultures, fed with the fermentation broth, and compared in terms of biomass and hydrogen production. 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), and higher hydrogen productivity when testing immobilized cultures rather than suspended ones.