Hydrogen production in anaerobic reactors during shock loads—influence of formate production and H2 kinetics

In this article the role of hydrogen as a process monitoring tool in methanogenic systems was studied by considering the influence of several key system parameters. Hydrogen production was found to be influenced mainly by the inoculaʹs source pH, and varied only slightly with external pH and HCO3− levels. When an inoculum adapted to above neutral conditions (pH>7) was shocked, reducing equivalents were selectively channelled through formate, while high hydrogen production was noticed with acidically (pH<6.5) adapted inocula. The results also revealed that the production of hydrogen or formate during shock loads was not strongly associated with microbial morphology (granules or flocs) as high electron fluxes were possible through either during acidogenesis. Shock load experiments in continuous reactors revealed that neither hydrogen nor formate accumulated to any significant degree, nevertheless digester recovery took a long time due to the slow kinetics of volatile fatty acid degradation. Selective formate production under neutral pH environments, coupled with high hydrogenotrophic activity, was found to be responsible for the dampened hydrogen response during the early phases of gradually shocked systems (step change). Based on these results it appears that the role of hydrogen as a process monitoring tool has been overemphasised in the literature.