Continuous fermentative hydrogen production from cheese whey e new insights into process stability

Continuous dark fermentation experiments for hydrogen production from synthetic cheese whey were conducted at different HRTs and OLRs. The study mainly aimed at developing a novel criterion to quantitatively assess stability and relating it to the evolution of microbial pathways and associated metabolic products. For HRTs = 6–8 h and OLRs = 65–97.5 g TOC/(L·d), the best hydrogen generation performance was attained, yielding 42–50 L H2/kg TOC. Instead of using a stability index for the entire test length, accounting for the fluctuations of hydrogen production over 1-HRT periods (dynamic stability index) provided a more accurate assessment of process stability showing a clear correlation with the hydrogen yield. The analysis of the metabolic reactions provided evidence of a competition among acidogenic, hydrogen-consuming and hydrogen-neutral microbial species. This explained the lower process performance in comparison to the theoretical yield expected, pointing out at the need for further investigation on suitable strategies to effectively inhibit undesired metabolic pathways.