Biohydrogen production by co-digesting whey and glycerin in an AnSBBR: Performance optimization, metabolic pathway kinetic modeling and phylogenetic characterization

The co-digestion of cheese whey with glycerin for biohydrogen production, in an anaerobic sequencing batch biofilm reactor (AnSBBR) with recirculation of the liquid phase (at 30 °C with 3.5 L of working volume and treating 1.5 L per cycle), was assessed. The feeding time (1.5 h) was equal to half of the cycle length (3 h). Two variables were studied: the ratio between whey and glycerin (100:0; 75:25; 50:50; 25:75; 0:100% on a COD basis) and the organic loading rate (from 10.1 to 23.9 kgCOD m−3 d−1) by altering the influent concentration (from 3 to 7 kgCOD m−3). The highest hydrogen productivity (129.0 molH2 m−3 d−1) and yield (5.4 molH2 kgCOD−1) with complete inhibition of methanogenisis were achieved when the reactor was fed with 75% cheese whey and 25% glycerin with an applied volumetric organic load of 23.9 kgCOD m−3 d−1; an increase in productivity of almost 145% and 27% when compared to the anaerobic digestion of cheese whey and glycerin separately, respectively, showing the significant benefit of glycerin addition for anaerobic whey digestion, most likely due to its buffering capacity. A kinetic metabolic model was efficiently fitted to the process to help understand the metabolic routes. Cloning analyses targeting 16S rRNA genes indicated the dominant microorganisms were phylogenetic affiliated to Ethanoligenens and Megasphaera genera.