Influence of hydrogen supply factors on in-situ biological biogas upgrading in thermophilic anaerobic digestion of sewage sludge

Abstract In-situ biological biogas upgrading has some challenges such as inhibition on digestion process and low water solubility of hydrogen (H 2 ). In this study, in-situ biogas upgrading was performed in laboratory-scale thermophilic anaerobic digestion of sewage sludge at 1.0 g-VS/L/day organic loading rate (OLR) with gas recirculation and a gradual increase in the H 2 load. Position of H 2 addition and stirring speed were tested as factors of H 2 supply. The results showed that no significant inhibition was apparent at a stirring speed of 75 rpm, and the maximum CH 4 concentration attained 93%. The position of H 2 addition did not affect the observed inhibition under the conditions of the present study. During stirring at 300 rpm, the growth of many microorganisms, especially Methanosarcina and Coprothermobacter , were strongly inhibited, associated with accumulation of volatile fatty acids (VFAs) [mostly acetic acid (1205 mg/L)], and a low biogas yield (13% that of the control reactor). The pH temporarily increased to 9.2, but the VFAs then produced lowered the pH to 8.1, which did not alleviate the inhibition. These findings suggest that reactor operation at a low OLR and a slow stirring speed, with gas recirculation, ensures stable in-situ biogas upgrading.