Feasibility of successive hydrogen and methane production in a single-reactor configuration of batch anaerobic digestion through bioaugmentation and stimulation of hydrogenase activity and direct interspecies electron transfer

Abstract

Two-stage anaerobic digestion is a promising way to increase the efficiency of decomposition of organic waste. However, spatial separation of stages makes this technology less economically attractive. In this work, we studied a model of two-stage anaerobic digestion with stages separated in time in one reactor. To implement this model, we used a combination of (1) bioaugmentation with a hydrogen-producing Thermoanaerobacterium thermosaccharolyticum SP-H2, resistant to low pH, (2) setting a low initial pH (5.5) to reduce the activity of hydrogenotrophic methanogens, (3) supplementing additives in the form of soluble iron (II) and granular activated carbon (GAC) to stimulate hydrogenase activity and direct interspecies electron transfer (DIET), respectively. For comparison, two methanogenic inoculums with different dominant microbial groups and initial iron concentrations were used. With the simultaneous addition of GAC and iron (II), sequential production of hydrogen and methane was observed for both methanogenic inoculums. Without the addition of GAC, methane formation was practically not observed, which, apparently, indicated the impossibility of syntrophic degradation of acidogenesis products due to the high partial pressure of hydrogen and the absence of conductive material for activation of DIET. Significant differences in hydrogenase activity and the kinetics of hydrogen and methane formation according to the modified Gompertz equation depending on the iron content in the system may indicate the importance of this additive. Further studies should be aimed at determining the optimal initial pH, concentrations of stimulating additives, bioaugmentation and methanogenic cultures, as well as confirming the stability of the proposed single-reactor two-stage anaerobic digestion system in semi-continuous mode.