Function of homoacetogenesis on the heterotrophic methane production with exogenous H2/CO2 involved

•H2 consumption in anaerobic digestion was studied under exogenous H2 stimulation.•Hydrogenotrophic methanogens consumed 60% H2 under high H2 partial pressure.•While homoacetogens contributed 40% H2 consumption under high H2 partial pressure.•Hydrogenotrophic methanogens had an upper capacity of H2 consumption.•Inhibiting homoacetogens did not help hydrogenotrophic methanogens consume more H2.

Biological H2 production from anaerobic digestion (AD) is not a sustainable pathway, based on both the efficiency of energy recovery and operation possibility in practice. Because of this, the use of AD for producing CH4 ought once again to be considered and some exopathic measures should be studied to enhance CH4 production. Among these, the intervention of exogenous CO2 and/or H2 has been demonstrated as an external approach to stimulating CH4 production. When exogenous H2/CO2 was intervened in mesophilic AD, the H2 consumed pathways were found to change due to increased H2 partial pressure (PH2PH2) in this study. At PH2PH2 = 0.96 bar, hydrogenotrophic methanogens (HMs) and homoacetogens (HAs) contributed 60% and 40% respectively to the H2 consumption, which resulted in an increased CH4 production via the combination of homoacetogenesis and aceticlastic methanogenesis. The enumeration experiments determined that the amount of HMs was two orders higher in magnitude than that of HAs, and that Ks (half-velocity constant) and Vmax (maximum specific substrate utilization rate) of HAs were respectively 10 times higher and 4 times lower than those of HMs. The determined kinetic parameters reveal that a low PH2PH2 as in normal AD makes HAs contribute much less (2–5%) to the H2 consumption. Based on the fact that there was an upper limit to the capacity of HMs for the H2 consumption due to their limited amounts in the reactor, any attempts to inhibit the pathway of HAs for H2 consumption would have no use in technology.

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