H₂ competition between homoacetogenic bacteria and methanogenic archaea during biomethanation from a combined experimental-modelling approach

Competition for H₂ between homoacetogenic bacteria and methanogenic archaea is commonly faced in biological biogas upgrading process reducing the potential for high methane production capacities. In the present work, different feeding regimes were examined using anaerobic inocula that were adapted and non-adapted to the gaseous feedstock. Adapted inoculum could compensate the increased pressure at all examined levels and especially, produced above 95% of the theoretical methane without accumulating acetate at 0.2 atm. On the contrary, acetate accumulation was detected when a non-adapted inoculum was used. Microbial adaptation to elevated pressures can be applied as a means to improve production capacities. Thermodynamic analysis showed that hydrogenotrophic methanogenesis had always lower permissive H₂ partial pressures (3.20-7.46 Pa) compared to homoacetogenesis (0.43-0.85 Pa) at the examined pH (neutral vs alkaline) and pressure levels (i.e., 0.2-2.0 atm). An unstructured kinetic model was developed to study the influence of the examined variables in the hydrogenotrophic, homoacetogenesis and aceticlastic pathways. The kinetic parameters of the proposed model were estimated from experimental results and the goodness of the fitting was assessed by the coefficient of determination which indicated that the model describes efficiently the dynamics of the different compounds involved in the investigated pathways.