Steering Product Formation in High-Pressure Anaerobic Digestion Systems: The role of elevated partial pressure of carbon dioxide (pCO2)

Anaerobic processes such as Anaerobic Digestion (AD) and mixed culture fermentation (MCF) are important technologies in the bioeconomy context since they can be used to convert (waste) biomass feedstock into gaseous energy carriers and chemical commodities, theoretically without the use of any additional energy source. AD is a multi-step bioconversion process pursuing organic matter stabilization whose final product, i.e., biogas, can be used as an energy source. On the other hand, MCF employs open mixed cultures under non-sterilized conditions to produce carboxylates, i.e., short and medium-chain organic acids, which will serve as chemical building blocks after downstream processing. Limitations of biogas production are associated with the low CH4 content (≈50-60%), presence of impurities (like H2S) and unsuitable final pressure for direct connection to national grids. Thus, in recent years, the topic of biogas upgrading to biomethane (i.e., CH4>90%) has gained momentum and in-situ and ex-situ alternatives have been proposed with differences in financial and technical viability as well as achieved final CH4 content. While for the carboxylate production, major limitations are associated with process selectivity, presence of trace pollutants and too low broth concentrations for direct application inducing a need for “wet” and energy-intensive downstream processing. High-Pressure Anaerobic digestion (HPAD) is an innovative technology designed for simultaneous digestion and biogas upgrading. HPAD takes advantage of the large differences in solubility between biogas constituents, i.e., CH4 and CO2. Consequently, CH4 will predominantly remain in the gas phase after a pressure increase, whereas ionisable gases like CO2 and H2S will increasingly dissolve in the liquid. Thus, from a biogas production perspective, the proposed technology accomplishes higher CH4 content in the gas phase at the cost of increased dissolved CO2 levels. The process's overall performance under elevated pCO2 has not been adequately addressed. Mechanistic explanations for the role of increased dissolved CO2 in the fermentation process remain speculative, partially due to the limited amount of published experimental work on high-pressure fermentation with open cultures. Since CO2 exerts multiple roles in biological systems, increased dissolved CO2 could impact the kinetic and energetic feasibility of the reaction chain in AD and MCF, as well as the microbial community dynamics. These effects constitute a notorious knowledge gap that requires urgent attention…