ENCOVER: Energetic utilization of CO2 to reduce the residual methane potential

Burning fossil fuels is the primary source of greenhouse gases. In order to become carbon neutral by 2050, energy production must increasingly shift to renewable sources. Biogas plays a key role in this, as it can be stored, unlike other renewable energies. However, when raw and residual materials are fermented, a significant portion of the methane potential remains in the fermentation residue. Studies show that CO₂ enrichment can increase methane production and improve process stability. The aim of this project is the energetic use of CO₂ to reduce the residual methane potential, especially for residues that have not been investigated much so far. These substrates are expected to be more efficient than sewage sludge. In addition to the quantification of methane production and stability, pH and redox measurements, isotopic analyses and microbiological investigations will be used to elucidate the mechanisms involved. This will be done in both batch and continuous pilot plant experiments. In the continuous tests with food waste, increased methane production was observed in the reactor enriched with CO₂ (R_CO2) (Figure 1A). By day 53, a methane increase of 96 L (+7%) was achieved compared to the control reactor (R0) (black solid line). However, the effect of CO₂ enrichment only became apparent at increased room load, whereas before, both reactors showed comparable methane production. The vertical lines mark the time of feed stop in each reactor. Improved process stability was observed in R_CO2, as the reactor continued to run stably despite continued substrate feeding, while substrate feeding had to be stopped in R0 due to the sharp drop in pH. Similar results were observed for hydrogen production (Figure 1B): About 0.2 L more H₂ was produced in R_CO2 than in R0. This indicates that not only was the existing H₂ efficiently converted, but additional H₂ was also produced. Therefore, CO₂ enrichment resulted in more methane, more hydrogen, and improved process resilience.