Increasing methane productivity in anaerobic digesters by addition of CO2 – the use of stable isotope techniques to identify the mechanisms

In recent years, the demand for renewable energy and the reduction of greenhouse gases such as CO₂ has become increasingly important within the framework of energy transition. Sato and Ochi showed in 1994 that CO₂ is converted into energetically usable methane (CH₄) without the addition of hydrogen (H₂). They observed increased CH₄ gas production in anaerobic sewage sludge digesters when maintaining high CO₂ gas concentration in the headspace. Although follow-up studies confirmed those findings, none of the few recently published studies were able to elucidate the basic effects leading to the bioconversion of CO₂ to CH₄. Our project (supported by the Fachagentur für Nachwachsende Rohstoffe) investigates bioconversion of CO₂ to CH₄ with the overall aim of reducing industrial CO₂ emissions and transformation of CO₂ into energetically usable CH_4. This is studied in a long-term anaerobic digestion experiment using different substrates (sewage sludge, maize silage, homogenized organic residuals) and applying stable isotope techniques to identify the underlying mechanisms leading to increased CH₄ formation observed within previous studies. For this purpose, two continuous 28 L anaerobic digestion systems were established (reactor with CO₂ injection and control) and δ¹³C-CH₄, δ¹³C-CO₂ and δ²H-CH₄ values in the headspace gas were measured to investigate the underlying CO₂ to CH₄ bioconversion pathways, targeting the mechanisms hypothesized so far.