Increasing methane productivity in anaerobic digesters with sewage sludge by addition of carbon dioxide

Background/Summary

Bioconversion of CO₂ to CH₄ with the overall aim of reducing industrial CO₂ emissions and transformation of CO₂ into energetically usable CH₄ was studied in a long-term anaerobic digestion experiment using sewage sludge as major substrate. The main goal of this project was the identification of the underlying mechanisms leading to increased CH₄ formation observed within previous studies. For this purpose, two continuous anaerobic digestion systems were established (one reactor with CO₂ injection and one control reactor without CO₂ injection).

Different experimental approaches were applied to investigate the underlying CO₂ to CH₄ bioconversion pathways, targeting the mechanisms hypothesized so far. In addition to standard process parameters (VFA, VOA/TIC, pH, gas quantity and quality), molecular biological and stable isotope analyses were used to reconstruct specific metabolic pathways as a consequence of CO₂ addition under varying organic loading rates.

The following major outcomes can be deduced from the long-time-monitoring results:

(i) A short-term boost of CH₄ yield after starting the CO₂ injection most probably resulted from increased acetate production by reductive acetogenesis which is utilized by acetoclastic methanogens at lower OLRs. The addition of CO₂ led to intensified conversion of partially oxidized intermediates such as acetate and, thus, supports the hypothesis of increased substrate turnover. In the course of ongoing discontinuous CO₂ injection, the microbiome seemed to adapt to the raised CO₂ concentrations.

(ii) A long-term effect of CO₂ injection resulting in enhanced CH₄ formation could not be observed until the organic loading rate increased to 3.0 kg_VS/(m³·d). Under those increasing stress conditions, a significant enhancement in CH₄ yield was observed for the test reactor compared to the control reactor. An acidification event even intensified the effect and, moreover, improved the process stability and resilience of the anaerobic digestion process for the reactor with CO₂ injection. Data from microbial and isotope analysis indicate an increasing fraction of hydrogenotrophic methanogenesis with the increasing organic loading rate. Surprisingly, this increase was more pronounced for the control reactor. Thus, CO₂ injection seems to lead to improved turnover rates for CH₄ conversion under higher loading rates without a significant impact on the microbial community.

Based on the observation, CO₂-injection seems to be most suitable to reduce the residual biogas potential during high VFA concentrations, and thus, lead to improved environmental conditions. Follow-up projects should focus on higher organic loading rates (> 4.0 kg_VS/(m³·d)) to verify and quantify the enhanced CH₄ yield and improved process stability under CO₂ enrichment. In a next step, results could be up-scaled to a pilot scale AD system for a final implementation of the approach in practice.

Isotopic values, expressed as apparent fractionation αC with αC = (δ13CCO2+1000)/(δ13CCH4+1000)), indicate that in comparison to the control reactor, R₀, acetoclastic methanogenesis plays a dominant role in the reactor with CO₂ injection, R_CO2