Increasing methane productivity in anaerobic digesters with sewage sludge by addition of carbon dioxide
Daniela Polag, Frank Keppler (project coordination and isotope analytics)
Konrad Koch, TU München (performance of continuous flow-through experiments)
Michael Lebuhn, Bayerische Landesanstalt für Landwirtschaft (microbial analyses)
2017 - 2019
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 kgVS/(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 kgVS/(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, RCO2