Increasing methane productivity in anaerobic digesters by addition of CO2

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The major goal of this project is the identification of the mechanisms leading to increased CH4 formation by the addition of CO2 during anaerobic digestion. While enough evidence of the effect has already been provided by various studies, the underlying mechanisms have only been hypothesized so far and are still far from being understood. In order to reproduce the effects and to utilize this great potential for energy production from waste streams, the mechanisms had to be fully uncovered. To achieve the above mentioned goals an interdisciplinary approach is required including the use of stable isotope methods as the key parameter for process identification, know-how of process engineering including the supply of a fully-equipped laboratory with expertise in the performance of continuous flow-through experiments, and accompanying microbial analyses. To identify the basic mechanisms of the observed bioconversion described in detail, we aim at establishing two continuous anaerobic digestion test systems at TUM. One system is enriched with CO2 while the other serves as the control. The reactors will be fed with sewage sludge, since it seems to be the substrate with the highest potential for CO2 enrichment. Tests will be carried out at various loading rates indicating different stress regimes which might have a significant influence on the mechanism of CH4-formation. During the tests, standard process parameters (TS, VS, COD, VFA, Alkalinity, pH value, gas quantity and quality) will be monitored. Additionally, the biocenosis in the digestate will be characterized by state-of-the-art molecular biological methods such as guild-specific quantitative real-time PCR on DNA and mRNA levels, and community sequencing approaches.As a special focus of the project, in order to retrace CO2 conversion to CH4, isotope techniques including isotope labeling experiments will be applied during the digestion tests. With this powerful technique it is possible to reconstruct precursors and specific pathways during CO2 addition.Once the underlying mechanisms are understood, the basis for implementing the approach in practice is provided.