Identification of methanogenic pathways in anaerobic digesters using stable carbon isotopes

Abstract

Abstract In a series of anaerobic batch experiments, the stable carbon isotopes, δ 13 C CH ?4? and δ 13 C CO?2? , were measured in biogas produced from various sources (maize, cellulose, inoculum) to identify the degradation kinetics and specific methanogenic pathways. Isotopic analysis was performed using a new absorption laser spectrometer in addition to conventional MS. A comparison of the isotopic evolution shows large isotope dynamics for maize and cellulose, indicating a temporal change in degradation pathways and/or a change in the relative contribution from different carbon fractions within the substrate. Further batch experiments with isotopically labelled acetate (either 13 CH 3 CO 2 Na or CH CO 2 Na) were carried out to study the degradation of acetate in inoculum systematically. The results suggest that the acetate is completely oxidized into CO 2 which in turn is partly reduced to CH 4 . Furthermore, the distinct isotopic signature CH 4 and CO 2 (for acetate‐methyl labelling as well as for acetate‐carboxy labelling) indicate that only a minor part of the produced methane derives from acetate. A substantial fraction of methane may have been produced at an earlier stage of the reaction chain or by other potential methane precursors such as formate or methanol.