Natural isotopic composition in substrate may be used to reveal the metabolic pathways of substrate transformation by microbial community. In this paper, a change in δ13CH4 during methanization of reconstituted municipal solid waste was described using a mathematical model based on stoichiometric chemical reactions, equation for the 13C isotope accumulation in products at the low natural C13/C12 ratio and microbial ecology. A set of experimental data used in the model was taken from Qu et al. (2009a). According to the model, during mesophilic municipal solid waste methanization initially hydrogenotrophic and further aceticlastic methanogenesis dominated. At the final stage hydrogenotrophic methanogenesis followed by acetate oxidation dominated again. In spite of rather high measured values of δ13C for CO2 above −21‰, a sharp decrease in δ13CH4 from −20‰ to −60‰ at the final stage was explained by a larger fractionation against 13C during methanogenesis from H2/H2CO3 due to a kinetic isotope effect when hydrogenotrophic methanogens preferentially take down light 12C. The model also confirmed that in thermophilic conditions a comparatively stable value of δ13CH4 about −60‰ measured earlier (Qu et al. 2009b) was due to a dominance of hydrogenotrophic methanogenesis during all methanization process of cardboard waste.

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