Stable isotopes of C and H in methane fermentation of agriculture substrates at different temperature conditions

Abstract

AbstractAgricultural substrates (maize silage and cattle manure) were used to carry out methane fermentation process in bioreactors under laboratory conditions. Identical mixtures of these substrates were incubated for 43 days at 20, 30 and 40ºC to determine how different temperature conditions affect the δ13C(CH4), δH(CH4), and δ13C(CO2) values. To ensure correct anaerobic digestion, the following parameters of the organic substrates and fermentation solutions were monitored: total organic carbon (TOC), volatile solids (VS), volatile fatty acids (VFA), chemical oxygen demand (COD) and carbon to nitrogen ratio (C/N). The variants with higher incubation temperature yielded higher amounts of biogas (20ºC=84.5, 30ºC=101.8 and 40ºC=133.3 dm3/kg VS). In the case of gas products of methane fermentation, it was observed that the higher temperature of incubation affects the depletion in heavy isotopes. At 20ºC, 30ºC, and 40ºC mean values of δ13C(CH4) reached −26.4, −29.7, and −35.4‰, respectively. Mean values of δ2H(CH4) were −311.6, −354.0, and −398.5permil, and of δ13C(CO2) +8.9, +3.7, and −2.3‰, respectively. Moreover, the apparent fractionation coefficient α13C(CO2-CH4) were calculated, which decreased when the temperature increased. This isotopic tool was used to identify acetoclastic reaction as a dominant methanogenesis pathway. Observed changes in the isotopic composition of gaseous products obtained at different incubation temperatures may indicate decomposition of different carbon sources (e.g. lactate, propionate) to acetate and its fermentation by acetoclastic methanogens. It is possible that this was also related to the observation of the various metabolic models due to the varied methanogenic community composition.