Anaerobic digestion of municipal solid waste and sewage sludge under mesophilic and thermophilic conditionsA thermal study in DSC associated to infrared spectroscopy

In this study, thermal analysis in differential scanning calorimetry associated to Fourier self-deconvolution (FSD) of infrared spectra was applied to the study of anaerobic digestion of the organic fraction of domestic solid waste and sewage sludge mixed in 1:1 ratio under mesophilic and thermophilic conditions. Curves showed an exothermic peak in the high-temperature range and a number of endothermic and/or exothermic peaks in the low- and medium-temperature range. The high-temperature exotherm was a common feature on curves of all samples. Digestates evidenced a shift of this exotherm toward higher temperature with respect to both substrates and feed-in materials as a result of a greater chemical complexity attained by organic matter (OM) during the process. A further shift toward higher temperature values was observed on digestates obtained under thermophilic conditions with respect to digestates obtained under mesophilic conditions. This result was associated to a higher recalcitrance of digestates produced under more drastic conditions. Based on evidence obtained by FSD spectra, the exotherm in the medium-temperature range was assigned to combustion of carbohydrates and to loss of aliphatic structures and carboxylic groups. Enthalpy data associated to the high-temperature exotherm were used to differentiate feed-in materials and their corresponding digestates and to distinguish digestates obtained under different operative conditions.