Low-temperature solubilization of carbohydrates, lipids, and proteins in livestock waste for anaerobic methane production

This study investigated (a) the impact of low-temperature thermal pretreatment (TPT) on the solubilization of three macromolecules (carbohydrates, lipids, and proteins) and (b) the anaerobic digestion (AD) efficiency of livestock waste substrates (swine manure (SM), cow manure (CM), and fodder (F)). TPT was carried out at two temperatures (80 °C and 90 °C), for 60 and 90 min each. The solubilization of the macromolecules and the chemical oxygen demand (COD) was evaluated during this process. The AD was carried out at an operating temperature of 37.1 °C. The volume of biogas was quantified using the water displacement method. The volatile solid removal (VS), pH, and methane production were monitored during the experiment. We also determined the synergistic effect of the substrates on methane yield by the co-digestion performance index (CPI). Tests were carried out with raw and pretreated substrates. All TPT conditions significantly enhanced the solubilization of macromolecules and COD. The highest COD solubilization was observed at 90 °C, 90 min (8.08% for SM, 7.30% for CM, 6.56% for F, and 5.80% for the combined substrates (SM + CM + F)). This high efficiency of the COD solubilization led to a methane yield reaching up to 1.03 L CH₄/g VS removed for the combined pretreated substrates. We also observed a high CPI (1.42). This indicates that the mixture of swine manure (SM), cow manure (CM), and fodder (F) can be subjected to a TPT and a co-digestion process to enhance their anaerobic digestion. These processes can be helpful in their management and energy recovery.