Stoichiometry and energy assessment comparison between thermophilic and mesophilic high-solid anaerobic co-digestion of lipids and food waste

High-solid anaerobic co-digestion of lipids and food waste at different lipid/TS ratios was investigated with an HRT of 30 days under thermophilic and mesophilic conditions. By co-digestion with food waste with thermophilic and mesophilic anaerobic digestion systems, the organic loading rate of lipids achieved 2.33 g/L/d and 1.67 g/L/d and the degradation efficiency of lipids reached 87 % and 85 %, respectively. As the lipid/TS ratio increased from 10 % to 50 %, the methane content in the biogas significantly increased from 60 % to 65 %, and the average methane yield of thermophilic and mesophilic anaerobic co-digestion was 288 mL-CH4/g-COD and 275 mL-CH4/g-COD, respectively. Meanwhile, the COD removal (80 % and 79 %) and lipids degradation (87 % and 85 %) kept high efficiencies with both thermophilic and mesophilic systems. The difference performance was attributed to the different microbial yield coefficients under different temperatures. Furthermore, an increase in lipid/TS ratio in the substrate inevitably resulted in lower microbial yield coefficients due to the low cell synthesis efficiency of lipids. In order to clarify the distribution of organic elements, stoichiometric equations were also illustrated. From the aspect of energy assessment, it was suggested that the energy surplus of mesophilic anaerobic co-digestion was 9 % higher than that of thermophilic anaerobic co-digestion when the lipid/TS ratio was less than 50 %, although the anaerobic system was energy and economy positive under both thermophilic and mesophilic conditions.