Life-cycle assessment of two food waste disposal processes based on anaerobic digestion in China

Abstract

In the Chinese food waste disposal industry, the choice between using liquid-phase anaerobic digestion or solid–liquid mixed anaerobic digestion to effectively dispose of food waste has become a recent source of debate. However, no previous studies have compared or assessed the relevant technological parameters of these two processes for the same feedstock. In this study, the two processes were evaluated from the perspective of energy consumption and environmental impact. Here, hydrothermal pretreatment, anaerobic digestion, combined heat and power, and waste treatment were considered in the system boundaries. First, a biochemical methane potential test was conducted to obtain the liquid-phase and solid–liquid mixed anaerobic digestion methanogenic potential. Then, using the actual operating parameters of the food waste disposal plant, a life-cycle assessment was performed for the two treatment processes. For solid–liquid mixed and liquid-phase anaerobic digestion processes, the total energy inputs are 182.81 kWh/t and 192.67 kWh/t, respectively, and the net energy inputs accounts for 26.2% and 30.7% of the total energy input, respectively. Compared with solid–liquid mixed anaerobic digestion, liquid-phase anaerobic digestion demonstrates a higher potential for global warming (77.91 vs. 29.24 kg CO2-eq./t), eutrophication (0.4 vs. 0.19 kg PO43--eq./t), acidification (2.61 vs. 1.22 kg SO2-eq./t), and photochemical ozone creation (0.077 vs. 0.03 kg NMVOC-eq./t). According to the sensitivity analysis, the increased efficiency in total-solids degradation of anaerobic digestion and the total-solids content of the biogas residue were found to have a significant effect on the reduction of eutrophication potential, acidification potential, and photochemical ozone creation potential in solid–liquid mixed anaerobic digestion. For liquid-phase anaerobic digestion, the total-solids content of the separated solid phase is an important factor affecting its environmental impact. These results provide important reference values for the development of food waste treatment processes.