Enhanced and Stabilized Methane Fermentation of Grease Trap Waste Using Granular Activated Carbon

Abstract This study investigated methane fermentation feasibility using grease trap waste as a sole substrate and evaluated the impact of granular activated carbon (GAC) on process stability and methane production. GAC has been shown to reduce volatile fatty acid (VFA) buildup in co-digestion systems, but its impact on methane fermentation when grease trap waste is used as a sole substrate remains unclear. In batch tests, increasing the substrate-to-inoculum (S/I) ratio from 0.5 to 2.0 increased the cumulative methane production by 1.8 times, reaching 520 mL at an S/I ratio of 2.0, although fermentation start-up was delayed. In the batch test with GAC addition, the methane production on day 40 reached four times that of the control (∼ 800 mL vs ∼ 200 mL), while methane concentration remained stable at around 70%. Furthermore, in the continuous test (single reactor for each condition), the reactor with GAC addition sustained organic matter degradation, achieving a decomposition rate of about 75% and maintained a stable pH between 7.0 and 7.5. These findings suggest that GAC may promote interspecies electron transfer and help stabilize methane fermentation, possibly reducing the accumulation of intermediate metabolites such as VFA and supporting long-term methane production. Importantly, this study demonstrates that grease trap waste, which has generally been considered to require pretreatment, can be used as a sole substrate for methane fermentation without complex pretreatment, and that GAC addition further promotes and stabilizes this process. This provides a novel and simplified strategy for utilizing grease trap waste as a sole substrate in methane fermentation. Graphical Abstract