The overall objectives of this study were to evaluate the effectiveness of utilizing manure vermicompost as an additive to enhance anaerobic digestion of post-consumer cafeteria food waste in a single-stage digestion system and investigate the mechanisms associated with such enhancement. Vermicompost was chosen because of its buffering capacity, abundance of humic substance, and variety of trace metals, all of which may enhance the digestion process. The experiment was first conducted using a batch-scale biochemical methane potential assay and found that manure vermicompost added to the food waste reactors at concentrations of 2 g/L and 6 g/L both significantly increased ultimate methane yield and methane production rate. Then, a long-term study was conducted using twelve semi-continuous single-stage reactors to confirm such enhancement and further investigate the associated mechanism. The specific methanogenic activity and trace metal (iron, nickel, and cobalt) bioavailability were also evaluated. Results showed that the food waste digester without any supplement (control) had unstable and low methane production (254 mL/g VS added/day and 455 mL/g VS destroyed/day). During the experimental period, the control reactor experienced a dramatic reduction in pH (less than 6) due to a significant accumulation of volatile fatty acids (more than 2,600 mg/L). The trace metal bioavailability tests further demonstrated that the control digester could be deficient in nickel and iron. In contrast, the food waste digesters supplemented with manure vermicompost (2 g/L), trace metals (a mixture of 0.01 mg/L nickel, 0.5 mg/L Fe, and 0.01 mg/L Co) or humic acids (0.4 g/L) all had stable and significantly greater methane production compared to the control. The pH was approximately 7 and volatile fatty acids were less than 200 mg/L. Among all treatments, the food waste digesters supplemented with manure vermicompost had the greatest methane production (625 mL/g VS destroyed/day). In comparison to the control, supplementation of manure vermicompost also nearly doubled the acetate utilization rate and enhanced the propionate utilization rate by 60%. It was found that such enhanced digestion performance was likely related to the trace metals (particularly iron and nickel) provided by the vermicompost. Humic acids, naturally presented in mature vermicompost, also contributed to the enhanced performance of food waste digestion. In summary, manure vermicompost (without any additional chemical amendments) stabilized and increased methane production from anaerobic digestion of food waste in the single-stage digestion system.
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