Hong Qiang
College of Resources&Environment al Science, Northwest A&F University

The amount of iron, cobalt and nickel required in the thermophilic methane fermentation of food waste was examined in this study. For this purpose, the effect of trace metals on the thermophilic methane fermentation of high-solid food waste was investigated using both batch and continuous experiments. The CSTR reactor was run with a working volume of 12L. The continuous experiment was comprised of three runs. During the first run, the HRT was stepwise decreased from 100 days to 30 days with an increase in the organic loading rate from 1.9 to 6.3 Kg-COD/m3.d. Stable methane fermentation was realized when the HRT was at both 100 days and 50 days. Four typical operation parameters, the methane production rate, the methane content, the pH and total VFA concentration, were stable in the digester at both HRT 100 days and 50 days, indicating that the digester was started-up successfully. From day 56 (HRT 30days) a sudden drop in the gas production rate, pH and methane content was observed, corresponding to the accumulation of VFAs, suggesting that there may have been some kind of inhibition resulting in unstable in digestion. It was inferred that the inhibition factor may have been either a lack of trace metals or a high ammonia concentration. The ammonia concentration, however, was not more than 1100mg/l, outside the ammonia inhibition range indicted by other researchers. As such, it was decided that the main reason for the gas inhibition must have been a lack of trace metals. According to the results of the batch experiment, a combination of iron, cobalt, and nickel, at 10, 1, and 1mg/l respectively, was added to the reactor once one and half months into the second run. The reduced gas production, methane content, and the pH all were restored to well-functioning levels after adding the trace metals.  During the third run, trace metals were added to the substrate to the effect that no inhibition was noted at all. Based on theoretical calculations and experimental results, it was found that the most suitable values for Fe/COD, Co/COD, Ni/COD are 276, 4.96 and 4.43 mg/Kg.COD, respectively, in the high-solid thermophilic methane fermentation of food waste.