Biogas production of fruit and vegetable wastes two-phase fermentation by batch and continuous feeding

Abstract: This study evaluated the biogas production of apple fruit wastes and vegetable wastes using batch test and continuous fermentation test. The biogas production of the mixtures of fruit wastes and vegetable waste with different mixing ratios (Fruit: Vegetable, F:V) was also evaluated. For the batch tests, the F:V was set as 5:0, 4:1, 1:1, 1:4, 0:5, respectively. The batch tests were employed for the evaluation of the biogas production process and the accumulated biogas production of these wastes, which were conducted with 500 mL digesters. During these tests, daily biogas productions were recorded. The results showed that the biogas production values of the mixture of vegetable and apple waste were significantly higher than those of fruit waste or vegetable wastes digested singly at 38 ℃, which suggested that synergistic effects occurred during the anaerobic digestion of the vegetable wastes mixed with fruit wastes. When the F:V was 4:1, the biogas production of the mixture of fruit wastes and vegetable wastes reached 914.6 L/kg, which was the highest among all the 5 tests. The modified Gompertz model had been proved well for fitting the accumulated biogas production curves of batch digestion of wastes. In this study, the modified Gompertz model was employed to fit the accumulated biogas production curves of the vegetable wastes, fruit wastes and their mixtures. The R2 values of the fitting curves ranged from 0.983 to 0.999, which was similar with the results of the literatures. This meant that the modified Gompertz model fitted the accumulated biogas production curves of fruit and vegetable wastes well. The continuous tests were conducted with the combined two-phase (CTP) reactors at 38℃, and the F:V values were 4:1 and 1:4, respectively. The hydraulic retention time (HRT) of the reactors was controlled in 24 d by adjusting the reflux of biogas slurry to the reactors. In general, the daily reflux of biogas slurry was about 400 mL. The organic loading rate (OLR) of the CTP reactors was 0.5 g/(L·d) during the first 18 days, and it rose to 1.5 and 2.5 g/(L·d) during the next days. The results showed that the OLR of CTP could reach 2.5 g/(L·d) when treating fruit wastes and vegetable wastes. Under this OLR, the daily biogas production and specific biogas production respectively reached 12.9 and 0.39 L/g. These results showed that the CTP reactor could be employed to treat fruit and vegetable waste for biogas production. However, the digestion rate of fruit and vegetable waste in CTP reactor was lower than continuous stirred-tank reactor, though the CTP reactor had simple structure and low cost.