Optimizing Methane Production from Anaerobic Batch Co-Digestion of Apple Pomace and Vegetable Waste

Co-digestion of apple pomace (AP) and vegetable waste (VW) was a feasible proposal to simultaneously eliminate their potential pollution to the environment and produce alternative biogas energy, whereas, it can be easily interrupted by rapid acidogenesis and inhibition of methanogen. Additionally, the methane yield could not be maximization due to the recalcitrant cellulose substance of AP. The inoculum regulation was employed for overcoming the acidogenesis problem and enhancing methane yield when conducting high-solids (TS of 10%) co-digestion of AP and VW under mesophilic condition. The chosen inoculum was the special acclimated anaerobic granular sludge (AGS) from the upflow anaerobic sludge blanket reactor treating VW acidification liquid. The AGS inoculum was preferable to the conventional inoculum in view of the multitudinous and abundant methanogens. Under the optimum inoculation conditions, the anaerobic batch co-digestion of AP and VW at different mixing TS ratios of 5:0, 4:1, 3:2, 2:3, 1:4 and 0:5 were conducted to obtain a suitable substrate mixing ratio. Results showed that the optimal AP/VW ratio was 2:3, which gave the highest methane yield of 336 mL/g-VS. The excellent performance of the co-digestion was attributed to the balanced C/N, reduced accumulation of volatile fatty acids (VFAs), stable pH value, as well as the high quality and quantity of methanogens included in the inoculum.