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The feasibility of the co-digestion of onion juice extracted from solid onion waste and aerobic sludge generated by an onion processor located in California was investigated in this study. Experiments were conducted to determine the anaerobic digestion requirements for onion juice and aerobic sludge mixture in a single-stage sequential batch anaerobic mixed biofilm reactor. The organic loading rate (OLR) to the digester was increased by increasing the concentration of onion juice, while maintaining the concentration of the aerobic sludge constant. At OLRs from 1.24 to 3.08 g volatile solids (VS)/L/d the biogas and methane yields were 0.73 0.09 L/gVS and 0.44 0.08 L/gVS, respectively. However, when the OLR reached 4.37 gVS/L/d, the biogas production and pH decreased rapidly, indicating digestion system failure. As the OLR increased from 1.24 to 4.37 gVS/L/d, the carbon to nitrogen ratio (C/N) of the mixture also increased from 13.7 to 20.3. The increased C/N was suspected to limit the digestion of the mixture. Therefore a second experiment was carried out to determine if higher OLRs (i.e., greater than 3.08 gVS/L/d) could be achieved if the C/N of the mixture was maintained at lower levels. In this second experiment, appropriate concentrations of onion juice and aerobic sludge were mixed to maintain C/N at about 15. The maximum OLR achieved in the second experiment with was 3.60 gVS/L/d, at which the biogas and methane yields were 0.65 and 0.40 L/gVS, respectively. However, when the C/N of the feed was increased to 18 at an OLR of 3.6 gVS/L/d, biogas production decreased by 48%. Thus the results of this study indicated that to achieve higher OLRs for highly biodegradable soluble organic material, low C/N values might be required.