The anaerobic digestion of onion residual from an onion processing plant was studied under batch-fed and continuously-fed mesophilic (35 +/- 2 degrees C) conditions in an Anaerobic Phased Solids (APS) Digester. The batch digestion tests were performed at an initial loading of 2.8 gVS L(-1) and retention time of 14 days. The biogas and methane yields, and volatile solids reduction from the onion residual were determined to be 0.69 +/- 0.06 L gVS(-1) 0.38 +/- 0.05 L CH(4) gVS(-1), and 64 +/- 17%, respectively. Continuous digestion tests were carried out at organic loading rates (OLRs) of 0.5-2.0 gVS L(-1) d(-1). Hydrated lime (Ca(OH)(2)) was added to the APS-Digester along with the onion residual at 16 mg Ca(OH)(2) gVS(-1) to control the pH of the biogasification reactor above 7.0. At steady state the average biogas yields were 0.51, 0.56, and 0.62 L gVS(-1) for the OLRs of 0.5, 1.0, and 2.0 gVS L(-1) d(-1) respectively. The methane yields at steady state were 0.29, 0.32, and 0.31 L CH(4) gVS(-1) for the OLRs of 0.5, 1.0, and 2.0 gVS L(-1) d(-1) respectively. The study shows that the digestion of onion residual required proper alkalinity and pH control, which was possible through the use of caustic chemicals. However, such chemicals will begin to have an inhibitory effect on the microbial population at high loading rates, and therefore alternative operational parameters are needed. (C) 2011 Elsevier Ltd. All rights reserved.