The objective of this study was to assess the feasibility of co-digesting grease sludge (GS) originating from domestic wastewater along with sewage sludge (SS) and to assess the effect of organic loading rate (OLR) and GS content on process performance. Three lab-scale semi-continuous fed mesophilic anaerobic digesters were operated under various OLRs and SS-GS mixtures. According to the results, addition of GS up to 60% of the total VS load of feed resulted in a 55% increase of biogas yield (700 vs. 452 m(3)/tVS(added)) for an OLR of 3.5 kg VS/m(3)/d. A stable and satisfactory operation of anaerobic co-digestion units can be achieved for a GS-OLR up to 2.4 kg VSGs/m(3)/d. For such values biogas yield is linearly proportional to the applied GS-OLR, whereas biogas yield is minimal for GS-OLR higher than this limit and acidification of the anaerobic digestion units is taking place. (C) 2013 Elsevier Ltd. All rights reserved.