The effect of two additives on anaerobic monodigestion of pig slaughterhouse waste was studied in three laboratory scale continuously stirred tank reactors (CSTRs) operated for 242-284 days at 35 degrees C. A higher organic loading rate (OLR), namely 2.25 kg volatile solids (VS)/m(3) d with a hydraulic retention time (HRT) of 30 d was feasible with an additive containing Fe, HCl and trace elements while an OLR of 1.5 kg VS/m(3) d (HRT 30 d) led to instabilities in CSTR with an additive consisting of Fe and HCl and in CSTR without additives. Methane yields of ca. 700 dm(3) CH4/kg VSfed were obtained in all reactors under stable conditions. Both additives decreased H2S concentration of the biogas below detection limit 0.5 ppm while in a control reactor H2S concentration was up to 1900 ppm. In conclusion, the results suggest the possibility that additives may facilitate higher OLRs in monodigestion of pig slaughterhouse waste. (C) 2012 Elsevier Ltd. All rights reserved.