Effect of additives on process stability of mesophilic anaerobic monodigestion of pig slaughterhouse waste

► Anaerobic monodigestion of pig slaughterhouse waste was studied in CSTRs at 35°C. ► Effect of two additives on process stability was studied as well. ► Additive of trace elements, Fe and HCl improved process stability. ► Additive of trace elements, Fe and HCl allowed higher OLRs to be used. ► Additive of Fe and HCl reduced H2S from biogas, but did not facilate higher OLR. 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 242–284days at 35°C. A higher organic loading rate (OLR), namely 2.25kg volatile solids (VS)/m3d with a hydraulic retention time (HRT) of 30d was feasible with an additive containing Fe, HCl and trace elements while an OLR of 1.5kg VS/m3d (HRT 30d) led to instabilities in CSTR with an additive consisting of Fe and HCl and in CSTR without additives. Methane yields of ca. 700dm3 CH4/kg VSfed were obtained in all reactors under stable conditions. Both additives decreased H2S concentration of the biogas below detection limit 0.5ppm while in a control reactor H2S concentration was up to 1900ppm. In conclusion, the results suggest the possibility that additives may facilitate higher OLRs in monodigestion of pig slaughterhouse waste.