Three mono-digestion experiments treating slaughterhouse waste with high TKN concentration (similar to 11 g/kg) were applied in lab-scale at mesophilic and psychrophilic conditions to study the impact of high ammonia concentrations and additives. Precipitation of sulphur by addition of ferrous chloride did not influence process behaviour, whereas supplementation of trace elements significantly improved process stability by reducing volatile fatty acid concentration towards zero. The limit of NH4-N concentration causing a rise of VFAs to 19,000 mg/l and reduction of methane by 25% was found between 7.7 and 9.1 g/kg which correspond to NH3 concentrations of 830-1060 mg/l. Psychrophilic operation (25 degrees C) lowered inhibitory NH3 concentration to 140 mg/l, but process performance was stable only at low OLR of 0.4 kg VS/m(3) d. Robust performance at highest possible NH4-N concentration (7.7 g/kg), low VFA accumulation and satisfying methane yield of about 280 Nm(3)/t COD was observed at OLR of 2.5 kg VS/m(3) d at 37 degrees C. (C) 2014 Elsevier Ltd. All rights reserved.