The work investigated why anaerobic digesters treating food waste and operating at high ammonia concentrations suffer from propionic acid accumulation which may result in process failure. The results showed deficiency of selenium, essential for both propionate oxidation and syntrophic hydrogenotrophic methanogenesis, leads to this while supplementation allows operation at substantially higher organic loading rates (OLR). At high loadings cobalt also becomes limiting, due to its role either in acetate oxidation in a reverse Wood-Ljungdahl or in hydrogenotrophic methanogenesis. Population structure analysis using fluorescent in situ hybridization showed only hydrogenotrophic methanogens. Critical Se and Co concentrations were established as 0.16 and 0.22 mg kg(-1) fresh matter feed at moderate loading. At this dosage the OLR could be raised to 5 g VS l(-1) day(-1) giving specific and volumetric biogas productions of 0.75 m(3) kg(-1) VS(added) and 3.75 STP m(3) m(-3) day(-1), representing a significant increase in process performance and operational stability.