The performance of two selected high rate anammox reactors, i.e. sequencing batch reactor (SBR) and an upflow biofilter (UBF) were compared operating them simultaneously at 30 degrees C. The sludge from anaerobic digester of a municipal wastewater treatment plant was used for inoculation and synthetic wastewater was fed to both reactors during the experiment. During start-up, the nitrogen loading rate (NLR) was increased gradually from an initial 140 to 560 mg/l day. The comparison of the performance revealed that the NLRs of SBR (2.7 g/l day) and UBF (2.5 g/l day) were far higher than those of the traditional nitrification/denitrification process. The biomass production of SBR outcompeted that of UBF. SBR took a longer time for start-up, i.e. about 57 days compared with 31 days of UBF Moreover, SBR tolerated smaller pulse of substrate concentration and hydraulic load, showing a weaker stability compared with UBF (C) 2007 Published by Elsevier B.V.