An upflow microaerobic sludge reactor (UMSR) was constructed in treating manure-free piggery wastewater with high ammonium concentration and a COD/TN ratio as low as 0.84. The UMSR offered an outstanding removal of NH4+-N and TN at 35 degrees C and hydraulic retention time 8 h subsequent to inoculated sludge acclimation. A short NO2-N accumulation phase was observed whenever there was a considerable increase in TN loading rate (NLR), but decreased rapidly along with an evident increase in TN removal. Fed with raw wastewater at a NLR of 1.10 kg/(m(3) d), the average COD, NH4+-N and TN removal reached 0.72, 0.76 and 0.94 kg/(m(3) d), respectively. Inference drawn from stoichiometry based on the potential nitrogen removal pathways and the C/N ratio required by denitrification indicated that anammox was the main mechanism for NH4+-N and TN removal in the UMSR. (C) 2015 Elsevier Ltd. All rights reserved.