The start-up of the completely autotrophic nitrogen removal process was examined in a sequencing batch reactor (SBR) using low activity anoxic ammonium oxidation (anammox) inoculum. The SBR received effluent from an upflow anaerobic sludge blanket (UASB) that treated low strength wastewater. The volumetric nitrogen loading rate (vNLR) was first 0.24 +/- 0.11 kg N m(-3) d(-1) and then reduced to 0.10 +/- 0.02 kg N m(-3) d(-1). The average specific anammox activity was 2.27 +/- 1.31 mg N (gVSS h)(-1), at 30 degrees C representing an increase of 161% compared to the inoculum. The decrease in vNLR did not significantly affect anammox activity, but resulted in a decrease of denitrifying heterotrophic activity to very low levels after the first 30 days owing to the decrease of organic loading rate (OLR). Fluorescence in situ hybridization (FISH) analysis confirmed the stable presence of anammox bacteria in biomass. Numerous filamentous microorganisms were present, several of which were in a state of endogenous respiration. (C) 2013 Published by Elsevier Ltd.