Iron (Fe) is often encountered in wastewaters. This study investigated the effects of iron on the single-stage deammonification process treating reject water from sludge dewatering. When Fe2+ and Fe3+ concentrations in the influent were below 1.3 mg/L and 0.4 mg/L, Fe2+ incorporation was found to be linearly correlated with NH4+-N removal. However, the excess Fe2+ and Fe3+ drastically deteriorated the deammonification performance. Both of the reactor performance and Fluorescence In Situ Hybridization results suggested higher sensitivity of autotrophic bacteria to iron than heterotrophs, the sequence of which was assessed to anammox bacteria > aerobic ammonium oxidizers > hetetrophic denitrifiers. With the excess existence of iron, extracellular substances were largely released by bacterial cells, which were the likely sites for iron uptake by scanning electron microscope-energy dispersive X-ray analysis. The information provided here would be useful to facilitate the application of deammonification process in the treatment of wastewater including metal iron. (C) 2011 Elsevier Ltd. All rights reserved.