Background Biological wastewater treatment by nitrification is widely used; however information on inhibition of nitrification by sulfide is still scarce. The aim of this study was to evaluate the inhibitory effect of different initial sulfide concentrations on ammonium and nitrite oxidizing processes. Results In the absence of sulfide, nitrification was completed after 24h with an ammonium consumption efficiency (ENH4+) of 100.0 +/- 2.1% and an ammonium to nitrate conversion yield (YNO3-) of 0.90 +/- 0.01g NO3--Ng-1 NH4+-N consumed. At sulfide concentrations lower than 13.5 +/- 0.7mg HS--SL-1, ENH4+ was maintained at 100% after 60h. At sulfide concentrations ranging from 3.1 to 112.0mg HS--SL-1, the YNO3- decreased from 0.9 to 0.3 due to an accumulation of nitrite, whereas specific rates decreased by 51 to 92% for NH4+ consumption and by 77 to 97% for NO3- production. The IC50 value (concentration causing 50% inhibition) for ammonium and nitrite oxidation was 2.6 +/- 0.3 and 1.2 +/- 0.2mg HS--SL-1, respectively. Conclusion Sulfide provoked an inhibitory effect on both ammonium and nitrite oxidizing processes, nitrite oxidation being the most affected step. This information is relevant for wastewater treatment, indicating that the presence of sulfide in effluents even at low concentration might strongly alter the nitrifying activity of activated sludge and provoke nitrite accumulation.(c) 2012 Society of Chemical Industry