Anaerobic ammonium oxidation coupled to Fe(III) reduction, i.e., Feammox, is playing an important part in nitrogen cycle of natural environments, which however has been rarely investigated in waste water/solid treatment processes. Ammonium as a byproduct of nitrogenous substance decomposition during anaerobic digestion of waste activated sludge usually presents a quite high content and poses a great risk to environment. This study focused on investigating the effects of supplementing Fe(III) compounds in anaerobic digestion of sludge and especially evaluating the roles in nitrogen removal. Supplementing magnetite, Fe2O3 and Fe(OH)(3) in anaerobic digesters all increased the methane production and sludge reduction. Importantly, these Fe(III) compounds induced Feammox to occur continuously. NO2- and NO3- were generated in the Fe(III)-added reactors, especially in Fe(OH)(3)-added reactor. Afterwards, NOx- would be reduced with organics or Fe(II) as electron donors. Consequently, 20.1% of total nitrogen was removed in Fe(OH)(3)-added reactor after 40 days. As a product of dissimilatory iron reduction (including Feammox), the Fe(II) content was far less than theoretical production through the stoichiometrical NH4+ removal in Feammox, implying that the Fe(II)/Fe(III) cycle likely occurred to trigger the successive nitrogen loss.