Autotrophic nitrogen removal through sequential partial nitritation and anammox reactions can be achieved in biofilm reactors by controlling the oxygen concentration in the bulk liquid in such a way that nitrite oxidizers are outcompeted by anammox bacteria. In the case of granular sludge reactors, the granule size may influence the optimal range of oxygen concentration, as has been confirmed in the present study by means of numerical simulations. The range of oxygen concentrations for which combined partial nitritation and anammox conversion is established becomes broader for larger particles and with increasing influent ammonium concentrations. At the same time the likelihood of nitrite accumulation in the reactor effluent also increases. © 2010 Taylor & Francis.