The dissolved oxygen (DO) concentration is known to be one of the most important factors affecting nitrous oxide (N2O) production, which might weaken the advantages of nitrogen removal in completely autotrophic nitrogen removal biofilm process. In this work, a mathematical model is applied to study the N2O production in a biofilm reactor performing nitritation followed by anaerobic ammonium oxidation (Anammox) for nitrogen removal. The nitrifier denitrification pathway through utilization of nitrite as the terminal electron acceptor under oxygen limiting conditions is used to predict N2O production. Simulations explicitly show that a large number of N2O is produced under conditions of low DO concentration for high nitrogen removal. A low ammonium concentration (<50 mg N L-1) and a suitable DO level (at around 0.5 mg O-2 L-1) could lead to high total nitrogen (TN) removal with a low N2O production. Biofilm has to be controlled to be in the optimal thickness (1000 mu m under the simulating conditions of this study), which allows relatively high TN removal, avoiding higher thickness that favors N2O production. (C) 2015 Elsevier B.V. All rights reserved.