In this study, a 1000 L pilot scale internal loop airlift bioreactor was operated and compared to a mathematical model to determine the best design for optimal supply of oxygen for nitrification and sufficient air for biomass fluidization. The design model is based on parameters such as geometry, carrier density, and airflow of the 1000 L pilot scale bioreactor. The model predicts a range of superficial air velocities (0.009-0.013 m/s) under which the airlift bioreactor was fluidized. Three superficial air velocities (0.009 m/s, 0.011 m/s and 0.013 m/s) were experimentally tested in the pilot plant and the obtained circulation velocities were compared with the predicted design scenarios. The predicted velocity was in agreement with the measured velocity. The aim of the mathematical model and the calculations of different geometry scenarios was to define the optimal geometry design for the physical model. The results show that the ratio of the cross-sectional area between the riser and the downcomer of 1.33 resulted in the lowest superficial liquid velocity of 0.076 m/s in the riser at a relative low superficial air velocity of 0.011 m/s and a carrier density of 1030 kg/m(3). This bioreactor design enabled longest retention time of particles in the oxygenated riser. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.