Abstract In urban waste water treatment plants one of the most developed processes to treat pollutants is the activated sludge basin. The aim of this paper is to present results on the hydraulics and aeration performance of an aerated basin at pilot plant scale. Local gas retention, gas velocity and bubble size have been measured and linked to the classical global measurements of oxygen transfer coefficient and of horizontal liquid velocity. Different operating conditions have been tested to show the impact of each parameter on hydraulics and aeration performance. The increase of air flow rate induces an increase of the local gas retention, of the gas velocity as well as of the Sauter diameter. Changes in these local parameters do exhibit a strong impact on the oxygen transfer coefficient. Hence these local measurements are used to discuss and gain a deeper understanding of the oxygen transfer phenomena. An augmentation of the horizontal liquid velocity results in higher values for the global oxygen transfer coefficient. This is confirmed by observation and local measurements, which show a stronger inclination of the air plume with increasing velocity. This translates to a larger course for the bubbles to cover before reaching the surface. With respect to global oxygen transfer measurements, the optimal position of the agitator was found to be near the bottom of the reactor for the type of pilot plant studied. In addition it can be stated that the horizontal position of the impeller has a large impact on the liquid horizontal velocity. Improved understanding is gained through local investigations, which reveal the increased inclination of the air plume as one major cause.