In biological wastewater treatment, the aeration process disperses air into the liquid phase in the form of air bubbles. It provides an anaerobic environment for microbial degradation of organic matter. However, aeration is also the most energy-consuming and cost-generating step which corresponds to 50 - 80 % of the total wastewater treatment plant (WWTP) energy budget and to about 1 % of total electricity consumption in developed countries. One way to achieve aeration energy-optimization is the implementation of intermittent gas injection. Therefore, the main objective of this thesis was to investigate the effect of pulsed aeration on the oxygen mass transfer. Experiments were performed with the help of an optical flow microscope in order to study bubble size distribution (BSD) dynamics, bubble size, and bubble rise velocity while the measurements of oxygen mass transfer coefficient obtained with the use of dissolved oxygen sensors. The overall volumetric oxygen transfer coefficient was calculated according to Higbie penetration theory. Local and global oxygen transfer coefficient were found and compared to values measured by a traditional gassing-out method. Based on the experimental BSD the population balance model (PBM) was formulated, calibrated, and then solved in MATLAB software. It was shown that the PBM can predict the BSD dynamics along the height of the column operated under different operating conditions. The Higbie oxygen mass transfer theory considers the single value of the Sauter mean diameter for the entire column. In this work, the modeling approach that takes into account the local BSD was implemented in order to calculate the oxygen transfer coefficient.