A simple experimental device has been assembled with the aim of increasing students’ understanding of multiphase reactors and to highlight the importance of economic considerations in real chemical engineering problems. The multiphase reactor studied is an activated sludge reactor. In this type of reactor, organic matter and nitrogen substrates, which are contained in wastewater, are oxidized by solid bacterial groups that employ oxygen. Gas dispersion is a factor of critical importance in the operating performance and cost of every multiphase reactor. In the work covered here, students determine the influence of this factor on the assembly while considering that energy consumption (economic cost) depends on both the type of aerator used and the operational conditions for a given aerator. A theoretical model is proposed that allows the interpretation of the results based on the assumptions that gas absorption and biochemical reactions are the limiting steps. Experimental data are fitted to this model, and the parameters obtained allow comparison of the behavior of each type of aerator and also provide an understanding of the process.