Respirometry is a technique based on measuring and interpreting the speed with which microorganisms involved in a biological process of degradation of an organic substrate or nitrogen elimination consume oxygen under well-defined experimental conditions. Its importance lies in the fact that oxygen consumption is directly proportional to both biomass growth and substrate degradation. It is a necessary tool for the control of the purification process and / or for the development of mathematical models that describe the process. It is very versatile and flexible and allows a multitude of different tests based on the measurement of dissolved oxygen to be carried out, allowing to know, analyze and diagnose what can happen to the active sludge inside the biological reactor during the actual purification process. Likewise, it allows the waste water to be analyzed and divided into its main components.