Biological wastewater treatment occupies an important place in sanitary schemes and therefore in environmental protection. To master the efficiency of this type of process necessitates the comprehension of the treatability which consists of the study of oxygen transfer between different phases and degradation of the carbonaceous nitrogen and phosphate loads. Therefore, in the first part of the study the oxygen transfer was considered in a batch reactor configuration in order to find the global oxygen transfer coefficient (K(L)a) from the gaseous phase to the liquid phase and from the liquid phase to the solid phase. Starting with the determination of the K(L)a (gas-liquid) of clear water by the variation of the air flow and mixing velocity. The experimental results were modeled with correlation reported in literature. In fact, the air flow and mixing velocity are related to energy consumption. The determination of the exact amount of oxygen and mixing needed could be of great effect on energy savings in wastewater treatment plants.