This study develops a coupling of energetic and experimental design approaches on a given configuration of wet air oxidation process (WAO), applied for wastewater containing a hard chemical oxygen demand (phenol for instance). Taking into account thermodynamic principles and process simulation, the calculation of minimum heat required by the process, exergetic efficiency and work balance is presented. Five parameters are considered: pressure (20–30 MPa); temperature (200–300 °C); chemical oxygen demand (23–143 g l−1); air ratio (1.2–2) and temperature of exiting steam utilities (160–200 °C). Using the surface response method, it appears that initial chemical oxygen demand and temperature are the two parameters that mainly influence the result. With the modelling, good conditions for the functioning of the presented process are the following: pressure of 19.4 MPa, temperature of 283 °C, chemical oxygen demand of 54.9 g l−1, air ratio of 1.7 and vapour temperature of 183 °C.