Sanitary landfilling is one of the most common ways to eliminate solid municipal/urban wastes. Despite many advantages, this method leads to the generation of contaminated leachates that remains an unavoidable consequence of the waste disposal. Membrane technologies, such as reverse osmosis, are frequently used for leachate treatment as they generate good quality permeate with a high recovery rate. However, their primary drawbacks are fouling, eliminated by chemical wash, and the production of highly polluted concentrates. This paper aims to assess and optimize the use of wet air oxidation to treat reverse osmosis concentrates in terms of bio-refractory organic pollutants removal. Wet air oxidation was performed at elevated pressure and temperature using experimental design methodology with a 70% oxygen excess for 6 h in a stirred batch reactor. The effect of operating conditions was investigated with an experimental design where 3 factors (two quantitative ones: pressure and temperature and a qualitative one: seasonality of the effluent) have been considered. The chemical oxygen demand and the total organic carbon removals increased with the increase of temperature (from 200 °C to 300 °C) and no effect of the pressure was observed within the range 18-21 MPa. Wet air oxidation could achieve up to 99% removals for chemical oxygen demand and total organic carbon for the seasonality of October. Experiments also showed that increasing the initial pollutant concentration increased initial kinetic rates. Finally, models were established to calculate and predict pollution removal rate and its kinetic, in the domain of study.