Laboratory experiments were carried out to evaluate the feasibility of the biological removal of nitrogen from mixed textile (80%) and municipal (20%) wastewater. Three modified “Ludzack Ettinger” bench scale plants were used. Wastewater characterization and process kinetic constants were evaluated experimentally. The results of this experience demonstrated that long sludge age is necessary in order to overcome the nitrification inhibitory effect of some compound(s) that were present in the treated wastewater. Moreover it can be concluded that experimental approaches are necessary to make reliable design of a full scale industrial wastewater treatment plant.