Bench scale experiments were conducted in order to finalize the design of the secondary treatment stage of Psyttalia Sewage Treatment Works that serves the Greater Athens area with a population equivalent of 3.5 million. Carbon and nitrogen removal efficiencies and activated sludge settling characteristics were determined through continuous flow experiments and determination of the maximum specific utilization rates of ammonia (AUR) and nitrate (NUR). According to the results of the pilot experiments the need arose for a significant increase in the initial design's anoxic volume to ensure a 70% average annual nitrogen removal. AUR experiments showed an insignificant inhibition to the nitrification process in the order of 0-30%, representative of typical domestic sewage. The findings of the pilot study were in good agreement with full scale data obtained during the start up of Psyttalia biological treatment plant. According to full scale data and AUR tests a very satisfactory nitrogen removal efficiency was obtained during start up. Finally both lab scale and full scale data indicated that the activated sludge plant operating on Psyttalia primary effluent that has a high oil and grease content, is prompt to experience foaming problems due to the excessive growth of filamentous microorganisms M. parvicella and Gordona amarae - like microorganisms.