The objective is the biological treatment area of the Wastewater treatment plant (unit 83), of the Elefsis Refinery Upgrade Project (ERUP), locating in Elefsina 27km western of Athens, owner of Hellenic Petroleum (ELPE), which is the greatest greek petroleum company. The Wastewater treatment plant consists of a square-plan tank divided at the inside at the following sub-sections: sselector basin, pre-denitrification basin, nitrification basin and defasification basin. The selector basin & degasification basin sections are further sub-divided. The Wastewater treatment plant is located at the northern side of unit 83, while it is connected with the clarifier unit. The clarifier unit consists of a concrete circular-plan tank with 10m diameter, while it is connected with three smaller rectangular-plan tanks (i.e. clarified water & sludge tanks). The shear walls of the circular-plan tank are of 3.92m free height.