H Rasouli Kenari - School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box ۱۱۱۵۵-۴۵۶۳, Tehran, Iran,
M. H Sarrafzadeh - School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box ۱۱۱۵۵-۴۵۶۳, Tehran, Iran,
O Tavakoli - School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box ۱۱۱۵۵-۴۵۶۳, Tehran, Iran,
M Nikbakht - Tehran Oil Refinery Company, P.O. Box ۱۸۷۳۵- ۴۱۷۳, Tehran, Iran

Nitrogen removal is currently an important constraint in treatment of any wastewater. Discharge of wastewater containing nitrogen can be toxic to aquatic life and human beings, causes oxygen depletion and eutrophication in receiving water, and affect chlorine disinfection efficiency. There are various methods of removing nitrogen, each with advantages and disadvantages. However, the biological treatment method is used most commonly. This investigation was performed on the biological removal of nitrogen from refinery's wastewater by the nitrification and denitrification process. With this method, organic nitrogen and inorganic nitrogen is converted into nitrate in an aerobic environment, and then it is converted in an anaerobic process to nitrogen gas and dispersed into the atmosphere. In a petroleum refinery, removing of hydrocarbons is the main concern and nitrogen content is supposed to be negligible. The aim of this work was to search for nitrogen in Tehran Refinery wastewater and employing a biological technology to reduce this pollutant. Samplings were done in different points of wastewater treatment plant; influent to aeration unit, effluent of aeration unit and effluent of clarifiers. The results showed that despite a high average removal efficiency of COD of above 93%, the nitrogen removal during conventional activated sludge process was not efficient and sludge rising due to denitrification was observed within clarifier. The analysis conducted in laboratory scale showed that a simultaneous nitrification and denitrification process could easily be realized in the same activated sludge plant by using the flocculating sludge and control of dissolved oxygen concentration.

Dissolved Oxygen, Industrial Wastewater, MLSS, Nitrogen Removal, SND