EVALUATION OF NUTRIENT REMOVAL PERFORMANCE FOR AN ORBAL PLANT USING THE ASM2d MODEL Güçlü Insel 1,2* , Dave Russell 3 , Bruce Beck 4 and Peter A.Vanrolleghem 1 1 BIOMATH, Department of Applied Mathematics, Biometrics and Process Control Ghent University, Coupure Links 653, B-9000, Gent, Belgium; guclu@biomath.ugent.be 2 Istanbul Technical University, Environmental Engineering Department, ITU Insaat Fakültesi, Ayazaga Kampusu, 80626, Maslak, Istanbul, Turkey. 3 Global Environmental Operations, Inc., 4642 Warrior Trail, SW, Lilburn, GA, 30047, USA. 4 Warnell School of Forest Resources 4-104, Athens, University of Georgia, Georgia, USA. ABSTRACT An Orbal plant achieving biological nutrient removal was modeled using nitrate, oxygen, ammonium- nitrogen and phosphate measurements for calibration. It was found that the oxygen and nitrate concentration in outer and middle ring is the bottleneck for modeling enhanced biological phosphorus removal (EBPR) whilst the system is under diurnal variation in the influent load. The simulation results showed that the overall system performances for total Kjeldahl-nitrogen and phosphate removal are around 77% and 94%, respectively. Also, the system is highly influenced by the operational changes rather than model kinetics. On the basis of this modeling exercise by using the ASM2d model, the necessary information for a trustable calibrated model that can be applied to upgrade the plant is discussed. KEYWORDS Modeling, Orbal, calibration, simultaneous nitrification-denitrification, EBPR INTRODUCTION In recent years, nutrient removal gained more importance in order to prevent eutrophication of receiving waters. As a solution, different plant configurations can be used for the design of wastewater treatment plants to achieve nutrient removal from domestic wastewaters. From an engineering point of view, the reliability of the treatment plant is important to meet nutrient discharge limits at all times. Extended aeration systems are associated with plants for small communities where reliability and simplicity of the operation are of prime importance (Grady et al., 1999; Orhon and Artan, 1994; Randall et al., 1992). The applicability of the system is good, because less sludge is to be disposed of, no further treatment is needed for the sludge, it maintains good effluent quality and allows flexibility and simplicity for the plant operators. However, an increase in the total volume (etc. for an upgrade) by addition of aerobic or anoxic reactors increases the investment and operational/maintenance costs. An Orbal plant is a type of extended aeration activated sludge plants which claims to achieve simultaneous nitrification and denitrification in a single reactor, offering reduced costs. Generally, three or four channels are recommended for design as shown in Figure 1-left (Drews et al., 1972; 1973). The simultaneous nitrification and denitrification in the outer loop that receives the influent wastewater give WEFTEC ® 2003 Copyright ©2003 Water Environment Federation. All Rights Reserved.