A New Interpretation of ASM2d for Modeling of SBR Performance for Enhanced Biological Phosphorus Removal Under Different P/HAc Ratios Nevin Yagci, 1 Guclu Insel, 1 Ruya Tasli, 1 Nazik Artan, 1 Clifford W. Randall, 2 Derin Orhon 1 1 Istanbul Technical University, Environmental Engineering Department, Istanbul, 34469, Turkey; telephone: þ90 212 285 65 40; fax: þ90 212 285 65 87; e-mail: yagcin@itu.edu.tr 2 Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia Received 11 January 2005; accepted 25 July 2005 Published online 31 October 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/bit.20701 Abstract: This study evaluated the prediction capability of Activated Sludge Model No. 2d (ASM2d), for the enhanced biological phosphorus removal (EBPR) performance of a sequencing batch reactor (SBR) receiving variable influent phosphate load. For this purpose, a laboratory-scale SBR was operated with a synthetic feed containing acetate as the sole carbon source. The experiments were conducted in four different Runs to ensure a range of different phosphate/acetate ratios in the influent. Model evaluations were carried out using concentration profiles measured throughout a representative cycle at steady state. An iterative calibration methodology was developed based on sensitivity analysis and applied to four different sets of experimental data on relevant model parameters reflecting SBR performance. ASM2d was able to predict the steady state behavior of the SBR system receiving variable influent phosphate loads only with the recalibrated para- meter set. The regular changing pattern of the coefficients could be interpreted with the ability of the SBR system to sustain glycogen accumulating microorganisms, GAOs, which can store substrate under anaerobic conditions without polyphosphate energy, but deriving energy from the degradation of glycogen. Thus they are capable of prevailing at lower P/Ac ratios. The results indicate the need to include glycogen and GAOs as model components for processes involving both phosphate accumulating organisms, (PAOs) and GAOs, in order to obtain a better prediction of X PHA and oxygen uptake rate (OUR) profiles in the system. ß 2005 Wiley Periodicals, Inc. Keywords: activated sludge; Activated Sludge Model No. 2d; enhanced biological phosphate removal; micro- bial storage; phosphate accumulating organism; glyco- gen accumulating organisms; sequencing batch reactor; model calibration INTRODUCTION Enhanced biological phosphorus removal, (EBPR), is one of the most intriguing metabolic activities of microbial cultures. It is commonly associated with a specific group of bacteria with the exclusive ability of converting short chain fatty acids, such as acetate, into internal storage products with ortho- phosphate release, (P i ), under anaerobic conditions. These bacteria require a subsequent aerobic stage where they grow utilizing the stored organics, and take up excessive amounts of orthophosphate as compared to conventional heterotrophic growth for energy storage. Although widely used as a biolo- gical treatment method for effective phosphorus removal from wastewaters, many aspects of the complex microbial mechan- ism in the process sequence still require better understanding and interpretation. Despite extensive research on the subject, there are still conflicting arguments concerning the basic stoichiometry and kinetics of the different processes involved (Filipe et al., 2001a; Murnleitner et al., 1997; Smolders et al., 1994; Wentzel et al., 1990, 1991). Modeling is now an integral part of biological wastewater treatment not only as a supporting tool for system design and performance prediction, but also as a valuable asset for a more accurate evaluation of the complex microbial mechanisms, which eventually leads to the development of more realistic models. Activated sludge as a wastewater treatment system has been the platform of a remarkable success on process modeling in the last two decades, first for organic carbon and nitrogen removal (Dold et al., 1980; Henze et al., 1987) and then for EBPR (Barker and Dold, 1997; Wentzel et al., 1985, 1991). The accumulated scientific information has been the basis of Activated Sludge Model No. 2, (ASM2) including the additional model components and processes defining EBPR for domestic sewage (Henze et al., 1995). Later, ASM2 was modified as Activated Sludge Model No. 2d (ASM2d) to include denitrification by phosphate accumulating organisms, (PAOs) with reduced P-uptake (Henze et al., 1999). In these models, (ASM2 and ASM2d), poly-hydroxyalkanoates (PHA) areinvolvedasthesinglecarbonaceouspolymerwhichisstored only by PAOs, utilizing acetate as the external fermentation product. ß 2005 Wiley Periodicals, Inc. Correspondence to: Nevin Yagci Contract grant sponsor: Foundation of TINCEL