Impact of Dissolved Oxygen and Anoxic Decay on the Longterm OModeling of Simultaneous Nitrification and Denitrification (SNdN) in a Pilot Scale MBR M.Sarioglu 1* , G.nsel 2 , N.Artan 2 , D.Orhon 3 1 MWH UK Ltd. Dubai, P.O.Box 3020, United Arab Emirates 2 Istanbul Technical University, Environmental Engineering Department, Maslak, 34469, Istanbul, Turkey 3 Turkish Academy of Sciences, Piyade sokak No.27, 06550, Çankaya, Ankara, Turkey *To whom correspondence should be addressed. Email: murat.sarioglu@uk.mwhglobal.com ABSTRACT This study is mainly focused on evaluating the impact of dissolved oxygen and anoxic decay on simultaneous nitrification and denitrification (SNdN) in a pilot scale membrane bioreactor (MBR). Nitrate utilization rates for anoxic and MBR tanks were calculated from a previously calibrated model to quantify the effect of each tank on the total nitrogen removal capability of the system. The total average nitrate utilization rate (NUR t ) in the membrane tank was 2.3 mg/L-h (4.6 gN/kgVSS-d) throughout the study where 65% of this was associated with the anoxic decay process having the major contribution to SNdN. The denitrification potential (N DP ) of the MBR tank corresponded to 75% of the total N DP of the system at an average DO level of 1 mg/L and decreased with increasing DO. The comparative steady state modeling revealed that MBR systems have the potential to provide 60% more total nitrogen removal efficiency at a DO level of 2 mg/L when compared to conventional systems with the same operating parameters. It was found that in a MBR system the existence of the anoxic tank is questionable at low DO concentrations because of the low levels of nitrate being recirculated back to the this tank due to the high N DP in the membrane tank. KEYWORDS: membrane bioreactor, simultaneous nitrification/denitrification, denitrification potential, nitrate utilization rate, denitrification potential, diffusion limitation INTRODUCTION Nitrogen removal requires the inclusion of an unaerated volume fraction into the activated sludge process to sustain anoxic conditions that will favor the uptake of nitrate as the final electron acceptor during the growth of heterotrophic microorganisms. This is done either before the aeration tank which is called pre-denitrification or after which is defined as the post denitrification scheme. Pre-denitrification is mostly favored as it takes advantage of the available carbon in the raw or settled sewage for the conventional activated sludge systems (CASS). Membrane bioreactor (MBR) systems have the same type of process scheme for nitrogen removal however it differs with its ability to sustain higher biomass concentrations as the solids-liquid separation is independent from the biomass settling properties. The complete biomass retention enables the system to be operated at higher sludge ages and thus resulting in lower excess sludge production with respect to CASS (Günder, 2001). The conceptual basis of nitrogen removal has been fully investigated and understood for conventional activated sludge systems in previous researches (Ekama et al.,1983, Avcıolu et al., 1998; Sözen et al., 2002; Artan et al., 2006). However studies on the nitrogen removal performance of MBR systems did not cover the fundamental aspects of process kinetics and modeling especially taking into account the higher biomass concentrations sustained in the WEFTEC 2009 Copyright ©2009 Water Environment Federation. All Rights Reserved. 2711