Effect of sludge age on simultaneous nitrification and denitrification in membrane bioreactor S. Murat Hocaoglu a , G. Insel b , E. Ubay Cokgor b , D. Orhon b,c,⇑ a TUBITAK Marmara Research Center, Environment Institute, 41470 Gebze, Kocaeli, Turkey b Environmental Engineering Department, Istanbul Technical University, ITU, Insaat Fakultesi, 34469 Maslak, Istanbul, Turkey c Turkish Academy of Sciences, Piyade Sokak No. 27, 06550 Çankaya, Ankara, Turkey article info Article history: Received 22 January 2011 Received in revised form 28 March 2011 Accepted 29 March 2011 Available online 2 April 2011 Keywords: Membrane bioreactor Simultaneous nitrification/denitrification (SNdN) Modelling Sludge age Black water abstract This study evaluated the effect of sludge age on simultaneous nitrification and denitrification in a mem- brane bioreactor treating black water. A membrane bioreactor with no separate anoxic volume was oper- ated at a sludge age of 20 days under low dissolved oxygen concentration of 0.1–0.2 mg/L. Its performance was compared with the period when the sludge age was adjusted to 60 days. Floc size dis- tribution, apparent viscosity, and nitrogen removal differed significantly, together with different biomass concentrations: nitrification was reduced to 40% while denitrification was almost complete. Modelling indicated that both nitrification and denitrification kinetics varied as a function of the sludge age. Cali- brated values of half saturation coefficients were reduced when the sludge age was lowered to 20 days. Model simulation confirmed the validity of variable process kinetics for nitrogen removal, specifically set by the selected sludge age. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Effective nitrogen removal by means of simultaneous nitrifica- tion and denitrification (SNdN) is one of the major specific assets of membrane bioreactor (MBR). The SNdN is widely observed in MBR systems operated under different conditions (Choo and Stensel, 2000; He et al., 2009; Sarioglu et al., 2009a,b, 2010; Hocaoglu et al., 2011). Nitrogen removal relies upon anoxic pro- cesses where the oxidized nitrogen serves as the electron acceptor for the utilization of the organic carbon. In conventional systems, it requires a separated anoxic volume/phase. Consequently, it utilizes only a fraction of the denitrification potential, N DP , i.e. the nitrogen equivalent of the biodegradable organic carbon potential and its efficiency is limited with the amount of available oxidized nitro- gen, N D introduced in the anoxic volume (Orhon and Artan, 1994; Artan et al., 2004). In this context, SNdN for nitrogen re- moval offers a competitive advantage to MBR as compared with conventional activated sludge (CAS) systems for a number of rea- sons: (i) all biodegradable COD of the influent becomes available as N DP , as the denitrification process is extended over the entire reactor volume. (ii) Operating conditions of the MBR can be ad- justed to secure nitrogen removal for domestic sewage, without the use of a separate anoxic volume and internal oxidized nitrogen recirculation as in CAS systems (Insel, 2007; Sarioglu et al., 2009b). (iii) Oxygen consumption may be greatly reduced because nitrate can be utilized to the fullest extent as the alternate electron accep- tor in both microbial growth and decay processes. Nitrite is an intermediary compound in both nitrification and denitrification, as denitrification through nitrite would require less organic carbon since oxygen equivalence of nitrite is 60% smaller than oxygen equivalence of nitrate. SNdN relies on the ability of system operation to keep the bio- mass partly aerobic and partly anoxic so that both nitrification and denitrification processes can be sustained on the microbial flocs (Daigger and Littleton, 2000; Munch et al., 1996; Daigger et al., 2007). While this seems conceptually possible, it requires full understanding and adjustment of two major parameters: (i) appro- priate set-point control of the dissolved oxygen at sufficiently low levels, i.e. 0.1–1.0 mg/L that would allow the outer portions of the flocs to nitrify while the inner portions sustain denitrification un- der anoxic conditions. Lowering the DO levels to this range does not appear to be a sustainable alternative for conventional acti- vated sludge as it favours filamentous growth and deteriorates sys- tem stability (Martins et al., 2003; Hashemi et al., 2005). Therefore, SNdN remains almost uniquely associated with the MBR among suspended growth activated sludge systems. In should be noted that biofilm processes also have a similar potential of performing SNdN for the very same reason: mass transfer limitations through the biofilm (Terada et al., 2003; Chung et al., 2007). (ii) The sludge 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.03.096 ⇑ Corresponding author at: Turkish Academy of Sciences, Piyade Sokak No. 27, 06550 Çankaya, Ankara, Turkey. Tel./fax: +90 212 285 3793. E-mail addresses: selda.murat@mam.gov.tr (S.M. Hocaoglu), inselhay@itu.edu.tr (G. Insel), ubay@itu.edu.tr (E. Ubay Cokgor), orhon@itu.edu.tr (D. Orhon). Bioresource Technology 102 (2011) 6665–6672 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech