Calibrating a side-stream membrane bioreactor using Activated Sludge Model No. 1 T. Jiang* , ** , X. Liu** , M.D. Kennedy** , J.C. Schippers** and P.A. Vanrolleghem* *BIOMATH, Ghent University, Coupure Links 653, B-9000 Gent, Belgium (E-mail: tao.jiang@biomath.ugent.be; Peter.Vanrolleghem@ugent.be) **UNESCO-IHE Institute for Water Education, PO Box 3015, 2601 DA Delft, The Netherlands (E-mail: liuxa@zhsyj.org.cn; m.kennedy@unesco-ihe.org; j.schippers@unesco-ihe.org) Abstract Membrane bioreactors (MBRs) are attracting global interest but the mathematical modeling of the biological performance of MBRs remains very limited. This study focuses on the modeling of a side-stream MBR system using the Activated Sludge Model No. 1 (ASM1), and compares the results with the modeling of traditional activated sludge processes. ASM1 parameters relevant for the long-term biological behaviour in MBR systems were calibrated (i.e. Y H ¼ 0.72 gCOD/gCOD, Y A ¼ 0.25 gCOD/gN, b H ¼ 0.25 d 21 , b A ¼ 0.080 d 21 and f P ¼ 0.06), and generally agreed with the parameters in traditional activated sludge processes, with the exception that a higher autotrophic biomass decay rate was observed in the MBR. Influent wastewater characterization was proven to be a critical step in model calibration, and special care should be taken in characterizing the inert particulate COD (X I ) concentration in the MBR influent. It appeared that the chemical – biological method was superior to the physical – chemical method. A sensitivity analysis for steady-state operation and DO dynamics suggested that the biological performance of the MBR system (the sludge concentration, effluent quality and the DO dynamics) are very sensitive to the parameters (i.e. Y H ,Y A ,b H ,b A m maxH and m maxA ), and influent wastewater components (X I ,S s ,X s and S NH ). Keywords Membrane bioreactor; modelling; parameter estimation; sensitivity analysis Introduction Membrane bioreactor (MBR) systems are one of the most promising biological waste- water treatment techniques. Many studies have been performed on the modeling of MBR fouling problems, but the modeling of the biological performance of MBRs is still lim- ited. The biological performance of a MBR and its description by the Activated Sludge model No.1 (ASM1) model might lead to characteristics that deviate significantly from the traditional activated sludge characteristics, due to the fact that: 1) membranes (micro- filtration or ultrafiltration (UF)) serve as a barrier that completely retains biomass, reducing the wash-out of non-flocculating biomass (and thus reduces the selection of biomass species); 2) the biomass is imposed to high shear rate conditions in MBRs (especially in the side-stream configuration). In this study, a lab-scale, side-stream MBR system is modelled using the ASM1, and the attention is focused on the comparison of the model characteristics of the MBR system with traditional activated sludge processes. Firstly, a steady-state MBR calibration was performed and the most sensitive parameters responsible for long-term behaviours were calibrated, i.e. the decay coefficients (b H and b A ), the yield coefficients (Y H and Y A ) and the inert particulate fraction of biomass (f P )(Nowak et al., 1999; Henze et al., 2000; Vanrolleghem et al., 2003). Meanwhile, the influent wastewater was completely character- ized using two methods (physical – chemical and chemical – biological) for comparison purposes. A sensitivity analysis of all ASM1 parameters was performed afterwards to con- firm that the calibrated parameters (b H ,b A ,Y H ,Y A and f P ) are indeed the most sensitive parameters for steady-state behaviours. And finally, simulations of the DO concentration Water Science & Technology Vol 52 No 10–11 pp 359–367 Q IWA Publishing 2005 359