Influence of selecting secondary settling tank sub-models on the calibration of WWTP models – A global sensitivity analysis using BSM2 Elham Ramin a,⇑ , Xavier Flores-Alsina b,c , Gürkan Sin b , Krist V. Gernaey b , Ulf Jeppsson c , Peter Steen Mikkelsen a , Benedek Gy. Plósz a,⇑ a Department of Environmental Engineering (DTU Environment), Technical University of Denmark, Miljøvej, Building 113, DK-2800 Kgs. Lyngby, Denmark b Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 227, DK-2800 Kgs. Lyngby, Denmark c Division of Industrial Electrical Engineering and Automation (IEA), Department of Measurement Technology and Industrial Electrical Engineering (MIE), Lund University, Box 118, SE-221 00 Lund, Sweden highlights  WWTP model outputs are very sensitive to settling parameters.  Sensitivity measures differ using 1st- or 2nd-order secondary settling sub-models.  Different calibration procedure proposed using either of settling sub-models.  2nd-order model: calibration focuses on measurable settling parameters.  1st-order model: high uncertainty by adjusting non-measurable settling parameter. article info Article history: Received 9 May 2013 Received in revised form 28 October 2013 Accepted 5 December 2013 Available online 12 December 2013 Keywords: Activated sludge models, ASMs Benchmark simulation model No. 2, BSM2 Good modelling practice Uncertainty analysis Morris screening Wastewater treatment plant, WWTP abstract This study investigates the sensitivity of wastewater treatment plant (WWTP) model performance to the selection of one-dimensional secondary settling tanks (1-D SST) models with first-order and second- order mathematical structures. We performed a global sensitivity analysis (GSA) on the benchmark sim- ulation model No. 2 with the input uncertainty associated to the biokinetic parameters in the activated sludge model No. 1 (ASM1), a fractionation parameter in the primary clarifier, and the settling parameters in the SST model. Based on the parameter sensitivity rankings obtained in this study, the settling param- eters were found to be as influential as the biokinetic parameters on the uncertainty of WWTP model predictions, particularly for biogas production and treated water quality. However, the sensitivity mea- sures were found to be dependent on the 1-D SST models selected. Accordingly, we suggest a different optimum parameter selection for the calibration of WWTP models when either of the 1-D SST models is used. Using first-order models, the calibration should give equal importance to the adjustment of the hindered settling and slow settling parameter values. The adjusted hindered settling parameters have, however, no physical meaning, and might additionally obtain unrealistic values. In contrast, using second-order SST models, the focus of calibration should be on providing measured values for the hindered settling parameters. This approach is in close agreement with the recommendations made in the good modelling practice protocol on activated sludge modelling. Besides, based on the GSA results, adjusting the non-measurable slow settling parameter produces significantly less variance in the model outputs using the second-order model than using the first-order model. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The simulation of a conventional activated sludge system in a wastewater treatment plant (WWTP) requires the mathematical models of bioreactors in combination with secondary settling tanks (SSTs) as solid–liquid separators. A good modelling practice (GMP) protocol [1] is available to guide wastewater engineers in the selection and calibration of the sub-models used for modelling of activated sludge systems. The sub-models describ- ing biokinetic processes are generally chosen from the activated sludge model (ASM) family [2]. With regard to SSTs, the GMP protocol recommends using simple point settlers and ideal 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.12.015 ⇑ Corresponding authors. Tel.: +45 45251600. E-mail addresses: elhr@env.dtu.dk (E. Ramin), xfal@kt.dtu.dk (X. Flores-Alsina), gsi@kt.dtu.dk (G. Sin), kvg@kt.dtu.dk (K.V. Gernaey), ulf.jeppsson@iea.lth.se (U. Jeppsson), psmi@env.dtu.dk (P.S. Mikkelsen), beep@env.dtu.dk (B.Gy. Plósz). Chemical Engineering Journal 241 (2014) 28–34 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej