Systems analysis of urban wastewater systems – two systematic approaches to analyse a complex system L. Benedetti*, F. Blumensaat**, G. Bo ¨ nisch**, G. Dirckx***, N. Jardin****, P. Krebs** and P.A. Vanrolleghem* *BIOMATH, Ghent University, Coupure Links 653, B-9000 Gent, Belgium **Institute for Urban Water Management, Dresden University of Technology, D-01062 Dresden, Germany ***Aquafin nv, Technology Department, Dijkstraat 8, B-2630 Aartselaar, Belgium ****Ruhrverband Essen, Kronprinzenstraße 37, D-45128 Essen, Germany Abstract This work was aimed at performing an analysis of the integrated urban wastewater system (catchment area, sewer, WWTP, receiving water). It focused on analysing the substance fluxes going through the system to identify critical pathways of pollution, as well as assessing the effectiveness of energy consumption and operational/capital costs. Two different approaches were adopted in the study to analyse urban wastewater systems of diverse characteristics. In the first approach a wide ranged analysis of a system at river basin scale is applied. The Nete river basin in Belgium, a tributary of the Schelde, was analysed through the 29 sewer catchments constituting the basin. In the second approach a more detailed methodology was developed to separately analyse two urban wastewater systems situated within the Ruhr basin (Germany) on a river stretch scale. The paper mainly focuses on the description of the method applied. Only the most important results are presented. The main outcomes of these studies are: the identification of stressors on the receiving water bodies, an extensive benchmarking of wastewater systems, and the evidence of the scale dependency of results in such studies. Keywords Systems analysis; integrated urban wastewater management; substance flow analysis; WFD; benchmarking; scaling problem Introduction The aim of this work was to perform a thorough and wide-focused systems analysis of the integrated urban wastewater system (catchment area, sewer, WWTP, river). The out- come of the study will ultimately serve as a basis for the development of a decision sup- port aid that gives assistance for the cost-effective development of urban wastewater systems for Water Framework Directive (WFD) compliance. The research work reported in this paper is carried out within the scope of the EU project CD4WC which is sup- ported by the European Commission under the 5th Framework Programme. Among the wide suite of tools available to perform a systems analysis (Finnveden and Moberg 2001; Balkema et al. 2002) substance flow analysis (SFA) combined with mass balances proved to be appropriate tools to highlight pressures on the environment, i.e. on the receiving water, and to pinpoint information gaps (Belevi 2002; Jeppsson and Hell- stro ¨m 2002). The main limitation of SFA lies in the uncertainty usually associated to the data used, which are also of different nature and origin (Danius and Burstro ¨m 2001). Using SFA as a tool for priority setting and follow-up is associated with considerable dif- ficulties. However, it is still a useful tool for screening in order to identify areas for further and more detailed investigation. Similar studies can be found in (Lampert and Brunner 1999; Larsen 1999; Lindqvist-O ¨ stblom et al. 2001). The evaluation of a list of indicators helped to characterise the behaviour of sewers and WWTPs in environmental and economic terms. A further objective was to recognize the information gaps in the system owed to the typical methods of data collection and Water Science & Technology Vol 52 No 12 pp 171–179 Q IWA Publishing 2005 171