Combined sewer overflows

SUDPLAN Newsletter 1 (December 2010): Climate changes will have crucial impacts to the aquatic environment.The Linz pilot focuses on the problem of combined sewer overflows (CSO) into receiving waters.

Mainly because of historical aspects most European cities are operating combined sewer systems which mean that waste water and storm water is drained in one sewerage system.

Due to the hydraulic limitation of waste water treatment plants (WWTP) it is not possible to treat the whole amount of the drained water at WWTPs during heavy rainfalls. Therefore, the storm water runoff in combined sewer systems has to be either spilled out at combined sewer overflows (CSOs) into receiving waters or stored temporarily in reservoirs (CSO tanks). 

It can be assumed climate changes will have crucial impacts to the aquatic environment. The Linz pilot focuses on the problem of combined sewer overflows (CSO) into receiving waters and within this pilot two main tasks will be carried out:

  • (1) Estimation of an overall CSO efficiency rates (η) for dissolved pollutants (ηd) and for particulate pollutants (ηp) in the total catchment area of the waste water treatment plant (WWTP) of Linz based on long-term simulations and future rain data provided by the common services of the SUDPLAN project.
  • (2) Estimation of the sedimentation efficiency rate (ηsed) for the primary clarifiers of the WWTP in Linz by the installation and operation of a novel sensor network in the inflow and outflow of the clarifiers in order to quantify the TSS and COD retention efficiency under different rainwater flow regimes The sedimentation efficiency rate of these clarifiers is crucial for calculating the overall CSO efficiency rate for particulate pollutants (ηp) of the whole catchment area.

By comparing the results of today’s and of future scenarios possible effects and changes can be recognised and located and proper strategic adaptions can be developed within the catchment area in time.
The Linz pilot will explore the suitability and applicability of an innovative sensor, communication and information systems for the improved operation of the sewerage system under possible climate change conditions.

Preparatory Work and Current Status of Linz Pilot after Year 1

1. Screening of Available Data

  • Screening of available catchment and measurement data in the area of the Linz pilot.
  • Decision on historical rainfall data: The available time series covers 13 years (01.01.1993 to 01.01.2007) with a time resolution of 1, 5 or 10 minutes. A version for SOS providing was prepared.
  • Downscaling of rainfall data for future scenarios was discussed with the SUDPLAN Common Service team. The above mentioned historical rainfall timeseries was submitted to the Common Service team for the development of the regional downscaling procedure within the SUDPLAN application.

2. Simulation Model Decision

Concerning software selection, two models were available for the Linz catchment in two different software packages to be used for evaluation of the efficiency rates: a hydrological model in the software CityDrain (Achleitner et al., 2007) and a hydrodynamic model in the software SWMM 5 (Rossmann, 2007).

Both models were set up by University of Innsbruck and represent an aggregation of the real network structure.

Figure 1 shows the model representation of the Linz catchment in the CityDrain modelling software, with representations of the subcatchments, main sewer conduits, overflow structures, dividers and pumps.

Figure 1: Linz catchment representation in CityDrain. Enlarge Image

Figure 2 shows the model representation of the Linz catchment in the SWMM modelling software, including subcatchments, manholes, main sewer conduits, overflow weirs and storage tanks. Compared to the CityDrain representation it can be seen that the subcatchments are represented according to their actual geometry.

Linz sewer
Figure 2: Linz catchment representation in SWMM 5. Enlarge Image

Based on the comparison of the two models TU Graz and LINZ AG decided to use SWMM as:

  • Computation time is slightly shorter in SWMM (based on the simulation of a one-year time series).
  • Data for subcatchments, sewer conduits, manholes etc. are geo-referenced in SWMM. This allows a better integration to the planned visualisation activities within the SUDPLAN project.
  • Due to the program structure of SWMM (stand-alone program, open-source, can be compiled on any platform), integration in the SUDPLAN Scenario Management System by wrapping the model will be easier than with CityDrain based on Matlab. In addition Matlab is proprietary and a license would have to be acquired.
  • SWMM is already being used in other projects by TU GRAZ.
  • SWMM can be linked to the optimization framework BlueM.OPT (Bach et al., 2009) that can be used for automated model calibration.

Model software SWMM is ready to be implemented in the SUDPLAN application.

3. Preparatory Work for the Installation of a Sensor Network at WWTP Linz

For the estimation of the sedimentation efficiency rate (ηsed) for the primary clarifiers of the waste water treatment plant (WWTP) in Linz a concept of a novel sensor network was developed and preparatory work for the installation of the sensor network in phase 2 of SUDPLAN has been started.


Achleitner, S., Moderl, M. and Rauch, W. (2007): CITY DRAIN (c) - An open source approach for simulation of integrated urban drainage systems. Environmental Modelling & Software 22(8), 1184-1195.

Bach, M., Froehlich, F., Heusch, S., Hübner, C., Muschalla, D. and Reußner, F.O., M. (2009): BlueM – a free software package for integrated river basin management. Proceedings: Annual meeting of the German hydrological society, Kiel, Germany, 109 - 116.

Rossmann, L.A. (2007): Storm Water Management Model User's Manual, Version 5.0, p. 265, EPA - United States Environmental Protection Agency, Cincinnati, Ohio (USA).