Preparatory work concerning model decision and available measurement data was mostly done within the first year. In the second year, additional preparatory work was necessary for preparing the local SWMM model and for the installation of the sensor network at Linz WWTP.
SWMM Model Component
Model set up
As local model an existing SWMM5 model of the Linz catchment was chosen as it was best suited for the SUDPLAN needs. The model was set up by University of Innsbruck and represents an aggregation of the real network structure. In order to provide a sound model basis for the SUDPLAN project, the available model was intensively analysed and evaluated in the work of Wendner (2011):
The model structure was checked on errors and the available measurement data was evaluated. Influential model parameters were determined and ranked by a Morris screening (Morris, 1991), a state-of-the-art global sensitivity analysis (GSA) method. Screening runs were performed to identify parameter sensitivities with respect to CSO efficiency according to the Austrian ÖWAV Regelblatt 19 (RB19) guideline (OEWAV, 2007). Based on the results from the GSA the model was calibrated to the available measurement data by coupling with an optimiser based on evolutionary strategies. Model calibration yielded different quality for different selected events. Overall, however, satisfying results were obtained as basis for the SUPLAN project (Gamerith et al., accepted-c). An exemplary calibration result for one optimisation run is shown in Figure 1.
Evaluation of Results According to Austrian RB19 Guideline
A script was developed in the software R (R Development Core Team, 2011) that allows calculating the required and actual efficiency rates according to the Austrian RB19 guideline based on the model run configuration. First the rainfall characteristic r720,1 for determining the required efficiency rates is calculated based on the German ATV A121 (ATV, 1985). The actual efficiency rates are then calculated based on the SWMM simulation results and given sedimentation efficiencies for the CSOs and storage tanks in the sewer system.
First Model Results for Climate Change Impacts
Based on the available historical rainfall time series and a downscaled predicted rainfall time series a first comparison and evaluation of possible climate change impacts for the Linz Pilot was carried out. The system’s performance was assessed by long term simulations with the two 14-year rainfall time series with the SWMM model. As shown in Figure 2 the predicted rainfall (climate model projection ECHAM5_A1B_3) shows a decrease of rainfall intensities in the summer period and an increase in winter. Strong precipitations intensities generally increase.
In a first assessment, 12 of 44 modelled CSO structures were identified as influential. The primary clarifiers at the WWTP proved to be the most important structure contributing with about 57% to the total overflow volume (Figure 2, right side). As shown in Figure 3 overflow volume increases at all relevant CSOs. The total overflow volume is increased by approximately 17%.
The requirements as defined in the Austrian RB 19 guideline, however, are met for both dissolved and particulate pollutants for the historical and the predicted time series (see Table 1). While the system efficiency decreases with the predicted time series so do the requirements with increased rainfall (based on the calculated r720,1).
For a more comprehensive assessment of the Linz pilot study in Phase 3, additional climate model projections will be applied to estimate an uncertainty range of predicted rainfall and different development scenarios. In addition, data from the installed measurement network at Linz WWTP will allow assessing the actual sedimentation efficiency in the primary clarifiers during storm water conditions.
Sensor Network at Linz wastewater treatment plant (WWTP)
The first model runs identified the primary clarifiers at Linz WWTP (Figure 4) that also function as CSO tanks during rainfall events to have a major influence on the efficiency rates as defined in the RB19 guideline. Especially the assumption of the sedimentation efficiency has an important impact on the overall efficiency for particulate pollutants. Therefore, an assessment of the actual sedimentation efficiency is crucial in order to evaluate the whole systems performance.
In order to address this question a sensor network was installed in the inflow and outflow channels of the primary clarifiers of the WWTP Linz, measuring hydraulic and water quality variables. An overview of the local situation at the WWTP Linz is given in Figure 5 and Figure 6.
Sensor network installation
For the estimation of the sedimentation efficiency of the primary clarifiers of WWTP Linz two measurement stations in the inflow and in the outflow channel of the primary clarifiers were installed and put into operation at the end of November 2011.
Both measurement stations are equipped with identical sensors: an UV/VIS spectrometer to estimate the pollution concentrations mainly for TSSeq and CODeq, an ultrasonic probe to measure the water levels in the channels and an industrial PC which records the data in 1 minute intervals.
Figure 7 shows the inflow installation site with the installed inflow sensors at the end of the inflow channel to the primary clarifiers and Figure 8 the enclosed outflow area with the installed outflow devices where the overflow weir of the CSO is also installed. Since the turbulences in the outflow channel especially during wet weather conditions are quite high it can be assumed that the wastewater quality of the spilled out overflow discharge is more or less the same than in the outflow channel.
While the UV/VIS probe in the inflow channel is fixed rigidly at the end of a metal bar some centimetres above the bottom of the channel (Figure 9) the outflow probe is fixed in a floating device (Figure 8).
The two industrial PCs are connected to an UMTS router which transfers the recorded data regularly to an Open Scientific Data Management System (OpenSDM) at TU Graz from where they are provided for the SUDPLAN SMS.
The additional data time series regularly measured and provided by LINZ AG which are relevant for the estimation of the sedimentation efficiency are submitted to TU Graz once a day via e-mail also with 1 minute intervals from where they are also imported to the OpenSDM and provided for the SUDPLAN SMS automatically.
Within the SUDPLAN project all by the sensor network measured or by LINZ AG provided variables (hydro- and pollutographs) shall be visualised in the SMS. In addition an event-detection algorithm will be implemented that allows automatically detecting overflow events from the measured data. Based on these events, the sedimentation efficiency of the CSO structure during storm events can be estimated.
ATV (1985) Arbeitsblatt ATV-A 121 Niederschlag - Starkregenauswertung nach Wiederkehrzeit und Dauer, Niederschlagsmessungen, Auswertung, GFA - Gesellschaft zur Förderung der Abwassertechnik e. V., Hennef, Germany.
Gamerith, V., Olsson, J., Camhy, D., Hochedlinger, M., Kutschera, P., Schlobinski, S. and Gruber, G. (accepted-a) Assessment of Combined Sewer Overflows under Climate Change - Urban Drainage Pilot Study Linz, IWA World Congress on Water, Climate and Energy, Dublin, Ireland.
Gamerith, V., Olsson, J., Hochedlinger, M., Camhy, D. and Gruber, G. (accepted-b) Abschätzung von zukünftigen Entlastungsfrachten nach dem ÖWAV Regelblatt 19 auf Basis von Klimamodellprognosen am Beispiel der Stadt Linz. Wiener Mitteilungen - Wasser, Abwasser, Gewässer Kanalmanagement 2012.
Gamerith, V., Wendner, M., Hochedlinger, M., Moederl, M. and Gruber, G. (accepted-c) Global Sensitivity Analysis and Multi-Objective Optimisation for Estimation of Combined Sewer Overflows – Case Study Linz, 9th International Conference on Urban Drainage Modelling (UDM), Belgrade, Serbia.
Morris, M.D. (1991) Factorial Sampling Plans for Preliminary Computational Experiments. Technometrics 33(2), 161-174.
OEWAV (2007). ÖWAV - Regelblatt 19 - Richtlinien für die Bemessung von Mischwasserentlastungen. Österreichischer Wasser- und Abfallwirtschaftsverband. Vienna, Austria.
R Development Core Team (2011) R: A language and environment for statistical computing, R Foundation for Statistical Computing, Vienna, Austria.
Wendner, M. (2011) Überprüfung, Anwendung und automatisierte Kalibrierung eines Kanalnetzmodells der Stadt Linz zur Bewertung nach ÖWAV Regelblatt 19 – Projekt SUDPLAN Master thesis, Graz University of Technology, Graz, Austria.