The new storage sewer in Graz Werner SPRUNG. Kanalbauamt Graz, Europaplatz 20, 8020 Graz, Austria

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1 The new storage sewer in Graz Werner SPRUNG Kanalbauamt Graz, Europaplatz 20, 8020 Graz, Austria Keywords Storage sewer, Graz, Mike Urban, RTC, user written control Abstract Graz (Austria) has inhabitants and a sewage system with a total approximate length of 840 km. The river Mur divides the city into two parts and is a receiving water body. The wastewater treatment plant is situated in the south of Graz. A new storage sewer for Graz is in progress to reduce the pollution of the river Mur caused by combined sewer overflows. The concept was designed using hydrologic and hydrodynamic models. The new storage sewer has redundancy to the main sewers as well as a storage function. The first two of three units of the project will reduce the pollution by 60%. Our models forecast a pollution reduction of 70% when a polluting load dependent storage management will be implemented. The sewer is divided into cascades by controlled weirs. The control system will be an integrated system using DCS, SCADA and HMI systems. This control system is developed in our models utilising user written controls of the RTC. The cleaning process of the new storage sewer starts within the storage process by generating waves and ends with a flush of water of the river Mur. P018-1

2 INTRODUCTION Sewage system of Graz Graz is a city with inhabitants in the south of Austria. The river Mur divides Graz into two parts. The sewage system of Graz is a grown storm sewage system and has a length of 849 km with a waste water treatment plant in the south of Graz. 37 combined sewer overflows (CSOs) are situated on the left and right river banks. The long term objective is to keep the river clean. Graz made the wastewater treatment plant better work and now Graz tries to reduce the sewage discharge during storm weather conditions. The strategy is on the one hand to reduce the inflow to the sewage system and on the other hand to store the wastewater as long as possible in the system. The concept of a central storage unit leads to the solution of a storage sewer. The key benefits of such a solution are: most of the CSOs can be connected to the storage sewer, it can be filled from both sides of Graz in order to use the whole volume even when it is raining only in parts of Graz, it can be used as main sewer in order to maintain the left or right main sewer and the maintenance of the storage sewer can be brought to a minimum. The storage sewer is a cascaded storage sewer with 10 cascades to provide a optimal utilisation of the storage volume. 90 percent of the storage volume can be discharged to the waste water treatment plant without pump, 10 percent has to be pumped. Graz decided to realise a central storage sewer (ZSK). This storage sewer will keep the combined waste water in the sewage system until the waste water treatment plant is able to take the water and clean it. The start of the project ZSK storage sewer was initialised by hydropower plant projects at the River Mur. COMBINED MODELS An effective pollution load dependent storage management system In order to get knowledge about the required volume of the storage sewer a hydrologic model was built with the software COSIM (ITWH). The model contains all CSOs of the sewage system Graz (more than 90), 130 catchments and the important volumes of the system. When it is raining the flow direction in parts of the main sewers turns back. Therefore it was necessary to enable the model to turn back flow direction. This was realised with fictive CSOs. The calibration of the hydrologic model was made using the hydrodynamic Mike Urban (DHI) model. All CSOs, also the fictive, were calibrated. Both models have the same catchment areas, both models are completely different and both models produce nearly the same results. Although the models are not connected the models are combined by hydrodynamic calibration. The next challenge was to implement the occurrence of a first flush in the hydrologic model. The first flush is an increase of the pollution load when the first water of a rain runs through the sewers. The pollution of the streets and sewers is transferred to the CSOs. It was possible to implement both effects, P018-2

3 pollution from the streets and pollution from the sewers, to a flush effect in the hydrologic model. The model was calibrated with a water quality online sensor. The model with the implemented first flush effect was compared with the model without this implementation. The results are remarkable. There are up to 20 percent difference in storage volume. The reason is that polluted water is caught in the storage sewer and spilled out again by the following low polluted water. There are two ways to solve this problem, to build more storage volume or to realise a pollution load dependent storage management system. So the next challenge was to implement a pollution load dependent control into the hydrologic model. And it was necessary to develop a working control strategy. The following statement is the conclusion of bitter experience: Never try to optimise a control without a working control strategy. Not even black box systems, neural networks and other modern things will work without a control strategy. In our case polluting load dependent controls were implemented in the hydrologic model, also a good working control strategy was found and the controls could be optimised. The results of the combined models not only tell us that a storage volume of m³ is necessary to transport 70% of pollution loads to the wastewater treatment plant during a rain, but they give us information how to handle such a sewer. And as an additional result an effective working pollution load dependent storage management system is now available for the ZSK. FEASIBILYTY Fundamentals and basic information When the river Mur hydropower plant projects started it was advantageous for both parties to build a combined storage and transport sewer. The hydro power plants will flood a couple of CSOs and water from the river will flow through the sewage to the wastewater treatment plant. To prevent this case a transport sewer has to be built. Now the question was: is it possible to combine both projects? The feasibility was studied with the adapted models. Both functions, storing and transport can be combined in one sewer. The cross section of the new storage sewer was optimised for transport and storing. The ZSK has a rectangular cross section with a width of 3.2 m and a height of 2.4 m. The part of the ZSK in the area of the second hydro power plant Graz has two sections with a width of 2 x 3.2 m and a height up to 4.0 m to transport up to a value of 80 m³/s. An ecological landscaping project called Mur Masterplan also was started. It was studied if it is possible to realise synergies with the Mur Masterplan. It was figured out that there are essential advantages for both projects. Therefore the planning process of the ZSK project and the Mur Masterplan project is coordinated. The concept of the ZSK project is based on the cascades. Every cascade has a controlled weir. It was studied which weir system is optimal for the requirements of the ZSK. The result of the study is a controlled weir plate diving into the ground. A product of ASA-Technik, Germany, will now be installed. The ASA- Technik controllable weir agrees with the requirements of the ZSK-project perfectly. It is possible to control the discharge of a cascade or a group of cascades in a wide range and with the required accuracy. Geology and groundwater can be a problem due to the rather high water permeability and the high groundwater levels. A study should find out if a cut P018-3

4 and cover method to build the sewer is suitable. The groundwater inflow into the excavation pit is expected with 2 l/s per m. So a cut and cover method is a practical solution. Operation and maintenance values the quality and benefit of the ZSK. On the one hand the ZSK has to be operated by the waste water treatment plant and has to meet the demands of sewage purification and on the other hand the maintenance has to run to a minimum. The results of the study are that the control of the ZSK is linked with the control system of the wastewater treatment plant. The wastewater treatment plant controls the ZSK and takes the water from it when the wastewater treatment plant is ready for the further cleaning process. So the water has to wait in the ZSK for a couple of hours. The particular material is settling to the ground of the ZSK. It is not practicable to clean the ZSK by manpower after every rain. To clean the ZSK and transport the sedimentations to the waste water treatment plant series of waves flush the particles to the treatment plant. The waves are generated by the ZSK control automatically. The economy of the ZSK was studied. The hydropower plants and the Mur Masterplan had been taken into account. Also effective strategies to keep the value of sewers (M. Stachowske) were considered. Long term investment versions were optimised and the budget for the ZSK was prepared. MODELLING THE STORAGE SEWER A high performance model of the storage sewer The concept was developed and a high performance model of the storage sewer was necessary. The model was made with the software Mike Urban. The storage sewer was cut off the model Graz. The model consists of 811 links, 57 weirs, 28 gates and a couple of sensors and connected time series. The controllable weirs and gates were implemented as RTC units. The main principle was to build reality in the model. This is the only way to make a model usable for a wide range of applications. Questions referring to a lot of different problems can be answered by the model. Additionally user written controls were prepared. The aim is to connect Mike Urban to other applications. THE STORAGE SEWER CONTROL A multiple level control system with the possibility of human intervention The first question is who will control the ZSK. Is it useful when the ZSK is controlled only by a human controller? This is definitely not useful because the ZSK is a complex system and complex systems cannot be solved by men. A system without any human control is not flexible enough to meet all requirements at all times. It makes sense when a human controller has the chance to take control over parts or the whole storage sewer. In the case of Graz the solution is a multiple level control system with the possibility of human intervention. There are four levels controlling the water flows. The levels are graduated. Level 1 is the control of one weir or one gate. The level 1 control has to know what to do when all other systems are not reachable. P018-4

5 Level 2 is the ZSK control. This control controls the whole ZSK with all weirs and gates and has to know what to do when weirs or gates or the wastewater treatment plant is not reachable. Level 3 is the wastewater treatment plant control which controls the ZSK control. Level 4 is the maintenance control; all other levels have to be silent when maintenance control is activated. To visualise the activities of the control levels a tool was developed that works like a HMI (Human-Machine Interface). This tool was connected to a MySQL database. Also the ZSK model was connected to this database using user written controls. So the tool is able to communicate with Mike Urban. SCADA Control Panel MySQL uwc Mike Urban Hydrologic model Figure 1: Using user written control (uwc) for communication with a MySQL database and a control panel. Blue: realised connections. The user written control is realised as a Delphi 2005 pe dll-file with a connection to Mike Urban and a connection to a simple table in a MySQL database. The control panel is realised as a VB 2008 exe-file also with a connection to the MySQL Database. Figure 2: The control panel during a simulation P018-5

6 The control panel seen in figure 2 is divided into two parts. The upper part shows the sewage system, the ZSK and the river Mur in a compacted way. The lower part shows a manual control panel. Red LEDs indicate that the object is controlled by Mike Urban, green LEDs indicate an object under manual control. Weirs can be moved by track bars. 29 minutes after the beginning of the rain the first cascade still is empty. The CSOs R7, L9, R1 and L3 spill out water into the Mur. At the CSO L9 the gate to the ZSK is manually closed and 15.1 m³/s are spilled out. At any time during the simulation the manual control can be activated or deactivated by a click onto the LED. Gates can be opened or closed by clicking onto the switch. Weirs can be moved by moving the track bars. The reactions of Mike Urban follow immediately. What are the benefits of the control panel? Control sequences will be developed with the control panel tool. The control panel tool is a kind of a sewer control simulator. Therefore it is useful that employees which will work with the ZSK control will be trained on this tool. In the next year the challenge will be to connect the hydrologic model to the database. This will open a wide field of optimisation options using fast long term simulations. Measured rain data can be taken to the database, can be extended by a statistic tool, and can be taken by the hydrologic model in real time and an optimised control sequence can be generated which can be sent to the SCADA system. The ZSK requires also control sequences to make waves to clean the ZSK. The better the effect of the waves the less maintenance is needed. TO MAKE WAVES Lift sediment material by waves Sedimentation of particles depends on the water velocity. In the ZSK the water velocity is near null for a couple of hours. Sedimentation will take place. When the waste water treatment plant takes water from the ZSK the water velocity will be less than 0.3 m/s. This is too less to lift sedimentations. The idea is to lift the sedimentations by waves. The controlled weirs shall produce the waves to raise the velocity in order to lift the sedimentations. During emptying the cascades groups of weirs shall interact to produce upsurge and down surge. The geometry of the storage sewer is a basic condition in our wave project. The rectangular cross section of the ZSK was chosen due to the expert knowledge and operating experience of ASA-Technik, Germany. The slope of the ZSK is 0.16% to 0.22%. The lengths of the cascades are 1300 m to 1900 m. The weirs can be opened and closed with a velocity of up to 8 m/s. Figure 3 demonstrates an example of a generated wave in a cascade. There are two waves visible. The second wave is running on the back of the first wave. The second wave has a higher velocity as the first wave. The second wave will catch up the first wave. P018-6

7 Figure 3: Generated waves. Two points are marked. Point blue is marked with the blue line and point red is marked with the red line. (W. Pirkner) Figure 4: Velocity-Time diagram for a single wave at the point blue and at the point red. (W. Pirkner) P018-7

8 Figure 5: Velocity-Time diagram for a double wave at the point blue and at the point red compared with a single wave. (W. Pirkner) The effect of a double wave is remarkable. A velocity increase of up to 35% can be expected in accompany with the effect that a higher velocity will act for a longer time. This simple example reflects the capability of generated waves in sewers. In future this application will become more and more important in old sewage systems. Additional storage volume can be activated, a pollution load dependent storage management can take place and maintenance can be reduced to a minimum at these parts of the sewer due to lifting and transport sediments. CONCLUSION Modelling There are three essential points referring modelling and the new storage sewer in Graz: Severs are complex systems and therefore they only can be solved by models. Models are essential for making decisions during planning a sewer. Sewer controls can be optimised by models. Planning with modelling makes investments effective. P018-8

9 REFERENCES Gamerith V., Muschalla D., Schneider S., Gruber G., Sprung W., (2008) Hydrologische Schmutzfrachtmodellierung des Einzugsgebiets Graz West, TU Graz, Austria Hochedlinger M., Sprung W., Kainz H., König K., (2006) CSO Modelling Considering Moving Storms and Tipping Bucket Gauge failures, TU Graz, Kanalbauamt Graz, Austria Manfred Weikopf, New techniques of controlled sewerage systems by the flush management of drain networks (CFD technique), ASA Technik GmbH Krefeld, Germany Günter Gruber, Harald Kainz, Werner Sprung, Norbert Flamisch, Alexander Pressl, Stefan Winkler (2006) Langzeiterfahrungen mit dem Betrieb von Kanal- Online_Messstationen in Österreich, TU Graz, Austia P018-9