Environmental Protection and Flood Prevention with the Modular h Combined Sewer Overflow Chamber

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1 Environmental Protection and Flood Prevention with the Modular h Combined Sewer Overflow Chamber Rising sea levels, increasingly severe and frequent rainstorms, plus an ever-growing population density keep influencing the ground water level in urban areas. While it is hardly possible to alter the actuators, we can do something to mitigate against the consequences: Drainage and flood prevention measures are becoming more and more important in urban infrastructure. The HOBAS Product Range comprises a variety of solutions for this purpose, from large-volume retention tanks to customized Combined Sewer Overflow (CSO) Chambers. Flood control is being practiced since ancient times. The methods used include planting vegetation as well as the construction of floodways, dams, and reservoirs to hold extra water during times of flooding. Nowadays, with more and more green areas and woodlands being paved and used to create parking lots, roads, and buildings, the amount of surface area available for water infiltration decreases. Instead, the stormwater, residential, commercial and industrial wastewater is redirected into sewage and storm drain systems. These are either combined sewers, in which all types of sewage flow together, or separate sewers containing a separate channel for wastewater and one for rain and drainage water. From an outside point of view, you would not note a difference during heavy rainfall though, combined sewers tend to overflow due to the often limited capacities of wastewater treatment plants. This is why combined sewer systems are increasingly equipped with overflow structures which lead parts of the combined sewage into a temporary storage system or a receiving water body (depending on national regulations) in order to reduce the load on the wastewater treatment plant and thereby also avoid flooding. Moreover, it is important to keep the discharge of suspended solids into the receiving water body as low as possible in order to prevent the ecosystem from suffering severe damage. 1

2 Hydrological and Ecological Efficiency The EU Water Framework Directive 2000/60/EC aims to reduce and manage the risks that floods pose to human health, the environment, and economic activities. This goal can only be reached through structures that ensure both an adequate hydrological capacity and ecological functionality. HOBAS has developed a particularly efficient GRP overflow system for combined sewers in cooperation with the Czech Tech University (CTU) Prague and in compliance with the EU Directive 2000/60/EC. The HOBAS CSO Chamber separates suspended solids from the wastewater very efficiently and directs them to the wastewater treatment plant, while the cleaned part of the water is discharged into a storage system, which can be directly integrated into the chamber or receiving watercourse. It does not require any maintenance or external supply of energy, can be quickly and easily installed, its operation is safe and unproblematic, and it can be adapted to different requirements. At first, a physical model was built by the CTU Prague, in the Department of Sanitary and Ecological Engineering, under the direction of Dr. Jaroslav Pollert. Then, a mathematical CFD (Computational Fluid Dynamics) model was calculated and calibrated on basis of the data gained from the physical model. This way, various realistic flow scenarios and chamber sizes were simulated and the design optimized in order to achieve a homogeneous distribution of flow. A Particularly Effective Solution The structure design resulting from these physical and calculatory models is a sophisticated combination of various components. The HOBAS CSO Chamber is directly connected to the existing sewer via an inlet pipe (1). The first section of the CSO Chamber is the so-called stilling chamber (2). This conic cross-sectional enlargement reduces the flow velocity significantly and thereby supports the sedimentation and accumulation of suspended solids. The size of the chamber can be increased, providing additional storage capacity. 2

3 A corrosion-resistant GRP scum board (3) which is mounted perpendicular to the flow direction serves as barrier for floating solids in rainfall events. As the flow velocity decreases in the stilling chamber, more solids sink to the bottom of the pipe at the scum board into the combined sewage flowing below. This way, the pollutants are safely transported to the wastewater treatment plant after the rain event. Thanks to the scum board s smooth surface, there is no danger of deposits. After passing the scum board, the wastewater reaches the separating chamber (4).In dry weather conditions, it simply flows through the chamber. During heavy rainfall, the water level rises and the combined wastewater with a low solids content rises up through the overflow slit (5). The overflow slit is aligned with the flow direction and thereby optimizes the separation process. It conveys the cleaned water to the receiving water body, a stormwater retention tank or an overflow tank. At the end of the HOBAS CSO Chamber, a throttled outlet (6) leads to the wastewater treatment plant. The throttled outlet can be individually supplemented with regulation and measuring equipment. The chamber can be accessed through factory-mounted GRP inspection manholes (7) that are equipped with ladders and manhole covers. They are particularly important in the area of the scum board and the throttled outlet. Additional manholes can ensure the accessibility of the measuring equipment. The retention of suspended solids can be improved by means of flexible GRP bars that function as cleaning elements (8). The bars do not reach down to the pipe bottom, which is why in dry weather, the combined sewage passes through to the wastewater treatment plant without touching them. After a rainfall event, the bars start to vibrate in the decreasing water thanks to their flexible mounting and thereby clean themselves automatically from pollutants, which are lead into the outlet pipe and further on to the wastewater treatment plant. The cleaning elements are maintenance-free and do not require any external power supply. The number of cleaning elements depends on the project specifications. Numerous Reference Projects Leave No Doubts The first HOBAS CSO Chamber was installed in April 2007 in the town of Děčín in the Czech Republic. The installation was completed in merely four hours overnight below a high traffic road. Over a period of four years, the CTU Prague measured the chamber s efficiency with regards to solids separation. The pollution of the sewage was determined by the water turbidity, which was measured at both the inlet pipe and the outlet pipe by means of special sensors. The sensors were triggered as soon as water entered the overflow pipe. The big challenge of the measurements was to have the measuring equipment ready when heavy rain sets in. Due to the lack of an external energy supply, the system ran on batteries, which had to supply enough power for an entire year. 3

4 The above chart describes the crucial 20 minutes of a single major rain event (X-axis: proceeding time, Y-axis: flow rate in l/s and suspended solids in the respective wastewater sample). The blue curve demonstrates the increasing flow rate at the inlet pipe in the event of heavy rain. The CSO Chamber was gradually filling up until at a certain point in time, water entered the overflow pipe. Then, the measuring system kicked in and water samples were taken at both the inlet pipe and the overflow pipe to measure the turbidity and pollution of the water. The triangles show the values at the inlet, the crosses at the overflow pipe. The overflowing water is clearly much less polluted than the inflowing water at the marked time stamp no less than 72 %. Thus, the measurement confirms the very good cleaning function of the HOBAS CSO Chamber. The whole setup was also reconstructed with the mathematical CFD model, different flow paths were simulated, and the model was thereby verified. Up to today, numerous HOBAS CSO Chambers have been successfully installed, covering various requirements from new constructions to the renovation of existing structures 4

5 The HOBAS CSO Chamber: Benefits At A Glance o Reliable storm water overflow o Low-maintenance, self-cleaning system o Optimum separation of suspended solids (design enhances secondary flow effects) o Additional storage function o Modular system o Quick installation o Little space and excavation required during installation o Withstands high traffic loads with minimum covering Fmd: 5