Abstract
Combined sewer systems are designed to transport stormwater surface run off in addition to the dry weather flows up to defined limits. In most European countries, hydraulic loads greater than the design flow are discharged directly into receiving water bodies, with minimal treatment (screening, sedimentation), or with no treatment at all. One feasible solution to prevent receiving waters from strong negative impacts seems to be the application of vertical flow constructed wetlands. In Germany, first attempts to use this ecological technology were recognized in early 1990s. Since then, further development continued until a high level of treatment performance was reached. During recent years the national “state-of-the-art” (defined in 2005) was adapted in other European countries, including France and Italy. Against the background of differing national requirements in combined sewer system design, substantial developmental steps were taken. The use of coarser filter media in combination with alternating loadings of separated filter beds allows direct feedings with untreated combined runoff. Permanent water storage in deep layers of the wetland improves the system’s robustness against extended dry periods, but contains operational risks. Besides similar functions (but different designs and layouts), correct dimensioning of all approaches suffers from uncertainties in long-term rainfall predictions as well as inside sewer system simulation tools.
Highlights
Introduction and ReviewWastewater treatment plants (WWTPs) maintain high standards in most member states of the European Community
Various end-of-pipe treatment techniques are designed to receive a specified dry-weather flow (DWF), which increases due to stormwater surface runoff conveyed in combined sewer systems (CSSs)
This correlation results in lower inlet concentrations for treatment facilities compared to WWTPs, but the hydraulic loads of single overflow discharges can exceed the typical loading of a vertical flow constructed wetlands (VFCWs) due to enormous volumes of water
Summary
Wastewater treatment plants (WWTPs) maintain high standards in most member states of the European Community. Compared to dry weather flows, flows from CSOs are usually diluted in terms of classical parameters like COD or NH4-N This correlation results in lower inlet concentrations for treatment facilities compared to WWTPs, but the hydraulic loads of single overflow discharges can exceed the typical loading of a VFCW due to enormous volumes of water. In Germany, the described overestimations of surface runoff led and still lead to many oversized CSO tanks, leading to increased return periods of storage overflows resulting in water stress in wetland plants due to the usual total drainage of RSFs. Development of CSO treatment filters in France and Italy started in the late 2000s. Fundamental differences can be found in the upstream CSO facilities: Direct discharges (without storage tanks) lead to shorter return periods of feeding events and increased particulate inlet concentrations by trend Both new designs can be considered as wetlands, because they provide permanent water layers. The original approach is currently under verification by simulating the first results from France (project “ADEPTE”) and Italy, but the necessary database needs to be realized first
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