Constructed Wetlands for Combined Sewer Overflow Treatment

Combined sewer systems are designed to transport stormwater surface runoff in addition to the dry weather flows up to defined limits. In most European countries, hydraulic loads greater than the design flow of wastewater treatment plants are discharged directly into the receiving water bodies with minimal treatment (screening, sedimentation) or with no treatment at all. One feasible solution to reduce negative impacts on receiving waters is the application of vertical flow constructed wetlands. In Germany, first attempts to use this ecological technology were made in early 1990’s. Since then, the further development let to a high level of treatment performance. 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 development 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. The constructions show similar functions despite different designs and layouts, but the correct dimensioning of all approaches (as well as inside sewer system simulation tools) suffer from uncertainties (e.g. impermeable surfaces, parasitic runoff and dry weather flow) in long-term runoff predictions. Current research studies aim to improve predictions of the system performance under varying conditions – both for classical wastewater parameters and emerging pollutants.