Constructed Wetlands for Combined Sewer Overflow Treatment—Comparison of German, French and Italian Approaches

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 OPEN ACCESSWater 2013, 5 2 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.

A Point Source of a Different Color: Identifying a Gap in United States Regulatory Policy for “Green” CSO Treatment Using Constructed Wetlands

“…Examples can be found mostly in Germany (several hundred [19]), the Netherlands (more than 5 [59]), Denmark (more than 5 [60]), and Belgium (more than 2 [61]). Single experimental full-scale applications currently under scientific monitoring are located in France [19], Italy [19], and Scotland [62].…”

“…In Germany, where CSO wetlands have been widely used since the 1990s (see Section 3.3 above), vertical down-flow systems are designed for a lifespan of more than 30 years. The biggest problem with their operation has been clogging over time, but this has been ameliorated by replacing older designs using fine filter media to ones specifying technical sands [19,65]. In general, key considerations for CSO wetland O&M include grit and floatables removal, invasive species removal/plant harvesting, and odor/vector control [50].…”

“…3/2006) [19]. Additional water storage before discharge to natural water bodies may be required for flood prevention.…”

“…European CSO wetland design is based broadly on the vertical down-flow type (originated from Germany in the 1990s), though different variations on this theme have been implemented (see Meyer et al [19] for an in-depth discussion on CSO wetland design parameters). Comparatively old approaches in the Netherlands and Belgium have horizontal free-water surface flow designs, and are either planted, unplanted, or equipped with floating plantation mats.…”

“…Conversely, only a handful of CSO treatment wetlands have been built in the U.S., although the technology has been applied in Europe [19]. Recently, CSO treatment wetlands have been included in LTCPs along with suites of GI solutions to mitigate the impacts of CSO discharges on receiving waters [20][21].…”