Intensified Constructed Wetlands for CSO/SSO Treatment to Achieve Immission-Oriented Thresholds for Phosphorus, Micro Pollutants and Priority Substances According to WFD

The German regulation for the protection of surface waters (OGewV) supplements and substantiate the requirements regarding the implementation of the Water Framework Directive (WFD) and contains detailed regulations regarding the classification, presentation and monitoring of the ecological and the chemical status of surface waters including thresholds for phosphorus. The quality of Germany’s water bodies has greatly improved in recent decades. Despite the considerable advances that have been made in the sewage treatment domain, point source pollution from certain nutrients and pollutants remains unduly high and thus needs to be reduced. This applies, for example, to phosphorus, as well as micro-pollutants (MPs) that in many cases cannot be adequately eliminated at wastewater treatment plants. In addition such inputs from combined sewer and rainwater (separate sewer) overflow (CSO/SSO) are also still a problem in certain water bodies. Sub-surface vertical flow constructed wetlands (SSVF) have been used since the late 1990s for CSO/SSO because of its high mechanical and biological efficiency. To achieve in particular a stable and long-term phosphorus reduction iron-containing substances are blended so far to large-scale SSVF as additives during the construction. However, some disadvantages in this type of substrate improvement must be stated. High and thereby costly material and installation costs are required and a subsequent melioration of a SSVF under continuous operation is impossible. The only possibility exists in the cost-intensive replacement of the entire soil body including replanting. All disadvantages can be avoided if instead of a substrate melioration a separate sorption stage, which is filled exclusively with iron-containing additives, is connected downstream of the SSVF. The material consumption is limited only to this sorption stage. This paper gives the results of an appropriate 3 year large-scale investigation including reduction rates and operational data concerning the standard parameters with the main focus on phosphorus as well as selected MPs and selected heavy metals as four of the six heavy metals investigated are classified as priority or even priority hazardous substances. The sorption stage consisted of two reactors, operated in series. The first reactor was filled with FerroSorp® (granular iron hydroxide), the second reactor with GAC. The P-removal of SSVF and sorption stage is in a load-related magnitude of 94% (oPO4-P), resulting in concentrations mainly less than 0.1 mg oPO4-P/L. Considering the FerroSorp® load capacity of 40% after two years of operation it can be estimated that a separated sorption stage might achieve a minimum lifetime of 4 years. Current German standards for meliorated SSVF generally recommend an amount of filter substrate of 10% by mass to achieve P-removal rates of 80% for two years of lifetime. This lifetime is half as long as with the investigated downstream sorption process and this with the thirtyfold increased use of iron-containing additive. Commenting the MP elimination, a substantial MP-loading of the CSO as well as a substantial MP-removal in the SSVF can be stated. Ten out of eleven MPs are reduced by the SSVF between 20% up to 80% (Metoprolol). The final downstream GAC sorption reactor eliminated all MPs almost completely as expected. The analyzed heavy metals cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) are retained in the entire three-stage plant (SSVF, two sorption reactors) with high efficiencies between 80% and 97%. Mercury (Hg) was analyzed only at very low concentration level.