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Vertical-flow constructed wetlands remove pollutants from combined sewer overflows (CSOs) and have been in operation for approximately 20 years in Germany. More than 150 sites are currently operated in the state of North-Rhine Westphalia, with most of them constructed according to the same guidelines. This number provides an adequate statistical population to evaluate the treatment efficiency of this design; however, most available data have not been collected in a standardized way. This study presents the results from a survey amongst operators of these plants, who provided long-term data for 24 sites. It also describes a statistical approach to harmonizing the data and evaluating the removal efficiencies of wastewater pollutants while considering errors and uncertainties with a Monte Carlo simulation. This method makes it possible to harmonize data from surveys and to draw generalized conclusions on the treatment efficiency of large-scale facilities. In this study, the evaluation could be adapted for the removal efficiencies of chemical oxygen demand, total suspended solids and total phosphorus.