Reduction of micropollutants and bacteria in a constructed wetland for combined sewer overflow treatment after 7 and 10 years of operation

Abstract

Repeated investigations on constructed wetlands for the treatment of combined sewer overflows, also named bioretention filters or retention soil filters, are necessary to provide information on their long-term performance. In this study, a sampling campaign was conducted on micropollutants, indicator microorganisms and standard parameters ten years after such filters were in operation and three years after the first investigation; it revealed that the filters lost capacity to remove chemical substances with no or only slow biological degradability. This was the case e.g. for phosphate (decrease from 29 to 11%), diclofenac (67 to 34%) and TCPP (34% to negative reduction). They continued to remove easily degradable parameters such as COD (stable around 75%) stably. The indicator microorganisms Escherichia coli (1.1/0.8 log10), intestinal enterococci (1.3/0.8 log10) and somatic coliphages (0.6/1.0 log10) showed comparably low process variations given the difficulties in sampling and analysing microbial parameters representatively as well as given natural variations in microbial behaviour and growth. Additionally, for bisphenol A, we found a temperature-related difference of removal efficiencies: while in the cold months (winter), the removal was only 53% on average, it increased to 90% in the warm months (summer).As for the long-term prospective of retention soil filters, decision-makers need to identify the most important pollutants in a specific catchment area and adapt the filter design accordingly. If pollutants are targeted that lead to an exhausted filtration capacity, post treatment or the exchange of charged filter material is necessary. However, for easily biologically degradable substances, so far, there is no limit in their use.