Distribution and degradation trend of micropollutants in a surface flow treatment wetland revealed by 3D numerical modelling combined with LC-MS/MS.

Conventional wastewater treatment plants are not designed to treat micropollutants; thus, for 20 years, several complementary treatment systems, such as surface flow wetlands have been used to address this issue. Previous studies demonstrate that higher residence time and low global velocities promote nutrient removal rates or micropollutant photodegradation. Nevertheless, these studies were restricted to the system limits (inlet/outlet). Therefore, detailed knowledge of water flow is crucial for identifying areas that promote degradation and optimise surface flow wetlands. The present study combines 3D water flow numerical modelling and liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS/MS). Using this numerical model, validated by tracer experimental data, several velocity areas were distinguished in the wetland. Four areas were selected to investigate the waterflow influence and led to the following results: on the one hand, the number and concentration of micropollutants are independent of the waterflow, which could be due to several assumptions, such as the chronic exposure associated with a low Reynolds number; on the other hand, the potential degradation products (metabolites) were also assessed in the sludge to investigate the micropollutant biodegradation processes occurring in the wetland; micropollutant metabolites or degradation products were detected in higher proportions (both number and concentration) in lower flow rate areas. The relation to higher levels of plant and microorganism metabolites suggests higher biological activity that promotes degradation.

Seasonal and ageing effect on the behaviour of 86 drugs in a full-scale surface treatment wetland: Removal efficiencies and distribution in plants and sediments.

The presence of human drugs in the aquatic environment is partly due to an incomplete and insufficient removal process of wastewater treatment plants (WWTPs). Thus, drug traces are observed at different concentrations in water bodies, sediments and aquatic plants all over the world. At the same time, Surface Flow Treatment Wetlands (SFTWs) at the outlet of WWTPs are commonly observed in small municipalities as complementary treatment. However, little is known regarding the role of SFTWs in the complementary mitigation of emerging contaminants, such as drugs, and the interactions between drugs, plants and sediment throughout the seasons. For that reason, we conducted sampling sessions over a period of two years on a full-scale SFTW downstream of a vertical-flow constructed wetland. At each session, the SFTW influent and effluent, as well as five different plant species and one composite sediment sample, were sampled. We detected more than fifty pharmaceutical compounds in the inflow and outflow water. The compounds most frequently detected were bisoprolol and ketoprofen. We emphasized that the SFTW removal ability was better in the summer than in the winter, due to the impact of weather on physicochemical parameters. Large variations of removal efficiencies were also observed when considering all of the detected compounds. Large seasonal variations were also observed for each compound. In addition, the study of the five plants showed their ability to uptake drugs from water and soil to the leaves in a species-specific manner. The pharmaceutical composition of the sediment was also correlated with the season: the maximum occurrence was reached in summer, and the minimum was reached in winter. Finally, the continuous decrease in removal efficiencies highlights the ageing effect on SFTW removal ability.