Performance of Vertical Flow Constructed Wetlands as Quaternary Treatment Systems

Abstract

This study evaluated the performance of laboratory-scale vertical flow constructed wetlands (VFCWs) as quaternary treatment systems for wastewater polishing. The experiment was conducted within the framework of the FORGOTTEN project at the University of Luxembourg and investigated the influence of vegetation and biochar amendment on treatment performance. Six lysimeters were operated under controlled indoor conditions and divided into three treatment configurations: unplanted systems, planted systems, and planted systems amended with activated biochar. All units were fed with synthetic treated wastewater and monitored for physicochemical parameters, nutrient dynamics, dissolved oxygen distribution, evapotranspiration, substrate characteristics, and microbial indicators. The results showed that all VFCW configurations improved water quality and achieved substantial reductions in organic matter and nutrient concentrations. Significant decreases in COD, TOC, nitrogen, and phosphorus were observed, confirming the capacity of the systems to provide effective wastewater polishing. Differences between treatment configurations indicated that vegetation and substrate composition influenced treatment performance, although no single configuration consistently achieved the highest removal efficiencies for all monitored parameters. Dissolved oxygen profiling confirmed the presence of predominantly aerobic conditions within the treatment media, supporting biological treatment processes. Microbial analyses further demonstrated the establishment of active microbial communities within the systems. Overall, the findings highlight the potential of vertical flow constructed wetlands as sustainable and low-energy nature-based solutions for wastewater polishing. The study contributes to a better understanding of the interactions between vegetation, substrate characteristics, oxygen availability, and microbial activity in VFCWs and provides useful insights for the optimization of nature-based quaternary treatment systems.