Performance of hybrid-constructed floating treatment wetlands in purifying urban river water: A field study

Abstract

Floating treatment wetlands (FTWs) are a new management method for improving urban stormwater runoff. However, most studies have been performed at laboratory, micro, and meso scales. Therefore, it is necessary to evaluate the purification effect of the ecological floating bed in urban water bodies under in situ conditions. The purpose of this study was to compare the treatment potential of two types of hybrid-constructed FTWs for urban river water, and to quantify the effects and concentration radii of dissolved oxygen (DO), water temperature (T), nitrate nitrogen (NO3−-N), ammonia nitrogen (NH4+-N), and total phosphorus (TP) under different rainfall conditions. The difference analysis in the first rainfall stage (p < 0.05) indicated that FTWs with multiple plant species (FTW-A) and FTWs with a single species (FTW-B) had significantly better treatment potential than the blank control for all water quality parameters except T. The second stage study showed that the hybrid system could effectively purify the river water during the rainfall period, and the effect of FTW-A was higher than that of FTW-B. After the initial rainfall, DO decreased, NH4+-N and TP concentrations increased, the removal rate of NH4+-N decreased to 55.88%–58.33%, and the removal rate of TP decreased to 5.00%–10.53%. During continuous rainfall, the removal rate of NH4+-N remained between 16.28% and 63.89%, while the removal rate of TP fluctuated, reaching 26.32%, although the final concentration of TP was not removed during the experimental period. Meanwhile, the hybrid constructed FTW continuously increased the DO concentration; however, denitrification was inhibited, resulting in ineffective removal of nitrate and nitrogen in the water body. Further study on the influence radius of two types of FTWs on DO, T, NO3−-N, NH4+-N, and TP found that the influence radius of FTWs varied with the concentration of each parameter. For the same parameter, the influence radius of FTWs with multiple plant species and FTWs with a single species was RFTW-A ≥ RFTW-B. In this study, the purification effect of hybrid-constructed FTW in urban water was evaluated, which provided guidance and support for the FTW site layout of actual river courses in the future.