Role of Seasons in the Fate of Dissolved Organic Carbon and Nutrients in a Large-Scale Surface Flow Constructed Wetland

Abstract

The role of seasons in the removal of dissolved organic carbon (DOC), nutrients and in changes in the spectral properties of dissolved organic matter (DOM) in a large-scale surface flow constructed wetland (SF-CW) receiving reclaimed water and composed of three basins with different vegetation patterns was studied. Dissolved nitrogen removal efficiencies within the three basins in summer (>50%) and winter (<30%) were significantly different. SF-CW water is enriched in DOC in spring and summer with average outlet concentrations above 8 mg·L−1. UV-visible indices, such as the specific absorbance at 254 nm or the spectral slope between 275 and 295 nm, did not vary over the seasons; thus, the basins did not change DOM aromaticity and average molecular weight. Synchronous fluorescence spectra showed variations in terms of protein-like and humic-like substances, the latter being more sensitive to photodegradation. A lab-scale photodegradation experiment confirmed that radiation from the sun was responsible for this decrease, showing this process could alter the composition of DOM at full-scale. DOM variations result from a seasonal competition between release by vegetation and photodegradation. These results validate the necessity for long-term monitoring of SF-CWs, and the utility of rapid optical methods to monitor DOC.