Effects of evapotranspiration on treatment performance in constructed wetlands: Experimental studies and modeling

Abstract

Evapotranspiration (ET) can affect treatment performance on constructed wetlands by enhancing constituent transport through the hydrosoil where treatment reactions occur. Additionally, ET can decrease volumetric flow thereby increasing hydraulic retention time and increasing concentrations of dissolved constituents. This research aims to assess the net effects of water loss attributed to ET on constructed wetland performance and determine the significance of plant transpiration on vertical transport of constituents. A flowing wetland lysimeter constructed using 265-L storage containers filled with sand and Typha latifolia was used to record ET and determine crop coefficient during summer 2011. Results indicate that ET from the lysimeter was 2.5 times greater than calculated reference ET (Kc=2.5; R2=0.96). The calculated crop coefficient was used in conjunction with a first-order tank-in-series model to predict removal of a conservative constituent (k=0.2d−1) and readily treatable constituent (k=1.2d−1) in a constructed wetland (20cm and 40cm water depths, 4-day nominal HRT, and 100mgL−1 constituent loading) operating under a range of ET (0, 10, 20, and 30mmd−1). The model predicts that removal efficiency of the conservative constituent decreases with increasing ET, while removal efficiency of the readily treatable constituent increases with increasing ET. In addition, eight vertical tracer tests were performed on wetland cells with either trimmed or untrimmed T. latifolia to measure transport time of tracer solution from the water surface to a depth of 5cm. Mean tracer arrival time differed significantly (p=1.2×10−8) between the untrimmed and trimmed cells (104min versus 450min, respectively) demonstrating that plant transpiration contributes significantly to vertical flow through hydrosoil.