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

Abstract

Evapotranspiration (ET) can affect treatment performance in 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 (K c = 2.5; R 2 = 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 = 0.2 d-1) and readily treatable constituent (k = 1.2 d-1) in a constructed wetland (20 cm and 40 cm water depths, 4-day nominal HRT, and 100 mg L-1 constituent loading) operating under a range of ET (0, 10, 20, and 30 mm d-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 5 cm. Mean tracer arrival time differed significantly (p = 1.2 x 10-8) between the untrimmed and trimmed cells (104 minutes versus 450 minutes, 3 respectively) demonstrating that plant transpiration contributes significantly to vertical flow through hydrosoil.

Research focusing on plant performance in constructed wetlands and agronomic application of treated wastewater – A set of experimental studies in Sicily (Italy)

“…australis , a commonly-used species in HSSFs CWs. When comparing the findings of other authors [48–49], the estimation of K c for C . alternifolius and T .…”

“…This relationship is highly significant in agronomic terms because it affects the choice of plant species in a CW in relation to our main aims. Some authors [48] claim that ET significantly affects pollutant RE in CWs and high ET values can determine higher pollutant concentrations at the outflow. In our research, in HSSFs CW(1), we found that when ET reached average values of over 20 mm d -1 , water loss increased at the outlet of the planted-units and we recorded higher BOD 5 and COD concentrations in the effluent.…”

Hydrodynamic modelling of free water-surface constructed storm water wetlands using a finite volume technique

“…However, according to Beebe [35], evapotranspiration can decrease the volumetric flow, thereby increasing the HRT and concentrations of dissolved constituents. Also investigating the effects of wind on the mixing factors on the solute tracer and hydraulic efficiency is suggested for further studies.…”

Estimated seasonal daily evapotranspiration rates for a horizontal subsurface flow constructed wetland

“…Freedman et al [12] measured similar values of 20 mm/day but observed 40 mm/day in certain times of the day. In another study, values of evapotranspiration in HSSF-CWs fluctuated between 19.5 and 40 mm/day [13]. Tuttolomondo et al [14] measured evapotranspiration in a constructed wetland in Italy and observed that on some summer days evapotranspiration reached 25-35 mm/day; this value was 20-30% of the hydraulic load.…”