Effect of plant species on water balance in a pilot-scale horizontal subsurface flow constructed wetland planted with Arundo donax L. and Cyperus alternifolius L. - Two-year tests in a Mediterranean environment in the West of Sicily (Italy)

In constructed wetland systems (CWs) for wastewater treatment, evapotranspiration (ET) is the most important water balance component in arid and semi-arid regions, where high performance levels are needed when treating contaminants and where it is also vital to preserve as much water as possible for reuse in irrigation. This study shows the results of a series of water balance measurements obtained between 2012 and 2013 from a pilot HSSF system in the West of Sicily (Italy). The system was made by two separate vegetated units: one with Arundo donax L. (giant reed) and the other with Cyperus alternifolius L. (umbrella sedge), and fed with urban wastewater following secondary treatment from an activated-sludge wastewater treatment plant. The aim of the study was to evaluate how two different macrophytes can affect the amount of water at the outflow of CWs under identical environmental, growth and hydraulic conditions. ET values were calculated by determining three components of a simplified water balance model without taking subsurface and surface water into consideration. Crop coefficients were estimated using the FAO 56 method analogous to herbaceous crops in open field cultivation. In the two years of tests, giant reed-unit was found to have higher cumulative evapotranspiration values than umbrella sedge-unit, with an average of 4273.6 mm. For both macrophytes, ET values were constantly found to be higher during the spring-summer season when the plants reached maximum vegetative growth. Crop coefficients were found to be higher than those of traditional crops grown in the Mediterranean area for all growth stages. Water use efficiency (WUE) was rather low on average, at 0.94 g/L for giant reed-unit and 0.66 g/L for umbrella sedge-unit. Results showed that greater or lesser amount of water at the outflow of the CWs is always dependent upon the ET rate of the species and it is essential to estimate the ET when designing a CWs in those regions where prolonged periods of drought can substantially reduce the amount of treated wastewater available for reuse in agriculture.