Water balance and pollutant removal efficiency when considering evapotranspiration in a pilot-scale horizontal subsurface flow constructed wetland in Western Sicily (Italy)

Abstract

In constructed wetland systems (CWs) for wastewater treatment, the performance of the system is affected by evapotranspiration (ET). This study shows the results of a series of water balance and pollutant removal efficiency analyses taken from a pilot horizontal-subsurface flow system (HSSFs) in the West of Sicily (Italy). The system comprised three separate units, one planted with Cyperus alternifolius L., one planted with Typha latifolia L. and an unplanted unit. The system was fed with urban wastewater from an activated-sludge wastewater treatment plant. The aims of the study were to determine water balance and pollutant removal rates when considering evapotranspiration in two root emergent macrophytes in typically Mediterranean climate conditions. ET values were calculated by determining three components of a simplified water balance model without taking subsurface and surface water into consideration. Crop coefficient values were estimated using the FAO 56 method. Removal efficiency (RE) of a pilot HSSFs was calculated using both inflow and outflow pollutant concentrations and mass loads. Biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) were the main pollutants examined. The T. latifolia-unit was found to have higher cumulative evapotranspiration rates (3579mm) than the C. alternifolius-unit (3142mm). Water-use efficiency (WUE) for C. alternifolius (0.66g/L) and T. latifolia (0.75g/L) was somewhat low on average compared to traditional open-field crops. Percent removal was calculated using mass loads was on average higher than that determined using concentrations for both the planted and unplanted units. Further knowledge on water losses could provide useful information when designing CWs. The estimate of ET is highly important for arid areas, especially where the water at the outflow of the CWs is required for reuse.