THE INFLUENCE OF EVAPOTRANSPIRATION ON WASTEWATER CONSTRUCTED WETLAND TREATMENT EFFICIENCY

The presence of plants (reed, rush, cattail, willow, etc) in constructed wetlands (CW) for wastewater treatment brings several benefits, including enhanced removal of pollutants (N, P, heavy metals) from treated wastewater, creating micro-aerobic conditions and providing carbon compounds in the rhizosphere, hence supporting a diverse subsurface microbial community. Aquatic macrophytes are also of great importance in evapotranspiration (ET), especially during hot periods, in both natural and constructed wetlands. ET increases the concentration of dissolved compounds due to decreasing water volume in a CW. This may result in zero-discharge from the system. Where ET is high, removal efficiency (calculated as the difference between influent and effluent concentrations) is lower than expected from mass balance (based on pollutant loadings). Case studies from Poland (lab scale) and Portugal (field scale) illustrate the influence of ET on volume reduction and on the calculation of pollutant removal efficiencies. Evapotranspiration can cause substantial reduction in volume of an effluent stream in a well vegetated constructed wetland, leading to reduced flow and increase in hydraulic retention time. This may lead to problems estimating pollutant loads and removal rates in field systems. Pollutant removal efficiency values calculated from pollutant concentrations may be unreliable in conditions of high evapotranspiration due to water loss within the constructed wetland system. Where evapotranspiration is high (warm dry climate, high macrophyte biomass), removal efficiency values based on concentrations may be greatly underestimated. Hence, inflow and outflow rates (hydraulic flux), should be measured so that pollutant mass loads can be used as the basis for calculating removal efficiency. In warm dry climates it may be feasible to increase the hydraulic loading rate of vegetated constructed wetlands , relying on enhanced water loss by evapotranspiration, but without reducing treatment performance.