Performance evaluation of first-order hydraulic models for COD removal in horizontal subsurface-flow constructed wetlands

The aims of the present study are to verify the performance of four first-order hydraulic models and identify the model that best described the kinetics of the chemical oxygen demand (COD) removal in constructed wetlands. Three horizontal subsurface-flow constructed wetlands (HSSFCWs) were examined: two were planted with Typha latifólia and one was left unplanted. Each HSSFCW received 50 L of secondary effluent daily, resulting in a hydraulic retention time of 3.2 days. The COD concentrations were determined throughout the HSSFCWs and compared in terms of the coefficient of determination with estimated values provided by hydraulic models. The performance of the models was verified under two different conditions: (1) without, and (2) with evapotranspiration (ET) effects. Among the studied models, the plug-flow hydraulic model revealed the poorest performance, whereas the modified plug-flow model (incorporating the coefficient of decay resistance) was the model that best described COD removal kinetics throughout the HSSFCW. The incorporation of water losses by ET improved the performance of all of studied models.