Influence of hydraulic loading rate and recirculation on oxygen transfer in a vertical flow constructed wetland

The oxygen transfer rate (OTR) has a significant impact on the design and operation of vertical flow constructed wetlands (VFCWs) intended for organic matter removal and nitrification. Despite its key role, the information on real oxygen input in VFCWs is limited, being usually estimated by mass balance (stoichiometry), through which is calculated only the oxygen consumption rate (OCR). In this study, for the first time, the gas tracer method was applied to evaluate the oxygen transfer capacity of a real-scale VFCW (24.5 m²) applied to the treatment of domestic wastewater. Propane was used as tracer. The OCR and the OTR were evaluated in VFCW under hydraulic loading rates (HLR) of 60, 90, and 120 mm d^-1 corresponding to recirculation rations of 0%, 50%, and 100%. The OTR in standard conditions (20 °C) ranged from 120 to 176 g O₂ m^-2 d^-1. The highest OTR was found for the lowest HLR. For the operating conditions tested, the OTR obtained with gas tracer were higher than the OCR calculated by stoichiometry in VFCW, which ranged from 20.6 to 27.8 g O₂ m^-2 d^-1. Besides, the OTR were sufficient to satisfy the VFCW oxygen demand for organic matter removal and nitrification. These results show that the gas tracer method for OTR determination may allow advances on the understanding of treatment processes and on the design of new VFCWs since its treatment performance requires aerobic conditions.