Intensifying Nutrient Removal in Hybrid-Constructed Wetlands Treating Urban Streamwater

This study investigated the influence of hydraulic retention time (HRT) variation and the presence of macrophytes on the efficiency of two pilot-scale hybrid-constructed wetlands (HCWs) treating urban streamwater contaminated with nontreated sanitary sewage contributions from the surrounding communities. Each HCW comprises a vertical unit (VF) and a horizontal unit (HF) filled with sand and gravel. HCW-P was planted with<i>Eichornia crassipes</i> onto the filtering media, and HCW-C was set up as a control unit with no macrophytes. The novelty of this study consists of evaluating the combination of these factors (HRT and macrophytes) in the operation of HCWs for removing organic matter and nutrients. The operation of the HCWs was divided into step I, with a hydraulic retention time (HRT) of 9 days for 133 days, and step II, with an HRT of 5 days for 131 days. Neither HRT variation (p-value = 0.7691) nor the presence of macrophytes (<i>p</i>-value = 0.0941) influenced the COD removal, as the HCWs achieved high removal efficiencies (>87%) during the operation. HCW-P achieved higher total nitrogen (TN) removal efficiencies in steps I and II (56% and 78%) compared to HCW-C (31% and 48%) during the operation, demonstrating the improvement in removing TN due to the presence of macrophytes (p-value ≤ 0.05). In addition, the shorter HRT promoted an increase of 22% in TN removal for HCW-P (p-value ≤ 0.05). The macrophytes and longer HRT enhanced total ammonia nitrogen (TAN) removal, as HCW-P (46% and 88%) achieved higher removal efficiencies than HCW-C (29% and 72%) in steps I and II, respectively (<i>p</i>-value ≤ 0.05). Regarding total phosphorus (TP), HCW-C and HCW-P achieved removal efficiencies of 63% and 89% in step I and 69% and 96% in step II, confirming the influence of HRT and macrophytes on TP removal. Finally, macrophytes demonstrated adaptability and resilience to the operational conditions, even when fixed in HCWs, which presented robustness in removing organic matter and nutrients from the urban streamwater via biofilm assimilation and adsorption under HRT variations.