Enhanced Long-Term Nitrogen Removal and Its Quantitative Molecular Mechanism in Tidal Flow Constructed Wetlands

Tidal flow constructed wetlands (TF CWs) have recently been studied as a sustainable technology to achieve enhanced nitrogen removal; however, the underlying mechanisms responsible for removing ammonium (NH4(+)) and nitrate (NO3(-)) have not been compared and quantified at the molecular level (genes) in controlled TF CWs. In this study, two TF CWs T1 (treating NH4(+) wastewater) and T2 (treating NO3(-) wastewater) achieved high removal efficiencies for chemical oxygen demand (COD, 92 ± 2.7% and 95 ± 2.4%, respectively), NH4(+)/NO3(-) (76 ± 3.9% and 97 ± 2.2%, respectively), and total nitrogen (TN, 81 ± 3.5% and 93 ± 2.3%, respectively). Combined analyses revealed that the presence of simultaneous nitrification, anammox, and denitrification processes and the coupling of dissimilatory nitrate reduction to ammonium, ammonia oxidation, and anammox were the primary reason accounted for the robust treatment performance in T1 and T2, respectively. Results from stepwise regression analysis suggested that the NH4(+) removal rate in T1 was collectively controlled by amoA, nxrA, and anammox, while the NO3(-) removal rate in T2 was governed by nxrA and narG gene.