Enhanced long-term nitrogen removal and its quantitative molecular mechanism in tidal flow constructed wetlands

Zhi, W., Yuan, L., Ji, G., & He, C. (2015). Enhanced long-term nitrogen removal and its quantitative molecular mechanism in tidal flow constructed wetlands. Environmental Science & Technology, 49(7), 4575-4583. doi: 10.1021/acs.est.5b00017

Abstract

• 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.

Figures

• Long-term dynamic transformations of nitrogen in tidal flow constructed wetlands
• Dynamic populations of nitrogen functional genes in T1 (black) and T2 (red)
• Table 1. Quantitative Relationships between Nitrogen Transformation Rates and Functional Gene Abundance (T1)
• Table 2. Quantitative Relationships between Nitrogen Transformation Rates and Functional Gene Abundance (T2)

Long-term dynamic transformations of nitrogen in tidal flow constructed wetlands

Throughout this study, little NO2− and NO3− were accumulated in T1, suggesting that denitrification at a C/N ratio of six was sufficient for nitrification to completely remove nitrogen from the wastewater. T2 achieved an NO3− removal efficiency of 91 ± 5.7% in the start-up stage, increasing to 97 ± 2.2% in the operational stage. Compared to the lagging NH4+ removal in T1, the NO3− removal in T2 was more rapid and efficient. This was because the shortcut nitrogen removal pathway in T2, the denitrification process, was unrestricted to the rate-limiting nitrification step.

Dynamic populations of nitrogen functional genes in T1 (black) and T2 (red)

The absolute abundances of bacterial 16S rRNA, archaeal 16S rRNA, anammox bacterial 16S rRNA, amoA, nxrA, narG, nirK, nirS, and nosZ were quantified eight times during the experimental period to determine their dynamic populations.

Table 1. Quantitative Relationships between Nitrogen Transformation Rates and Functional Gene Abundance (T1)

Results from stepwise regression analysis suggested that the NH4+ removal rate in T1 was collectively controlled by amoA, nxrA, and anammox.

Table 2. Quantitative Relationships between Nitrogen Transformation Rates and Functional Gene Abundance (T2)

Results from stepwise regression analysis suggested that the NO3− emoval rate in T2 was governed by nxrA and narG gene.