Zhi, W., & Ji, G. (2014). Quantitative response relationships between nitrogen transformation rates and nitrogen functional genes in a tidal flow constructed wetland under C/N ratio constraints. Water Research, 64, 32-41. doi: 10.1016/j.watres.2014.06.035
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- The present study explored treatment performance and nitrogen removal mechanisms of a novel tidal flow constructed wetland (TF CW) under C/N ratios ranging from two to 12. High and stable COD (83–95%), (63–80%), and TN (50–82%) removal efficiency were simultaneously achieved in our single-stage TF CW without costly aeration.
- Results showed that a C/N ratio exceeding six was required to achieve complete denitrification without and accumulation in the system. Molecular biological analyses revealed aerobic ammonia oxidation was the dominant removal pathway when the C/N ratio was less than or equal to six. However, when the C/N ratio was greater than six, anammox was notably enhanced, resulting in another primary removal pathway, in addition to the aerobic ammonia oxidation.
- Quantitative response relationships between nitrogen transformation rates and nitrogen functional genes were established, and these relationships confirmed that different nitrogen transformation processes were coupled at the molecular level (functional genes), and collaboratively contributed to nitrogen removal in the TF CW. Specifically, transformation rates were collectively determined by amoA, nxrA, anammox, narG, nirS, nirK, and nosZ; and TN removal was influenced primarily by amoA and anammox.
- Nitrogen transformations and nitrogen transformation rates different C/N ratios
- Absolute abundance of microbial communities and functional genes
- Pearson correlation coefficients between nitrogen transformation genes
- Nitrogen transformation pathway with functional genes
The synthetic wastewater used in this study was derived from Beijing groundwater, in which the NO3- concentration fluctuated between 1.2 and 5.3 mg/L throughout our study. Results showed negligible nitrification but NH4- adsorption occurred during the first 35 d of the Start-up stage. Nitrification (with accumulated NO2-) began contributing to removal in the second half of the Start-up period. Results also that the increased C/N ratio had a significant positive impact on TN removal (One-way ANOVA analysis, P < 0.05). However, this positive effect became attenuated when insufficient nitrification became the limiting factor for TN removal under a C/N ratio exceeding six.
The absolute abundance of bacterial 16S rRNA, archaeal 16S rRNA, anammox bacterial 16S rRNA, amoA, nxrA, narG, nirK, nirS, and nosZ were quantified 8 times during the entire operation period to determine their dynamic population shifts. The anammox bacterial 16S rRNA, amoA gene, and nxrA gene, which are the three functional genes involved in NH4+ transformation. A clear trend among narG, nirK, and nosZ genes provided direct evidence of an ecological association and symbiotic relationship among the denitrifying communities.
Pearson correlation coefficients between anammox-narG, anammox-nirK, anammox-nosZ, narG-nirK, narG-nosZ, nirK-nosZ, and nirS-nosZ all exceeded r = 0.72 (P < 0.05), confirming an interaction and ecological association among the nitrogen transformation microbial communities. The potential explanations for the associated enrichment of anammox, narG, nirK, and nosZ were their similar environmental adaptations, and interrelated but distinct ecological niches. The associated enrichment of anammox, narG, nirK, and nosZ genes demonstrated that multiple (anammox and denitrification) and complete (reduction to N2) nitrogen removal pathways successfully developed in the TF CW, assuring functional stability in nitrogen removal performance and effective control of N2O greenhouse gas emission.
NH4+ transformation rate was collectively determined by amoA, nxrA, anammox, narG, nirS, nirK, and nosZ; and TN removal was primarily influenced by amoA and anammox.
