Quantifying nitrogen transformation process rates using nitrogen functional genesin a multimedia biofilter under hydraulic loading rate constraints

• Hydraulic loading rate showed significant effects on N removal pathways.
• Nitrification and anammox are the dominant N removal pathways.
• NH4+-N removal pathways was jointly determined by six functional genes.
• Different N removal pathways were coupled at molecular level (functional genes).

The present study explored the treatment performance and nitrogen removal pathways in a multimedia biofilter treating micro-polluted source water under hydraulic loading rate (HLR) constraints, ranging from 0.5 to 3.0 m3/m2 d. High and stable chemical oxygen demand (COD) (97.7–99.1%) and ammonium (NH4+-N) (76.3–90.9%) removal efficiency were simultaneously achieved. Results showed that an HLR exceeding 2.5 m3/m2 d was required to achieve complete denitrification without NO3−-N accumulation in the biofilter. Molecular biological analyses showed that nitrification and anaerobic ammonium oxidation were the dominant NH4+-N removal pathways in the biofilter. Quantitative analysis demonstrated that the key functional gene groups for NH4+-N transformation process rate were amoA/archaea, nxrA/archaea, and (nirS + nirK)/anammox. Furthermore, nxrA/archaea contributed the most to the NH4+-N transformation rate, followed by (nirS + nirK)/anammox, and amoA/archaea. The results support that archaea potentially play vital roles in the nitrogen removal pathways, and nitrogen transformation pathways are coupled at the molecular level (i.e., the functional gene level).

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