Nitrogen transformations and associated microbial characteristics in tidal flow constructed wetlands based on ANAMMOX operated at different temperatures

This study explored nitrogen transformations and associated microbial characteristics in TFCWs based on ANAMMOX operated at different temperatures. The four tested temperatures exhibited different effects on nitrogen removal performances and transformation pathways, since the competitive relationships among the nitrogen-transforming microorganisms and their long-term acclimatization capacities varied with temperature. According to the contributions of functional gene groups characterizing the dynamic processes of nitrogen transformation at each temperature, the multi-path coupled nitrogen removal process based on ANAMMOX dominated in the TFCWs at 15 and 20 °C, achieving high TN removal rates. However, at 10 and 5 °C, the abundance and activity of anaerobic ammonium oxidation bacteria were significantly lower, resulting in lower nitrogen removal rates and different nitrogen transformation pathways. Nitrification-denitrification was the primary route for TN removal in the TFCW at 10 °C due to the proliferation and relatively higher activity of nitrite oxidizing bacteria, causing a low TN removal rate of (16.24 ± 3.01) mg/(L·d). Partial denitrification-ANAMMOX was limited in the TFCW at 5 °C due to restriction in the first step of denitrification; however, a TN removal rate of (25.49 ± 3.03) mg/(L·d) was driven by partial nitrification-ANAMMOX and partial nitrification-denitrification. • Different temperatures led to variations in N conversion of TFCW based on ANAMMOX. • Integrated N removal process of PN-A and PD-A was dominant in TFCWs at 20 and 15 °C. • N-D played a major role in TN removal of TFCW at 10 °C due to overgrowth of NOB. • N conversion in TFCW at 5 °C relied on both the PN-A process and the PN-D process.