Achieving high-rate autotrophic nitrogen removal via Canon process in a modified single bed tidal flow constructed wetland

•High-rate N removal is achieved via Canon route in a modified TFCW.•Recirculation frequency can be manipulated to control oxygen supply in the TFCW.•Setup of a saturated zone benefits the stability of the Canon route.•High IC provides a strong selective pressure to trigger the Canon pathway.

This study attempts to explore the Complete Autotrophic Nitrogen removal Over Nitrite (Canon) process to enhance the nitrogen removal in constructed wetlands (CWs). To achieve a stable and high-rate nitrogen removal via Canon, a single stage tidal flow constructed wetland (TFCW) system was modified by adopting internal upflow recirculation, shorter unsaturated time, and creating a pre-saturated zone. It was found that reduction of recirculation reduced oxygen supply, which favored partial nitrification and anammox process but decreased the overall nitrogen conversion rate. On the contrary, increasing the recirculation cycles enhanced the overall ammonium oxidization, but partial nitrification and anammox process could no longer be maintained. The setup of a pre-saturated zone in CW was beneficial to maintaining the stability of the Canon route. The effect of inorganic carbon (IC) on Canon was tested and the results revealed that high inorganic carbon (IC) could promote both partial nitrification and anammox activities but had no substantial effect on nitrite oxidization, which provided a strong selective method to maintain the Canon pathway. By integrating the control of oxygen supply (internal recirculation and pre-saturated zone) and high influent IC concentration, this study finally achieved total inorganic nitrogen (TIN) removal of >80% under high nitrogen loading rate of 15 g N/m2/d from long-term stored livestock wastewater, which was characterized as having extremely low BOD5/TN ratio of <0.2.

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