The successful integration of anammox to enhance the operational stability and nitrogen removal efficiency during municipal wastewater treatment

• The stable anammox performance enhanced the stability of nitrogen removal. • Synergize of endogenous denitrification and anammox was critical to 87.1% NRE. • AnAOB enriched from 4.25×10 6 and remained at 3.1±0.9×10 8 copes/gSS. • Anammox activity was maintained despite temperature decreasing from 28 to 17 ℃. • The anammox contribution to nitrogen removal was above 50% under varied C/N ratio. The anaerobic ammonium oxidation (anammox) process is a promising method for carbon reduction and energy conservation. However, the advantages of its application in municipal wastewater treatment have not been completely revealed. Stable nitrogen removal was successfully achieved in the anaerobic/aerobic/anoxic (AOA) process by directly integrating stable anammox. Over a 200+ day operating period, the abundance of anammox bacteria (3.1 ± 0.9 × 10 8 copies/gSS) and anammox activity (0.32 ± 0.07 mgN/(gSS·h)) remained high, despite a temperature decrease (27.8-17.2°C) and fluctuating carbon-nitrogen (C/N) ratio (1.6-5.0). A stable nitrogen removal efficiency of 87.1 ± 2.5% was successfully maintained, because the anammox activity compensated for the effect of low temperatures and low C/N ratios on the nitrification and denitrification. Further analysis found that anammox accounted for 52.6%-57.5% of N 2 production, and the rest was through nitrification-denitrification. The stable anammox reaction was mainly due to the limitation of oxygen and carbon sources and the stable supply of nitrite by endogenous denitrification. Overall, these findings confirmed that the stability and efficiency of nitrogen removal from municipal wastewater could be increased by integrating anammox under appropriate conditions.