Synchronous achievement of rapid self-induced anammox and advanced nitrogen removal from low C/N wastewater via gas-permeable membrane-mediated aeration system

Anammox process is a promising biological nitrogen removal (BNR) technology that offers substantial energy and resource savings. Despite widespread application of anammox for low carbon-to-nitrogen (C/N) ratio wastewater treatment, research on self-induced anammox under complex wastewater conditions remains limited, especially regarding its integration with the shortcut BNR process (partial nitrification (PN)/denitritation) in a single-stage system. Herein, we pioneer a strategy for achieving rapid, self-induced anammox via introducing gas-permeable membrane-mediated aeration in the treatment of mature landfill leachate (C/N ratio = 0.92 ± 0.05). The nitrogen removal efficiency reached 82.1 ± 3.3 %, with PN/anammox and PN/denitritation pathways contributing to approximately 75 % and 25 %, respectively, based on ¹⁵N-stable isotope tracing and mass balance analysis. Notably, the self-induced anammox process rapidly occurred within 34 days, and the absolute and relative abundances of Candidatus_Kuenenia increased by 65.8-fold and 9.2-fold, respectively, compared to the inoculum, by day 160. The rapid onset of self-induced anammox and efficient nitrogen removal was driven by two key mechanisms: (i) the stable nitritation controlled by sequencing batch operational pattern and (ii) the optimized biofilm stratification induced by membrane aeration. Specifically, Nitrosomonas and Candidatus_Kuenenia individually occupied the inner layer and outer layer of biofilms, respectively, effectively alleviating oxygen inhibition on anammox metabolism. Further, the biofilm provided dual protective benefits for Candidatus_Kuenenia via enhanced biomass retention and sequestration of toxic organics (e.g., humic acid), greatly improving the resilience of this single-stage anammox system. This study provides a efficient and sustainable approach for accelerating self-induced anammox and deciphers mechanisms of the robust cooperation between anammox and the shortcut BNR process in counter-diffusional biofilm, paving the way for large-scale implementation of this sustainable technology in complex, ammonium-rich wastewater treatment.