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[Display omitted] • Ultra-high nitrogen removal efficiency (NRE) of 95.5% at C/N of 2.7 was achieved. • Lower NAR (range 40 %∼70 %) instead resulted in better anammox contribution and NRE. • High-efficiency EPDA with less NO X - loss dominated in ammonium removal than PN/A. • Enriching NOB and GAOs in PN/A process instead favored co-metabolism stability. • The nitrate introduction effectively attenuated the competition of ED for nitrite. Higher nitrite accumulation, which is challenging to achieve reliably, is always sought to obtain better nitrogen removal performance in traditional partial nitritation-anammox (PN/A) process. This study developed a modified PN/A process by introducing nitrite-oxidizing bacteria and endogenous metabolism. Advanced nitrogen removal performance of 95.5 % was achieved at a low C/N ratio of 2.7 under nitrite accumulation ratio (NAR) fluctuations. Higher nitrate accumulation at lower NAR (70 ∼ 40 %) resulted in superior anammox contribution (60 ∼ 75 %) and nitrogen removal performance (93 ∼ 98 %). This was attributed to the higher nitrogen removal efficiency of the post-anoxic endogenous partial denitrification coupling anammox process, although the PN/A process occurring first possessed a faster anammox rate of 2.0 mg NH 4 +-N /(g VSS⋅h). The introduction of nitrate allowed more nitrite flow to anammox, promoting a high enrichment of anammox bacteria (Ca. Brocadia, 0.3 % to 2.8 %). This study provides new insights into the practical application of the PN/A process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Complete nitrogen removal was achieved treating actual low C/N (2.8) sewage. • The double nitrite shunt from PN and EPD guaranteed the thorough anammox reaction. • ED and DPR pathways jointly contributed 37% to TIN removal via nitrate respiration. • Coexistence of enriched AnAOB, GAOs and DPAOs favored co-metabolism stability. • Directional transfer of electrons and substrates radically supported superior performance. Autotrophic nitrogen removal processes based on anammox are facing huge challenges of high effluent quality in wastewater treatment. In this study, a novel hybrid system of anammox coupled with heterotrophic endogenous metabolisms was developed in a single-stage bioreactor to treat real domestic wastewater. Complete nitrogen removal was achieved at a low carbon to nitrogen (C/N) ratio of 2.8 and more than 95.2% nitrogen removal was maintained under an extremely low C/N ratio of 1.9. The nitrite shunt supplied by dual partial pathways (partial nitritation and endogenous partial denitratation) guaranteed the anammox activity with a high contribution to nitrogen removal above 60%. While endogenous denitrification (ED) and denitrifying phosphorus removal (DPR) both participated in nitrate respiration, contributing more than 28% and 9% to nitrogen removal, respectively. The coexistence and enrichment of anammox bacteria (Ca. Brocadia, 0.9%), glycogen accumulating organisms (Ca. Competibacter, 6.4%) and phosphorus accumulating organisms (Dechloromonas , 3.9% and Ca. Accumulibacter, 0.7%) contributed to the co-metabolism of carbon, nitrogen and phosphorus. The functional genes encoding the above metabolism were highly correlated with the dominant strains, indicating that the multiple pathways of carbon and nitrogen metabolism supported the continuous supply of electrons required for nitrogen removal. This study puts forward a novel complete nitrogen removal process by successfully coupling autotrophic and endogenous heterotrophic metabolism. [ABSTRACT FROM AUTHOR]

[Display omitted] • EPD can synergize with PNA or even provide NO 2 − alone during PN deterioration. • The EPD capability of GAOs can further benefit the stabilization of PNA process. • Anammox contributed 80% to the excellent nitrogen removal of 92% with all NO 2 − supplied by EPD. • Outstanding anammox stability was obtained via a flexible NO 2 − supply by EPD. • Coexistence of enriched GAOs (6.0%, EPD) and anammox bacteria (1.5%) favored anammox stability. The application of partial nitritation-anammox (PNA) process suffers severe obstacles due to the instability of partial nitritation (PN) process. This study sought to evaluate the feasibility of endogenous partial denitratation (EPD) as a remediation or alternative to supply nitrite for the unstable PNA process. A novel strategy of optimal organics utilization through pre-anaerobic carbon storage and post-anoxic endogenous denitrification coupled with anammox was developed in a single-stage bioreactor treating actual municipal wastewater with low C/N (∼3.2). Specifically, the undesired NO 2 −/NH 4 + ratio (2.4 to 0.04) and nitrate accumulation were obtained by increasing the aeration rate (0.6 to 1.8 L/min) to simulate the PN instability. Delightedly, advanced nitrogen removal efficiency (92.1%) was maintained despite a dramatic decrease in nitrite accumulation ratio from 97.6% to 2.6%. This was attributed to the significant increase in anammox contribution to total nitrogen removal from 30.2% to 80.5%. The steady nitrite flux supplied from EPD coupled with PN (EPD contribution increased from 0 to 97.0%) was assumed to be the main reason for the continually increasing abundance and bioactivity of anammox bacteria. Both the anammox bacteria (1.5%, Ca. Brocadia) and glycogen accumulating organisms (6.0%, responsible for EPD) were enriched and coexisted stably in the single reactor. Our study confirms that coupling EPD with anammox has great potential as a remediation for the unstable mainstream PNA process. [ABSTRACT FROM AUTHOR]

• Excellent NRE of 97.5% was achieved with TIN below 2.0mg/L by selective enhanced GAOs. • Intermittent phosphorus exclusion with poly-P/VSS below 0.01(w/w) enhanced GAOs. • GAOs gained organics outcompeting PAOs at low poly-P and strengthened ED. • PNA was unaffected by the enhanced ED and coupled to remove nitrogen in the anoxic stage. • EPDA was promoted by the enrichment of GAOs that reduced NO 3 − to NO 2 − exclusively. Integrating endogenous denitrification (ED) into partial nitrification-anammox (PNA) systems by adequately utilizing organics in municipal wastewater is a promising approach to improve nitrogen removal efficiency (NRE). In this study, a novel strategy to inhibit phosphorus-accumulating organisms (PAOs) by inducing phosphorus release and exclusion was adopted intermittently, optimizing organics allocation between PAOs and glycogen-accumulating organisms (GAOs). Enhanced ED-synergized anammox was established to treat real municipal wastewater, achieving an NRE of 97.5±2.2% and effluent total inorganic nitrogen (TIN) of less than 2.0 mg/L. With low poly-phosphorus (poly-P) levels (poly-P/VSS below 0.01 (w/w)), glycogen accumulating metabolism (GAM) acquired organics exceeded that of phosphorus accumulating metabolism (PAM) and dominated endogenous metabolism. Ca. Competibacter (GAO) dominated the community following phosphorus-rich supernatant exclusion, with abundance increasing from 3.4% to 5.7%, accompanied by enhanced ED capacity (0.2 to 1.4 mg N/g VSS /h). The enriched subgroups (GB4, GB5) of Ca. Competibcate r established a consistent nitrate cycle with anammox bacteria (AnAOB) through endogenous partial denitrification (EPD) at a ∆NO 2 −-N/∆NH 4 +-N of 0.91±0.11, guaranteeing the maintenance of AnAOB abundance and performance. These results provide new insights into the flexibility of PNA for the energy-efficient treatment of low-strength ammonium wastewater. [Display omitted] [ABSTRACT FROM AUTHOR]