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A novel two-stage partial nitrification-anammox (PN-A) process was developed, achieving nitrogen removal from low carbon/nitrogen ratio municipal sewage under two typical operational modes and seasonal ambient temperatures. When complete nitritation-anammox was performed at temperatures greater than 19.4 °C, the effluent concentration of total inorganic nitrogen (TIN) was 4.1 mg/L, corresponding to a nitrogen removal efficiency (NRE) of 94.3 %. In contrast, when partial nitritation-anammox was performed at temperatures below 19.4 °C, the effluent TIN was 12.3 mg/L, corresponding to a NRE of 83.6 %. The relative abundance of Nitrosomonas and Nitrosomonadaceae increased from 0.02 % to 0.28 %, while Ca. Brocadia decreased from 1.85 % to 1.30 %, with the contribution of anammox to nitrogen removal being highest under low temperatures (19.4℃ to 13.8℃), at 59.0 %. This novel two-stage PN-A process provides a new approach for the stable operation of wastewater treatment plants (WWTPs) under low ambient temperatures., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

[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]

[Display omitted] • Rapid start-up of partial nitrification in EBPR was attained within 15 days. • Faster recovery of AOB than NOB was observed after simultaneous elutriation. • Improved AOB activity after PO 4 3− uptake facilitated a NO 2 − accumulation ratio of 95.8%. • PAOs and AOB were enriched simultaneously in a stable EBPR-induced PN community. Stable nitritation is the major challenge for short-cut nitrogen removal from municipal wastewater. This paper demonstrated a rapid achievement of partial nitrification (PN) in an enhanced biological phosphorus removal (EBPR) reactor treating domestic wastewater. Polyphosphate accumulating organisms (PAOs) were enriched operated at a short aerobic HRT (2.0 h) and SRT (10 d), with satisfactory phosphorus removal efficiency (95.9%). Both of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were elutriated simultaneously. Interestingly, AOB recovered much faster than NOB by a subsequent extension of aerobic HRT and SRT, resulting in a rapid development of PN within 15 days. Ammonia oxidation rates of AOB significantly increased by 44.2%, facilitating a high nitrite accumulation rate (NAR) of 95.8%. Genus Tetrasphaera, Halomonas, Paracoccus and Candidatus_Accumulibacter belonging to PAOs accounted for 4.6%. The proliferation of heterotrophs, typically as PAOs, maximized the microbial competition against NOB by favoring AOB activity and synergy with functional bacteria. [ABSTRACT FROM AUTHOR]