Enhancing the Stability of Partial Nitritation Anammox in Mainstream Municipal Wastewater Treatment: Biological Foundations, Control Strategies, and Future Perspectives

Hong Wang; Yanzeng Li; Shiyu Liu; Xiaohu Dai

Abstract

Mainstream anammox has attracted much attention as an energy-efficient nitrogen removal technology. However, its stable operation remains challenged by complex mainstream conditions, limiting engineering implementation. This review synthesized the biological foundations, operational control strategies, and engineering practices underlying stable mainstream anammox operation. Process stability depends on effective coupling between ammonia-oxidizing bacteria and anammox bacteria (AnAOB), sustained suppression of nitrite-oxidizing bacteria (NOB), and efficient enrichment and retention of AnAOB. Accordingly, key intensification strategies were highlighted, including fine-tuned dissolved oxygen control, decoupling of hydraulic and solids retention times, reinforcement of biofilm and granular sludge structures, and the use of physical selectors and membrane separation. Application cases demonstrate that, despite proven feasibility, autotrophic nitrogen removal in mainstream anammox remains limited and may decline during long-term operation. Finally, future research directions were proposed, including multi-omics-based elucidation of AnAOB metabolic regulation under low-temperature and low-substrate conditions, quantitative modeling of organic matter and anammox interactions, cross-scale linkages between microscale structure and macroscopic performance, and data-driven intelligent operation and control. This review provides a theoretical basis and practical guidance for the optimization and engineering application of mainstream anammox processes.

References

1.
Qu, J.; Dai, X.; Hu, H.; et al. Emerging Trends and Prospects for Municipal Wastewater Management in China. ACS ES&T Eng. 2022, 2, 323–336.
2.
Qu, J.; Wang, H.; Wang, K.; et al. Municipal wastewater treatment in China: Development history and future perspectives. Front. Environ. Sci. Eng. 2019, 13, 88.
3.
Zhang, D.; Liu, J.; Wang, H.; et al. Advancing carbon-neutral wastewater treatment: Artificial intelligence-driven strategies for emission mitigation and process optimization. Environ. Res. 2026, 290, 123449.
4.
Wu, H.; Li, A.; Gao, S.; et al. The performance, mechanism and greenhouse gas emission potential of nitrogen removal technology for low carbon source wastewater. Sci. Total Environ. 2023, 903, 166491.
5.
Hu, K.; Li, W.; Wang, Y.; et al. Novel biological nitrogen removal process for the treatment of wastewater with low carbon to nitrogen ratio: A review. J. Water Process Eng. 2023, 53, 103673.
6.
Hu, K.; Liu, X.; Jia, X.; et al. Advances in sulphur-iron autotrophic denitrification research and exploration of its future application in wastewater treatment: A critical review. Bioresour. Technol. 2026, 439, 133391.
7.
Zhao, Q.; Peng, Y.; Li, J.; et al. Sustainable upgrading of biological municipal wastewater treatment based on anammox: From microbial understanding to engineering application. Sci. Total Environ. 2022, 813, 152468.
8.
Cao, Y.; Van Loosdrecht, M.C.M.; Daigger, G.T. Mainstream partial nitritation-anammox in municipal wastewater treatment: Status, bottlenecks, and further studies. Appl. Microbiol. Biotechnol. 2017, 101, 1365–1383.
9.
Xu, Y.; Xu, Y.; Li, T.; et al. Two-step partial nitrification-anammox process for treating thermal-hydrolysis anaerobic digester effluent: Start-up and microbial characterisation. J. Clean. Prod. 2020, 252, 119784.
10.
Du, R.; Li, G.; Zhao, R.; et al. Adaptation of pilot-scale one-stage partial nitritation and anammox process to kitchen waste digestion liquid and mature landfill leachate. Bioresour. Technol. 2025, 434, 132820.
11.
Ren, Z.; Wang, H.; Zhang, L.; et al. A review of anammox-based nitrogen removal technology: From microbial diversity to engineering applications. Bioresour. Technol. 2022, 363, 127896.
12.
Strous, M.; Kuenen, J.; Jetten, M. Key Physiology of Anaerobic Ammonium Oxidation. Appl. Environ. Microbiol. 1999, 65, 3248–3250.
13.
Han, N.; Jin, J.; Yang, J.; et al. Polystyrene nanoparticles regulate microbial stress response and cold adaptation in mainstream anammox process at low temperature. J. Hazard. Mater. 2024, 480, 135860.
14.
Liu, Y.; An, T.; Xie, J.; et al. The influencing mechanisms and optimization strategies of organics on anammox process: A critical review. Chem. Eng. J. 2024, 493, 152743.
15.
Wang, L.; Gu, W.; Liu, Y.; et al. Challenges, solutions and prospects of mainstream anammox-based process for municipal wastewater treatment. Sci. Total Environ. 2022, 820, 153351.
16.
Li, S.; Zeng, W.; Jiang, W.; et al. Dual strategies for enhancing anammox predominance in municipal wastewater treatment: Micro-granule-driven multi-pathway nitrite supply and enrichment synergy. Chem. Eng. J. 2025, 524, 169413.
17.
Sethi, S.; Gupta, R.; Sahu, R.; et al. Temperature variations help in-situ anammox self-enrichment in a single-stage partial nitrification-anammox system from unacclimatized biomass. J. Water Process Eng. 2025, 76, 108281.
18.
Ma, X.; Zhang, X.; Sun, Y.; et al. Reason and control strategy for denitrification and anammox sludge flotation in nitrogen removal process: Mechanisms, strategies and perspectives. Environ. Res. 2024, 258, 119456.
19.
Takeda, P.Y.; Paula, C.T.; Borges, A.D.V.; et al. A critical review of the mainstream anammox-based processes in warm climate regions: Potential, performance, and control strategies. J. Environ. Chem. Eng. 2024, 12, 113691.
20.
Chen, Y.; Guo, G.; Li, Y. A review on upgrading of the anammox-based nitrogen removal processes: Performance, stability, and control strategies. Bioresour. Technol. 2022, 364, 127992.
21.
Cao, S.; Koch, K.; Duan, H.; et al. In a quest for high-efficiency mainstream partial nitritation-anammox (PN/A) implementation: One-stage or two-stage? Sci. Total Environ. 2023, 883, 163540.
22.
Wang, H.; Yang, M.; Liu, K.; et al. Insights into the synergy between functional microbes and dissolved oxygen partition in the single-stage partial nitritation-anammox granules system. Bioresour. Technol. 2022, 347, 126364.
23.
Chen, H.; Wang, H.; Yu, G.; et al. Key factors governing the performance and microbial community of one-stage partial nitritation and anammox system with bio-carriers and airlift circulation. Bioresour. Technol. 2021, 324, 124668.
24.
Wu, S.; Gao, F.; Lin, C.; et al. A critical review of oxygen supply and control strategies in single-stage partial nitritation-anammox system for autotrophic nitrogen removal from wastewater. J. Environ. Chem. Eng. 2025, 13, 117613.
25.
Liang, Z.; Han, H.; Yi, J.; et al. Modified integrated fixed-film activated sludge process: Advanced nitrogen removal for low-C/N domestic wastewater. Chemosphere 2022, 307, 135827.
26.
Gong, H.; Ding, J.; Wang, S.; et al. Optimizing granular anammox retention via hydrocycloning during two-stage deammonification of high-solid sludge anaerobic digester supernatant. Sci. Total Environ. 2021, 791, 148048.
27.
Cao, Y.; Kwok, B.H.; Loosdrecht, M.C.M.V.; et al. Mainstream partial nitritation and anammox in a 200,000 m3/day activated sludge process in Singapore: scale-down by using laboratory fed-batch reactor. Water Sci. Technol. 2016, 74, 48–56.
28.
Yuan, Q.; Wang, K.; He, P.; et al. Spontaneous mainstream anammox in a full-scale wastewater treatment plant with hybrid sludge retention time in a temperate zone of China. Water Environ. Res. 2021, 93, 854–864.
29.
Ni, B.; Yuan, Z. A model-based assessment of nitric oxide and nitrous oxide production in membrane-aerated autotrophic nitrogen removal biofilm systems. J. Membr. Sci. 2013, 428, 163–171.
30.
Hellinga, C.; Schellen, A.A.J.C.; Mulder, J.W.; et al. The sharon process: An innovative method for nitrogen removal from ammonium-rich waste water. Water Sci. Technol. 1998, 37, 135–142.
31.
Yang, Q.; Shen, N.; Lee, Z.M.P.; et al. Simultaneous nitrification, denitrification and phosphorus removal (SNDPR) in a full-scale water reclamation plant located in warm climate. Water Sci. Technol. 2016, 74, 448–456.
32.
Wang, W.; Zhou, S.; Ye, M.; et al. Characterization of nitrogen transformation and microbial interactions of floc sludge and biofilms in single-stage gas-lift partial nitritation/anammox reactor with hollow cylindrical carriers. Environ. Res. 2025, 279, 121729.
33.
Liu, T.; Yu, L.; Liao, C.; et al. Integrated fixed-film activated sludge partial nitritation-anammox (IFAS-PN/A) for landfill leachate treatment rapidly established through the thermal inactivation of nitrite oxidoreductase. Environ. Res. 2025, 285, 122548.
34.
Xiong, X.; Jiang, J.; Yu, H.; et al. Achieving rapid granulation and long-term stability of partial nitritation /anammox process by uniquely configured airlift inner-circulation partition bioreactor. Bioresour. Technol. 2025, 428, 132474.
35.
Wang, H.; Yang, D.; Chen, Y.; et al. Improving the anammox performance in municipal wastewater treatment based on the powder functional carriers: A critical review. Chem. Eng. J. 2023, 470, 144167.
36.
Huang, D.; Fu, J.; Li, Z.; et al. Inhibition of wastewater pollutants on the anammox process: A review. Sci. Total Environ. 2022, 803, 150009.
37.
Chen, Y.; Feng, G.; Tamaishi, M.; et al. Low temperature impact and HAP-enhanced one-stage partial nitritation/anammox process: Long-term stability, high-rate nitrogen removal, and operational strategies at 15 °C. Water Res. X 2025, 28, 100340.
38.
Li, J.; Wang, H.; Li, Z.; et al. Enhanced nitritation through bubbleless aeration-promoted AOB growth and environmental selective pressures-induced NOB suppression in membrane aerated biofilm reactors. Chem. Eng. J. 2025, 507, 160519.
39.
Li, J.; Zhang, L.; Peng, Y.; et al. NOB suppression in partial nitritation-anammox (PNA) process by discharging aged flocs: Performance and microbial community dynamics. Chemosphere 2019, 227, 26–33.
40.
Kent, T.R.; Sun, Y.; An, Z.; et al. Mechanistic understanding of the NOB suppression by free ammonia inhibition in continuous flow aerobic granulation bioreactors. Environ. Int. 2019, 131, 105005.
41.
Jiang, H.; Liu, G.; Ma, Y.; et al. A pilot-scale study on start-up and stable operation of mainstream partial nitrification-anammox biofilter process based on online pH-DO linkage control. Chem. Eng. J. 2018, 350, 1035–1042.
42.
Liu, T.; Li, X.; Wang, Y.; et al. Synergistic control of SRT and DO in independent zoned sludge systems: A model-guided PDA-PN/A process achieving low-carbon nitrogen removal in municipal wastewater environment. Chem. Eng. J. 2025, 521, 166841.
43.
Wang, D.; Wang, Q.; Laloo, A.; et al. Achieving Stable Nitritation for Mainstream Deammonification by Combining Free Nitrous Acid-Based Sludge Treatment and Oxygen Limitation. Sci. Rep. 2016, 6, 25547.
44.
Liu, Y.; Ngo, H.H.; Guo, W.; et al. The roles of free ammonia (FA) in biological wastewater treatment processes: A review. Environ. Int. 2019, 123, 10–19.
45.
Peng, S.; Yang, D.; Liang, Z.; et al. Effect of influent C/N ratios on high-concentration powder carrier bio-fluidized bed (HPB) process: Performance, sludge characteristics, and microbial community. J. Water Process Eng. 2024, 62, 105375.
46.
Chen, R.; Ji, J.; Chen, Y.; et al. Successful operation performance and syntrophic micro-granule in partial nitritation and anammox reactor treating low-strength ammonia wastewater. Water Res. 2019, 155, 288–299.
47.
Pijuan, M.; Ribera-Guardia, A.; Balcázar, J.L.; et al. Effect of COD on mainstream anammox: Evaluation of process performance, granule morphology and nitrous oxide production. Sci. Total Environ. 2020, 712, 136372.
48.
Li, C.; Liu, Q.; Fan, J.; et al. Metagenomics-based interpretation of selective bioaugmentation promoting partial-denitrification coupling with anammox process reactivation in suspended sludge system. Chem. Eng. J. 2023, 454, 139977.
49.
Zhao, Q.; Li, X.; Zhang, L.; et al. Partial denitrifying phosphorus removal coupling with anammox (PDPRA) enables synergistic removal of C, N, and P nutrients from municipal wastewater: A year-round pilot-scale evaluation. Water. Res. 2024, 253, 121321.
50.
Chen, H.; Liu, K.; Yang, E.; et al. A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal. Sci. Total Environ. 2023, 857, 159462.
51.
Xiao, R.; Ni, B.; Liu, S.; et al. Impacts of organics on the microbial ecology of wastewater anammox processes: Recent advances and meta-analysis. Water Res. 2021, 191, 116817.
52.
Wang, H.; Chen, H.; Chen, Y.; et al. Revealing the response of community structure and metabolic pathway to varying organic matter stress in a dissolved oxygen-differentiated airlift internal circulation partial nitritation-anammox system. Sci. Total Environ. 2023, 886, 164002.
53.
Zheng, Z.; Huang, S.; Bian, W.; et al. Enhanced nitrogen removal of the simultaneous partial nitrification, anammox and denitrification (SNAD) biofilm reactor for treating mainstream wastewater under low dissolved oxygen (DO) concentration. Bioresour. Technol. 2019, 283, 213–220.
54.
Chen, R.; Takemura, Y.; Liu, Y.; et al. Using Partial Nitrification and Anammox to Remove Nitrogen from Low-Strength Wastewater by Co-immobilizing Biofilm inside a Moving Bed Bioreactor. ACS Sustain. Chem. Eng. 2019, 7, 1353–1361.
55.
Gu, X.; Wang, F.; Ding, Y.; et al. Two-stage partial nitritation/Anammox (PN/A) for mainstream wastewater treatment: Achieving high nitrogen removal at elevated loading rates under low-temperature conditions. J. Environ. Manag. 2026, 397, 128265.
56.
Qian, F.; Cui, S.; Liu, F.; et al. Effect of hydraulic selection pressure on the characteristics of partial nitritation/anammox granular sludge in a continuous-flow reactor. Environ. Technol. Innov. 2021, 24, 102042.
57.
Awata, T.; Goto, Y.; Kuratsuka, H.; et al. Reactor performance and microbial community structure of single-stage partial nitritation anammox membrane bioreactors inoculated with Brocadia and Scalindua enrichment cultures. Biochem. Eng. J. 2021, 170, 107991.
58.
Chen, H.; Wang, H.; Chen, R.; et al. Unveiling performance stability and its recovery mechanisms of one-stage partial nitritation-anammox process with airlift enhanced micro-granules. Bioresour. Technol. 2021, 330, 124961.
59.
Gholami-Shiri, J.; Azari, M.; Dehghani, S.; et al. A technical review on the adaptability of mainstream partial nitrification and anammox: Substrate management and aeration control in cold weather. J. Environ. Chem. Eng. 2021, 9, 106468.
60.
Liu, L.; Wu, J.; Sun, H.; et al. Combination of sludge free nitrous acid treatment in the presence of ammonium with intermittent aeration for rapid control of nitrate production in mainstream partial nitritation/anammox processes. J. Environ. Chem. Eng. 2025, 13, 119400.
61.
Wei, Y.; Xia, W.; Ye, M.; et al. Optimizing hydraulic retention time of high-rate activated sludge designed for potential integration with partial nitritation/anammox in municipal wastewater treatment. Bioresour. Technol. 2024, 401, 130710.
62.
Podmirseg, S.M.; Gómez-Brandón, M.; Muik, M.; et al. Microbial response on the first full-scale DEMON® biomass transfer for mainstream deammonification. Water. Res. 2022, 218, 118517.
63.
Wang, J.; Zhou, J.; Feng, Y.; et al. Construction of acyl-homoserine lactone-producing engineered bacteria for activating low-temperature anammox process. Environ. Res. 2026, 290, 123478.
64.
Huang, K.; He, Y.; Wang, W.; et al. Temporal differentiation in the adaptation of functional bacteria to low-temperature stress in partial denitrification and anammox system. Environ. Res. 2024, 244, 117933.
65.
Gong, Q.; Zeng, W.; Meng, Q.; et al. Integration of mixotrophic denitrification and anammox in Thiothrix-hydroxyapatite coupled particles treating municipal wastewater against low temperature and organic load shocks. Chem. Eng. J. 2024, 497, 154577.
66.
Chen, Y.; Zhang, C.; Chen, Z.; et al. Achieving nitrite shunt using in-situ free ammonia enriched by natural zeolite: Pilot-scale mainstream anammox with flexible nitritation strategy. Water Res. 2024, 265, 122314.
67.
Guo, Y.; Sanjaya, E.H.; Rong, C.; et al. Treating the filtrate of mainstream anaerobic membrane bioreactor with the pilot-scale sludge-type one-stage partial nitritation/anammox process operated from 25 to 15 °C. Bioresour. Technol. 2022, 351, 127062.
68.
Rong, C.; Luo, Z.; Wang, T.; et al. Biomass retention and microbial segregation to offset the impacts of seasonal temperatures for a pilot-scale integrated fixed-film activated sludge partial nitritation-anammox (IFAS-PN/A) treating anaerobically pretreated municipal wastewater. Water Res. 2022, 225, 119194.
69.
Pedrouso, A.; Morales, N.; Rodelas, B.; et al. Rapid start-up and stable maintenance of the mainstream nitritation process based on the accumulation of free nitrous acid in a pilot-scale two-stage nitritation-anammox system. Sep. Purif. Technol. 2023, 317, 123851.
70.
Cao, Y.; Kwok, B.H.; van Loosdrecht, M.C.M.; et al. The occurrence of enhanced biological phosphorus removal in a 200,000 m3/day partial nitration and Anammox activated sludge process at the Changi water reclamation plant, Singapore. Water Sci. Technol. 2017, 75, 741–751.
71.
Zhang, S.; Li, Q.; Gu, P.; et al. Full-scale mainstream partial nitritation/anammox process: A three-year demonstration for municipal wastewater treatment. Water Res. X 2025, 29, 100419.
72.
Li, J.; Peng, Y.; Zhang, L.; et al. Quantify the contribution of anammox for enhanced nitrogen removal through metagenomic analysis and mass balance in an anoxic moving bed biofilm reactor. Water Res. 2019, 160, 178–187.
73.
Zheng, M.; Li, H.; Duan, H.; et al. One-year stable pilot-scale operation demonstrates high flexibility of mainstream anammox application. Water Res. X 2023, 19, 100166.
74.
Cao, Y.; Kwok, B.H.; van Loosdrecht, M.C.M.; et al. The influence of dissolved oxygen on partial nitritation/anammox performance and microbial community of the 200,000 m3/d activated sludge process at the Changi water reclamation plant (2011 to 2016). Water Sci. Technol. 2018, 78, 634–643.
75.
Izadi, P.; Sinha, P.; Andalib, M.; et al. Coupling fundamental mechanisms and operational controls in mainstream partial denitrification for partial denitrification anammox applications: A review. J. Clean. Prod. 2023, 400, 136741.
76.
Gilbert, E.M.; Agrawal, S.; Schwartz, T.; et al. Comparing different reactor configurations for Partial Nitritation/Anammox at low temperatures. Water Res. 2015, 81, 92–100.
77.
Im, H.; Jeong, S.; Kim, H.; et al. Mainstream application of the partial denitrification-anammox process for carbon-neutral wastewater treatment: A review. J. Water Process Eng. 2025, 72, 107558.
78.
Wu, Z.; Mu, J.; Li, X.; et al. A strategy for starting and controlling nitritation-denitrification in an SBR with DO and ORP online monitoring signals. Desalin Water Treat. 2019, 151, 365–371.
79.
Wen, X.; Gong, B.; Zhou, J.; et al. Efficient simultaneous partial nitrification, anammox and denitrification (SNAD) system equipped with a real-time dissolved oxygen (DO) intelligent control system and microbial community shifts of different substrate concentrations. Water Res. 2017, 119, 201–211.
80.
Ai, W.; Shang, J.; Wang, Y.; et al. Constructing a robust mainstream Anammox process via cold shock adaptation: Nitrogen removal performance and microbial community. J. Environ. Chem. Eng. 2025, 13, 119443.
81.
Wan, J.; Wang, X.; Ren, Y.; et al. Physiological adaptations of anammox bacteria to cold stress in wastewater treatment: Bioaugmentation and process intensification strategies. J. Environ. Chem. Eng. 2025, 13, 118550.
82.
Chen, H.; Tu, Z.; Wu, S.; et al. Recent advances in partial denitrification-anaerobic ammonium oxidation process for mainstream municipal wastewater treatment. Chemosphere 2021, 278, 130436.
83.
Yan, H.; Du, G.; Sun, F.; et al. Rapid enrichment strategies for anaerobic ammonium oxidation (anammox) bacteria: A critical review. Process Saf. Environ. 2025, 204, 108120.
84.
Ma, J.; Sun, H.; Ji, Y.; et al Niche differentiation drives microbial community assembly in an anaerobic/oxic/anoxic-aerobic granular sludge (AOA-AGS) system: Insights into Anammox self-enrichment. Water Res. 2026, 288, 124615.
85.
Du, Y.; Lei, T.; Jin, J.; et al. Harnessing machine learning for energy optimization and intelligent process control in wastewater treatment. J. Water Process Eng. 2025, 80, 109210.
86.
Fisher, O.J.; Wang, Y.; Ahmed, A. Making waves: Transforming biofilm-based wastewater treatment using machine learning-driven real-time monitoring. Water Res. 2025, 287, 124491.

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