Superior nitrogen removal and sludge reduction in a suspended sludge system with in-situ enriching anammox bacteria for real sewage treatment

Highlights: • A/O/A operational mode favors in-situ enrichment of anammox bacteria. • TIN removal efficiency was 94% with influent and effluent TN of 61 and 3.4 mg/L. • External sludge reduction rate of 39% was achieved in the AOA-SBR system. • Oxic SNDA and anoxic anammox contributed 35% and 35% of TIN removal, respectively. • Low DO (0.6 mg/L) and short HRT_oxic (6.5 h) reduced energy consumption. Mainstream partial anammox provides a cost- and energy-efficient alternative for wastewater treatment. This study provided a new strategy to achieve mainstream partial anammox in a single-stage suspended sludge system. The novel method coupling external excess sludge fermentation with simultaneous partial nitritation-anammox-denitrification process (SF-SPNAD) was established for 202 days in an anaerobic-aerobic-anoxic sequencing batch reactor (AOA-SBR) with real sewage and actual sludge fermentation products. Under the condition of low DO (0.6 ± 0.2 mg/L), short oxic and long anoxic hydraulic retention time (HRT_oxic = 6.5 h, HRT_anoxic = 8 h), the average total inorganic nitrogen (TIN) concentration in the influent and effluent during 110-day operation were 61.0 and 3.4 mg/L, respectively, and the TIN removal efficiency was 94.56%. Under the inhibitory effect of continuous sludge fermentation products addition, nitrite accumulation ratio reached 99.1% and the external sludge reduction ratio reached 38.75%. ¹⁵N-stable isotope tracing tests showed the great potential of nitrogen removal by anammox pathway in the system. High-throughput sequencing confirmed that Candidatus Brocadia (not detected to 0.50%) and Candidatus Kuenenia (not detected to 0.06%) were successfully in-situ enriched. Nitrogen conversion pathways based on stoichiometry and cycle tests show that 34.69% of the TIN removal was obtained by simultaneous nitritation denitrification and anammox under oxic stage and 35.21% of the TIN removal was carried out by anammox under anoxic stage. Overall, the SF-SPNAD process provides a new possibility for coupling autotrophic and heterotrophic nitrogen removal with excess sludge utilization.