In Situ Rapid Start-Up and Enhanced Nutrient Removal in a Coupled Partial Nitritation/Anammox and Denitratation/Anammox System: Insights into Multivariate Drivers of Process Performance

Anammox-based processes are energy- and cost-efficient for wastewater nitrogen removal, but practical application is limited by unavoidable nitrate production, slow start-up, and instability under fluctuating influent conditions. Here, a novel hydroxyapatite-enhanced coupled partial nitritation and anammox (PNA) and partial denitratation and anammox (PDA) system was developed for simultaneous high-rate nutrient removal and phosphorus recovery. Compared with a standalone PNA reactor, the integrated system increased nitrogen and phosphorus removal efficiencies from 77% and 76% to 91% and 85%, respectively, achieving a nitrogen removal rate of 2.8 ± 0.2 kg N/m 3 /d. The reuse of discharged PNA sludge facilitated the operation and early establishment of the PDA reactor, supporting rapid system development and stable performance. Notably, anammox ultimately contributed over 95% of total nitrogen removal, confirming highly efficient autotrophic nitrogen conversion. A trade-off between maximizing anammox contribution and overall PDA removal efficiency was identified. Mechanistically, multivariate analysis revealed that nitrogen species composition, reactor interactions, and carbon supply jointly governed system performance. This study demonstrates a robust and energy-efficient integrated anammox-based system and provides mechanistic and operational insights for advanced nutrient removal and resource-oriented wastewater treatment.