Upgrading the two-stage partial nitritation/anammox process: high-rate partial nitritation with hydroxyapatite-enhanced anammox granular sludge

Abstract

Two-stage partial nitritation/anammox (PN/A) provides a low-energy, low-carbon pathway for nitrogen removal. In recent years, high-rate partial nitritation coupled with hydroxyapatite (HAP)-enhanced anammox granules has attracted growing attention. This review summarizes current progress and outlines future directions for this process. We first examine the fundamentals of PN and anammox separately, covering reaction stoichiometry and kinetics alongside microbial ecology and practical operating windows. We then discuss the role of HAP in granulation and biomass stabilization, clarifying the proposed mechanisms (e.g., nucleation/templating, Ca–P surface chemistry, and EPS interactions) and the rationale for selecting the HAP-enhanced granules. Next, we compile laboratory and pilot evidence under low-temperature and real-wastewater matrices and assess two-stage coupling, with emphasis on inter-stage load balance and control strategies that deliver the desired NO₂⁻-N : NH₄⁺-N ratio to the anammox stage. Finally, we identify key limitations and future needs, which center on the PN step and end-to-end process engineering: robust NOB control with sustained AOB activity at low temperature; predictive, site-transferable approaches for community composition design and process control; and full-chain integration, including pretreatment for complex industrial wastewaters and adaptive cascade control. Overall, this review provides a comprehensive, systematic summary of advantages and research gaps to guide future study and implementation of high-rate PN with HAP-enhanced anammox.