Performance stability and recovery strategy of pre-denitrification coupled partial nitritation/anammox-hydroxyapatite processes for enhanced organic matter, nitrogen and phosphorus removal from swine manure digestate

Achieving efficient treatment of high concentrations of NH₄⁺-N and organic matter, while addressing NO₃^--N residue, poses a challenge for the Anammox process. Herein, a novel two-stage pre-denitrification coupled partial nitritation/anammox-hydroxyapatite (PN/A-HAP) process was developed for treating swine manure digestate. The pre-denitrification reactor eliminated the inhibition of organic matter and NH₄⁺-N on anammox bacteria while converting NO_x^- to N₂. Furthermore, it enhanced PN/A-HAP particles, achieving a mean particle size of 1184 μm along and an excellent settling velocity of 2.6 m/h. Short-term exposure to a low Ca/P ratio of 0.17 led to the initial fragmentation of PN/A-HAP particles, while subsequent fragmentation caused by the inhibition of anammox bacteria and shear stress ultimately resulted in the collapse of the two-stage process. A strategy involving the addition of mature Anammox-HAP sludge demonstrated a restoration of process stability. This study advocates for the application of the anammox process in swine wastewater treatment.