Efficient and stable removal of organic matter, nitrogen and phosphorus from swine manure digestate by coupling high-rate activated sludge and single-stage partial nitritation/anammox-hydroxyapatite processes

Organic matter stress restricts the efficiency and broader applicability of anammox process for high-nutrient digestate treatment. A high-rate activated sludge (HRAS) process coupled a single-stage partial nitritation/anammox-hydroxyapatite (PN/A-HAP) process was developed to treat swine manure digestate. The coupled processes achieved stable removal efficiencies of 92 % (N), 63 % (P), and 66 % (chemical oxygen demand, COD), respectively. The HRAS process reduced the five-day biological oxygen demand/COD ratio to 0.31, effectively mitigating the organic stress of anammox process. The synergistic effect of HAP crystallization and external forces induced structural fragmentation of PN/A-HAP granules. The HAP microcrystals in fragmented sludge (<100 μm) function as nucleation cores, driving PN/A-HAP granules reformation. Ca. Kuenenia dominated in the PN/A-HAP microbial community and demonstrated a positive correlation with the saturation index of HAP. This study offers a significant reference for the implementation of the HRAS-PN/A HAP process aimed at achieving COD, N, and P removal.