Understanding Partial Denitrification-hydroxyapatite (PD-HAP) coupled granules to enhance process stability and phosphorus removal by investigating the size effect of granules

Abstract

The innovative Partial Denitrification-hydroxyapatite (PD-HAP) coupled granule offers a promising biotechnology for supplying nitrite to Anammox while simultaneously removing phosphorus. This study systematically examined the characteristics, biomass activity, and microbial community of PD-HAP granules of various sizes to enhance system stability and phosphorus removal efficiency. The results showed that larger granules had higher inorganic mineral and extracellular polymeric substances (EPS) content. These factors contributed to their denser structure, smoother surfaces, and better settling velocities compared to smaller granules. In contrast, smaller granules exhibited higher denitrification activity due to their greater biomass content and more efficient mass transfer. Calcium and phosphorus were identified as the primary inorganic constituents, mainly in the form of HAP, and were especially abundant in larger granules. Granules larger than 3.0 mm demonstrated a higher capacity for phosphorus removal. This was due to their significantly greater tightly bound EPS (T-EPS) content, which led to increased HAP and phosphorus levels. All granule sizes maintained a nitrate-to-nitrite transformation ratio (NTR) above 80 %, primarily due to the dominance of Flavobacterium (65.8 %-74.9 %). Although larger granules had lower denitrification activity, they still achieved an excellent NTR (above 85 %). They also showed superior settling velocity (174.9 m/h) and stronger phosphorus removal capacity (54.1 mg/g SS) compared to smaller granules. Maintaining a dominant proportion of granules larger than 3.0 mm could be an effective strategy to enhance both system stability and phosphorus removal. This study provides valuable insights into the PD-HAP coupled granular process, promoting efficient nitrite production and effective phosphorus removal.