Calcium-driven adaptive microbial succession and metabolic shift towards chemical-biological synergistic phosphorus removal in anaerobic-aerobic-anoxic-hydroxyapatite (AOA-HAP) system.

Coupling chemical precipitation with biological processes enhances phosphorus (P) removal, yet the metabolic response of anaerobic-aerobic-anoxic (AOA) process to calcium (Ca2+) regulation remains unclear. This study investigated a novel AOA-hydroxyapatite (AOA-HAP) system under gradient Ca2+/P ratios (0, 7, 10, 20) to elucidate these chemical-biological synergistic mechanisms. Rather than merely acting as a precipitant, elevated Ca2+ induced adaptive microbial responses, substantially enhancing sludge granulation and settleability by upregulating the secretion of structural extracellular polymeric substances (EPS). Crucially, high-Ca2+ stress triggered a profound ecological niche shift. Although polyphosphate-accumulating organisms (PAOs) were partially inhibited, chemical phosphorus removal (contributing up to 91.71 %) seamlessly compensated for the biological decline. Meanwhile, the system sustained robust endogenous denitrification by enriching denitrifying glycogen-accumulating organisms (DGAOs: 11.91 %) and organic-degrading bacteria (ODB: 12.87 %). Ultimately, the system achieved excellent P (96.39 %) and nitrogen (85.32 %) removals. Multi-factor mathematical fitting further established an optimal Ca2+/P ratio of 8.8---17.8. These findings provide vital insights for optimizing resilient AOA-HAP systems for stable nutrient removal and P recovery.