Spatial Evolution and Dual-Mode Mechanism of Calcification in the CANDAN Granular Sludge

The Complete Ammonium and Nitrate Removal via Denitratation–Anammox over Nitrite (CANDAN) process offers a low-carbon route for simultaneous ammonium and nitrate removal, yet elevated calcium commonly introduced during upstream treatment can trigger granule calcification and undermine system stability. This study elucidates the spatial evolution and mechanistic basis of calcium-induced mineralization in CANDAN granules under long-term Ca2+ exposure. Despite partial calcium reduction in the influent, sustained surface precipitation of hydroxyapatite (HAP) led to a progressive ammonium removal decline and eventual reactor failure. Multiscale characterization (scanning electron microscopy (SEM)–energy-dispersive spectroscopy (EDS), microcomputed tomography (μ-CT), and X-ray diffraction (XRD)) revealed a dual-mode calcification pathway governed by two spatially distinct alkalinity centers: acetate-driven denitrification at the surface and anammox-mediated alkalinity in the core. Their concurrent activity produced both external HAP shell formation and internal nucleation, generating a trilayered structure that imposed severe diffusion constraints and was closely associated with progressive deterioration of coupled nitrogen conversion and reactor performance. These mechanistic inferences are based on performance trends and multiscale structural evidence, rather than direct microscale concentration or pH profiling. The proposed dual-alkalinity-center model provides a foundation for managing calcification in CANDAN systems and offers broader implications for predicting mineralization dynamics in other calcium-rich granular sludge processes.