Transient calcium crusts in PD-HAP granules: Mechanisms and self-regulatory recovery

Calcium precipitation in granular sludge frequently threatens the stability and efficiency of biological nitrogen removal, yet most studies focus on slow, progressive calcification during long-term operation. Here, we report for the first time a transient calcification phenomenon in PD-HAP coupled granular sludge, where nitrite accumulation activity collapses within 24 h under fluctuating influent conditions (NO₃^--N deficiency). Microstructural analyses revealed that amorphous calcium phosphate (ACP) initially deposits on the granule surface and migrates inward through axial mineral channels driven by microbial metabolism. Through the Ostwald ripening process, ACP gradually transforms into hydroxyapatite (HAP), simultaneously forming the characteristic Liesegang ring patterns. When microbial activity declines and migration driving forces are insufficient, surface deposition outpaces inward transformation, generating a transient calcium crust that blocks substrate-bacteria contact and triggers rapid activity loss. Over time, the repeated occurrence of such surface deposits and mineral phase transformations can ultimately form a more compact and stable mineral coating, exhibiting characteristics of progressive long-term calcium accumulation. Notably, this granular system achieves self-regulation by fracturing into functional micro components (d < 0.2 mm), thereby restoring the system's exceptional nitrite accumulation capacity. These results elucidate the mechanistic basis of transient calcification, highlight the critical role of microbial metabolism in controlling calcium migration, and provide actionable strategies for mitigating calcification shocks in industrial wastewater treatment. Understanding these processes offers a foundation for designing granular sludge systems with enhanced resilience against sudden influent fluctuations and high-calcium wastewater.