Promoting the butterfly transformation of residual sludge: Specific adsorption of heavy metals and in-situ cultivation of granular sludge

To address low sludge resource utilization and high treatment costs, this study uses low-temperature hydrothermal oxidation to pretreat residual sludge, generating calcium salt sludge-induced nuclei (Ca-SC). It demonstrates Ca-SC’s dual role in heavy metal adsorption and low-temperature in-situ cultivation of aerobic granular sludge (AGS) in continuous-flow systems. Hydrothermal oxidation forms a hierarchically porous mixed-crystalline structure. Ca(ClO)₂ increases oxygen-containing groups (16.8 %) and Zeta potential (27.6 %) while preserving surface groups. Notably, Ca-SC achieves a 91.2 % adsorption rate for Pb²⁺, while its removal efficiencies for phenol (24.3 %), NH₄⁺–N (20.1 %), and NO₃⁻–N (9.0 %) are relatively low. Density functional theory attributes this to –OCH₃/–CHO/–COOH coordination and electrostatic interactions. In a 6℃ A/O system, Ca-SC promotes continuous-flow AGS (CFAGS) formation within 15 days, achieving 33.2 % mature particle size and 150-day stability under low-temperature/toxic stress. It enhances extracellular polymeric substance secretion (1.22-fold protein increase), enriches cold-adapted microbes ( Flavobacterium/Psychrobacter ) and nutrient-removing bacteria, improving COD/TN/TP removal by 6.7 %/12.5 %/5.4 %. This work proposes a sludge valorization strategy via functional material engineering, enabling sustainable low-temperature wastewater treatment with efficient pollutant removal and resource recovery.