Effect of multi-morphology eggshell-based hydroxyapatite on the freezing recovery and structural stability of aerobic granular sludge

The key to ensure the treatment efficiency of wastewater is to realize the rapid recovery of the performance of frozen storage aerobic granular sludge and maintain its long-term stable operation. In this study, needle-like, petal-like and needle-petal-petal-like hydroxyapatite were synthesized by solvothermal method using waste egg shells as precursors. The physicochemical properties of these samples were characterized by SEM-EDS, BET, XRD, etc. Frozen aerobic granular sludge was inoculated and 0.3 g/L hydroxyapatite was added in a three-plate sequencing batch reactor (SBR, A0 group: needle tip shape, A1 group: petal shape, A2 group: needle tip-petal shape). The effects of hydroxyapatite on the start-up regeneration, pollutant degradation efficiency and extracellular polymer (EPS) secretion of frozen aerobic granular sludge were systematically investigated. At the same time, the internal mechanism of needle-petal hydroxyapatite-mediated aerobic granular sludge regeneration was preliminarily elucidated. The results show that the micro-morphology of needle-like, petal-like and needle-petal-petal-like hydroxyapatite presents slender and sharp, fluffy porous aggregates and composite hierarchical structure, respectively. The specific surface areas were 17.4142, 22.8686 and 28.1209 m²/g, respectively, and the average zeta potentials were −8.64, −5.64 and −4.53 mV, respectively. Tip-petal hydroxyapatite has high porosity, high specific surface area and weak negative charge. The addition of hydroxyapatite can increase the particle size of the regenerated granular sludge, promote the secretion of EPS, and reconstruct the particle skeleton structure. The average particle sizes of sludge in A0, A1 and A2 groups reached 1.83 ± 0.05 mm, 2.02 ± 0.02 mm and 2.25 ± 0.03 mm, respectively, and the corresponding EPS contents were about 80.31, 99.80 and 109.28 mg/g, respectively. The stability of sludge particle structure was significantly improved. During the operation of SBR, the average removal rates of COD, NH₄⁺-N and TP in the wastewater of A0 group were 82.10%, 71.93% and 60.36%, respectively. A1 group was 85.15%, 71.52% and 64.44%. A2 group was 88.12%, 76.51% and 70.01%. Pin-tip-petal hydroxyapatite enhanced the removal of pollutants. Based on the triple synergistic mechanism of material adsorption start-up, EPS regulation and particle structure reconstruction, system pH self-stabilization and long-term stable operation, the efficient and rapid start-up of frozen aerobic granular sludge can be realized, and the sludge re-agglomeration and molding can be completed. This study verified the feasibility of introducing hydroxyapatite with different morphologies to mediate the recovery of frozen aerobic granular sludge. This strategy not only improves the pain points such as easy disintegration of granular sludge structure and lagging recovery of biological activity in the recovery stage, but also provides reference value for the stable operation of aerobic granular sludge process.