Preparation of hydroxyapatite (HAP) from waste eggshells for enhancing the granulation and treatment performance of aerobic granular sludge: Enhancement effects and mechanism insights

Hydroxyapatite (HAP), an inorganic mineral that is generated in situ within the core of aerobic granular sludge (AGS), plays a pivotal role in both the granulation process and system stability. Due to the slow precipitation rate of HAP, waste eggshells were utilized as a readily available source for preparing HAP by using the conventional hydrothermal (C-HAP) and microwave hydrothermal (M-HAP) methods. Subsequently, the prepared HAP was employed to promote AGS formation. Results showed that the prepared HAP exhibits superior morphology, crystallinity, functional groups, and specific surface area compared to the commercial grade HAP, whereby M-HAP outperforms C-HAP for functional groups and specific surface area. The addition of 0.5 g/L HAP facilitated the granulation process (within 20 days), leading to the development of structurally compact AGS, which was attributed to the microbial attachment onto HAP particles. Moreover, the presence of HAP greatly improved the nutrient removal efficiency (80.8 % of TN and 82.4 % of PO43−-P), most likely due to the enrichment of denitrifying and phosphorus-accumulating microorganisms (e.g., Candidatus_Competibacter and Flavobacterium). Further analysis revealed that an increase in the relative abundance of denitrification-related genes (e.g., narG and nirK) was responsible for the intrinsic mechanism behind the enhanced nitrogen removal, while phosphorus removal occurred through biomineralization and abiotic precipitation pathways. Overall, the present study highlights the potential of utilizing waste eggshells to enhance AGS systems for wastewater treatment, thus realizing a circular economy approach.