Calcium-loaded activated carbon microtubes as a structural framework of aerobic granules: crosslinking networks...

Aerobic granular sludge (AGS) technology is often constrained by slow granulation and structural instability, issues largely attributed to imbalances in filamentous bacteria and extracellular polymeric substances (EPS). In this study, calcium-loaded activated carbon microtubes (ACMTs-Ca) were developed as novel frameworks to enhance AGS formation and stability. The interfacial energy barrier between microorganisms and ACMTs-Ca was reduced by 67.43 % compared to that between microorganisms and conventional sludge. Incorporation of ACMTs-Ca significantly accelerated granulation kinetics, achieving mature granulation (89.36 % granules >200 μm, SVI₃₀ = 48.35 mL·g^-1, SVI₃₀/SVI₅ = 99.15 %) within 30 d, in contrast to 140 d required by the control. AGS with ACMTs-Ca as the primary framework exhibited an 80 % enhancement in structural strength relative to the control. The improved structural integrity was attributed to the formation of a calcium-alginate coordination network with β-polysaccharides, thereby reducing dependence on filamentous bacteria for granule stability. Enhanced pollutant removal efficiencies were observed, with AGS-ACMTs-Ca system achieving COD, TN, and TP removal rates of 96 %, 80 %, and 99.48 %, respectively, facilitated by the enrichment of functional genera such as Candidatus_Competibacter, Thauera, and Flavobacterium. Metagenomic analysis revealed increased relative abundance of algE genes (associated with β-polysaccharide synthesis) alongside decreased abundance of genes involved in amino acid biosynthesis pathways. These abundance shifts suggest potential microbial regulation favoring EPS compositional changes toward calcium-stabilized frameworks. This study demonstrates the dual role of ACMTs-Ca in structural reinforcement and microbial niche regulation, offering a sustainable strategy for rapid and robust AGS cultivation.