Iron-functionalized carbonized melamine sponge reinforces aerobic granular sludge and reconfigures nitrogen-metabolic...

Aerobic granular sludge (AGS) is limited by slow granulation and structural instability under variable municipal wastewater conditions. Here, an iron-functionalized carbonized melamine sponge (Fe³⁺@CMS) was developed as a three-dimensional scaffold to accelerate granulation and improve nitrogen removal. Fe³⁺@CMS reduced the time required for complete granulation by 20 d and became embedded within the granules as a persistent internal skeleton. It promoted extracellular polymeric substance (EPS) production during granule nucleation, whereas mature Fe³⁺@CMS-supported granules maintained greater structural integrity with lower EPS accumulation, suggesting partial substitution for the native EPS-dependent framework. After transition to real municipal wastewater, the Fe³⁺@CMS reactor maintained an average soluble COD removal of 92.4% and >97% ammonium removal, and achieved a final total nitrogen removal efficiency of 84.45%, compared with 44.10% in the control. Fe³⁺@CMS also suppressed excessive Thiothrix proliferation and favored denitrifying and iron-redox-associated taxa. Metagenomic analysis indicated selective redistribution rather than overall enrichment of nitrogen-cycle genes: napA/napB and nirS showed higher relative abundances, while the DNRA/(nirK + nirS) abundance ratio decreased from 0.184 to 0.107. These findings demonstrate that Fe³⁺@CMS acts as both a persistent structural scaffold and a selective ecological niche, integrating aerobic-granule stabilization with enhanced denitrification under fluctuating wastewater conditions.