Toward scalable and sustainable solid-phase denitrification: Materials, interfacial regulation, and mass transfer-microbiology coupling in bioreactors

Nitrogen removal from low C/N wastewater is often limited by insufficient electron donors. Liquid carbon sources can rapidly supplement carbon but suffer from dosing, storage, and operational stability concerns, promoting interest in solid-phase denitrification (SPD) based on solid carbon sources (SCSs). However, SCSs still face limitations such as uncontrolled carbon release, inadequate mechanical durability, limited substrate accessibility, and high polymer cost. Modification strategies such as physical blending, chemical modification, and surface functionalization have been developed to improve carbon release regulation, structural stability, substrate accessibility, and microbial attachment. Nevertheless, conventional fixed-bed or packed-bed SPD systems remain constrained by diffusion resistance, biofilm overgrowth, clogging, and inefficient carbon utilization. To address these challenges, this review proposes an Advanced Carbon Source-Particle Enhanced Bioreactor (ACS-PEBR) framework, which couples engineered ACS particles with fluidized operation. Within this framework, ACS particles serve as both carbon donors and biofilm carriers, while fluidization enhances liquid-solid contact, biofilm renewal, and interfacial mass transfer. This review critically summarizes SCS materials, modification strategies, microbial enrichment, reactor engineering, and the proposed mass-transfer and kinetic basis of ACS-PEBRs. Finally, key research priorities are outlined for translating ACS-PEBRs from conceptual design toward scalable and sustainable SPD applications.