Aerobic granular sludge development and microbial responses to FPB-mediated influent conditioning in a coupled FPB-SBR system

With the expanding full-scale implementation of aerobic granular sludge (AGS), understanding how influent characteristics shape aggregate development, microbial organization, and nutrient-removal function has become increasingly important. Upstream physicochemical conditioning can simultaneously alter organic loading, chemical oxygen demand (COD) distribution, and nutrient availability, but the integrated responses of downstream AGS to these coupled influent changes remain insufficiently resolved. This study compared a standalone sequencing batch reactor (SBR, R1) with a fluidized-bed pretreatment (FPB)–SBR coupled system, in which R2 served as the downstream SBR. FPB pretreatment reduced total COD from approximately 500 to 250–300 mg/L while dissolved COD remained around 250 mg/L, and decreased total phosphorus (TP) entering R2 from approximately 5 to 1 mg/L. Under these integrated influent changes, R2 reached the predefined settling-based granulation benchmark 31 days earlier than R1. By Day 41, the mean particle diameter reached 241.3 μm in R2 versus 116.9 μm in R1, accompanied by greater biomass retention, denser granule morphology, and lower filamentous representation. Microbial and physiological analyses showed greater relative representation of Candidatus Competibacter and Nitrospira and a higher autotrophic-to-heterotrophic respiration ratio (R A/H ) in R2 (0.35 vs. 0.09 on Day 75), indicating a greater relative contribution of autotrophic respiration. Extracellular polymeric substances (EPS) characteristics, local surface elemental composition, and thermo-oxidative behavior also differed between reactors. Overall, these findings support FPB-assisted influent conditioning as a potential process-level strategy for regulating AGS development, microbial organization, and physicochemical characteristics.