Abstract:In view of the widespread challenge of structural instability that limits the engineering application of aerobic granular sludge (AGS), this study proposes a synergistic strategy combining extended hydraulic retention time (HRT) and nitrate addition to enhance the structural stability of AGS. The aim is to elucidate the synergistic effects of these two factors on enhancing the structural stability of AGS. Four sequencing batch reactors (SBRs) were operated under distinct conditions: R0 (control), R1 (external nitrate addition only), R2 (extended HRT-induced starvation), and R3 (combined extended HRT and external nitrate addition). Synthetic wastewater was utilized as the substrate to systematically investigate the impact of different operational conditions on AGS performance. Experimental results indicated that extending HRT effectively induced starvation conditions, leading to effective consumption of polysaccharides (PS) within the extracellular polymeric substances (EPS). Consequently, the protein (PN) to PS ratio in EPS was significantly increased, promoting a denser and more structurally stable granule formation. Specifically, the granule integrity coefficients in reactors R0, R1, R2, and R3 were 84.26%, 85.69%, 95.13%, and 97.12%, respectively. Corresponding EPS concentrations were 78.06, 96.3,0.00, and 91.42 mg/g (based on VSS), with PN/PS ratios of 4.7,5.5,1.12, and 9.30, respectively. These findings highlight that the combined strategy of extended HRT and nitrate supplementation effectively accelerated granulation and significantly enhanced structural strength. Regarding pollutant removal performance, the average chemical oxygen demand (COD) removal efficiencies for reactors R0, R1, R2, and R3 were 89.01%, 88.25%, 83.94%, and 88.56%, respectively. Similarly, average total nitrogen (TN) removal efficiencies were 74.49%, 82.50%, 81.02%, and 81.41%, respectively. Among the reactors, R3 exhibited the best nitrogen removal efficiency and sludge stability. Microbial community analyses revealed that Proteobacteria (56.49% relative abundance) dominated the microbial consortium in R3. Notably, under nitrate-induced starvation stress, the enrichment of the functional genus Zoogloea (16.13% relative abundance) significantly increased EPS secretion (97.40 mg/g), thus effectively driving the granulation process. These results further confirm that targeted microbial community optimization through nitrate regulation represents an effective approach to improve the structural stability of AGS.
