Operational strategy matters: How stepwise versus shock increases in COD/N ratio affect aerobic granular sludge stability and microbial community dynamics

The stability of aerobic granular sludge (AGS) is frequently compromised by fluctuating organic loading rates during high-strength wastewater treatment; however, the impacts of different COD/N transition patterns on AGS stability remain inadequately characterized. This study systematically investigated the effects of COD/N transition patterns (shock vs. stepwise) on the stability of AGS during high-strength wastewater treatment. Three independent laboratory-scale SBRs (R1-R3) were operated for 150 days, with each reactor representing a distinct COD/N adjustment strategy: R1 (shock increase from 12 to 24), R2 (stepwise increase from 12 through 16 and 20 to 24), and R3 (constant 24). Results demonstrate that shock loading triggered catastrophic granule disintegration (SVI₅: sludge volume index after 5 min = 72.0 mL/g, IC: integrity coefficient = 51.8 ± 1.1 %), microbial diversity collapse (36 % reduction), and severe, persistent deterioration in nitrogen removal (40 % TN efficiency decline). In contrast, stepwise adaptation maintained structural stability through balanced extracellular polymeric substance (EPS) composition (PN/PS=3.7 ± 0.3) and robust microbial networks (440 edges; 52 % positive correlations). Direct cultivation at a COD/N ratio of 24 was found to potentially induce systemic instability. Notably, the variation pattern of the COD/N, rather than its final steady-state value, was identified as the determining factor for long-term system resilience. These findings demonstrate that a stepwise adjustment of the COD/N ratio represents the optimal operational strategy and provide practical implementation guidelines for the full-scale application of AGS technology in the treatment of organic-rich wastewater.