Insight into the internal microscopic structure and composition of aerobic granular sludge under high salinity based...

Aerobic granular sludge (AGS) is a self-aggregated three-dimensional microbial aggregate and has great potential for treating high salinity wastewater, but quantitative, non-destructive analysis of its internal structure is limited. This study employed novel high-resolution micro-computed tomography (Micro-CT) to non-destructively quantify the internal structural response of AGS under salinity. Results showed that increasing salinity from 0 % to 5 % raised AGS porosity from 57 % to 72 %. The total pore count decreased markedly while average pore size increased from 76.02 μm to 157.64 μm. Conversely, biomass aggregate size declined significantly with increasing salinity. The increased porosity and reduced biomass demonstrated that salinity ≤2.5 % preserved structural integrity while salinities above 3.5 % led to notable internal damage. The robust internal structure at moderate salinity (≤2.5 %) created a microenvironment that stimulated the secretion of protective EPS, which in turn reinforced the structure by fostering dense internal biomass aggregates. Based on the internal structure and composition, the average particle size decreased from 2855.13 μm at 1 % salinity to 1186.78 μm at 5 % salinity, indicating granulation reduction. The internal structure of the granules directly dictated operational stability. Although all reactors removed pollutants effectively, the severe structural collapse at high salinity (≥3.5 %) caused prolonged recovery periods and greater fluctuations in removal efficiency, signifying a profound loss of functional resilience. By providing a non-destructive method for internal analysis, this work elucidated that internal structural failure preceded macroscopic collapse and was a key determinant of AGS instability in hypersaline environments, offering crucial insights for the robust design and operation of AGS systems for industrial applications.