Visual indicator changes of aerobic granular sludge during filamentous bulking induced by reduced aeration intensity

Granular bulking and structural destabilization caused by filamentous overgrowth are technical bottlenecks limiting the application of aerobic granular sludge (AGS) technology. Traditional monitoring methods (e.g., SVI measurement, high-throughput sequencing) suffer from high latency and cost, whereas quantitative image analysis enables real-time quantification of granule morphology, color, and texture, offering a promising alternative. Therefore, in this study, the superficial gas velocity of a sequencing batch reactor was gradually reduced from 1.2 cm/s to 0.17 cm/s over 124 days, and seven visual indicators covering shape, color, and texture were extracted from granule images. The results showed that filamentous bulking occurred after the gas velocity was reduced to 0.17 cm/s, with Feret diameter increasing by 78% and circularity dropping sharply to 0.037. Due to coverage by white filamentous hyphae, lightness L* increased by 14.1%, while yellowness b* shifted from positive to negative. Surface roughness Sa showed the largest increase (+133%), textural entropy increased by 19%, and contrast decreased significantly by 67.7%. Feret diameter and circularity can serve as threshold-type alarm indicators for macroscopic structural destabilization, i.e., they remain stable during the stable period and undergo a sudden change after the critical point of structural failure, while Sa provides the earliest warning signal for initial microscopic filamentous growth.