Operational variables and microbial community dynamics affect granulation stability in continuous flow aerobic granular sludge reactors

Retrofitting wastewater treatment plants with continuous aerobic granular sludge reactors is a promising alternative to enhance treatment capacities and reduce footprint. This study investigates the main variables influencing granulation and microbial dynamics in two reactor configurations (25 L): stirred tanks in series (R1) and a plug-flow-like system (R2). Granule formation was achieved by increasing the organic loading rate (OLR) from 0.7 to 4.1 kg COD/(m³·d) and the up-flow velocity in the biomass selector from 1.4 to 6.9 m/h. However, irreversible granule destabilization occurred at day 68 for R1 and day 108 for R2. Principal component analysis and examination of food-to-microorganisms (F/M) ratio medians identified the F/M ratio as the primary variable associated with instability. Microbial analysis revealed that a high F/M ratio induced significant increases in the abundance of specific genera such as Arcobacter, Cloacibacterium, Rikenella, Aquaspirillum and Sphaerotillus, whose overgrowth may negatively impact granule stability. Based on these findings, maximum F/M ratio thresholds were obtained to establish operational conditions allowing the maintenance of stable aerobic granules on continuous flow reactor configurations.