Phenol-induced shifts in aerobic granular sludge: Linking granule properties, microbial dynamics, and treatment performance

This study investigated the effects of increasing phenol concentrations (10–200 mg.L -1 ), a xenobiotic compound, on the physical properties and microbial community structure of an aerobic granular sludge (AGS) sequencing batch reactor (SBR), and their association with overall reactor performance. Despite the gradual replacement of acetate by phenol as the primary carbon source, the reactor sustained chemical oxygen demand (COD) removal above 85% throughout the experiment, while complete phenol removal was achieved after microbial adaptation. However, a pronounced shift in the COD degradation profile was noticed, especially at the highest phenol concentration tested (200 mg.L -1 ), with organic matter consumption progressively migrating from the anaerobic feeding to the aerated phase of the SBR cycle. Along with phenol toxicity, this scenario markedly inhibited nitrification and biological phosphorus removal, substantially reducing ammonium and phosphate removal efficiencies to around 10% at 200 mg.L -1 phenol. Increasing phenol concentrations promoted granule enlargement, reduced density, and impaired settleability due to the proliferation of filamentous organisms. Protein-to-polysaccharide (PN/PS) ratios of the granular biomass increased from approximately 2 at 100 mg.L -1 phenol to around 3 at 200 mg.L -1 phenol, suggesting shifts in extracellular polymeric substances (EPS) composition associated with a protective biochemical response to phenol toxicity. High-throughput 16S rRNA sequencing revealed a sharp community shift from Firmicutes and Bacteroidetes dominance at low phenol concentrations to Proteobacteria predominance under high phenol loads. At the genus level, Pseudomonas dominated at 50–100 mg.L -1 phenol, while Klebsiella prevailed at 200 mg.L -1 (41% relative abundance), indicating selective enrichment of robust phenol-degrading bacteria. Anaerocolumna , Clostridium sensu stricto , and Clostridioides -like organisms exhibited increased relative abundance over time, supporting the resilience of the system under phenol stress. The findings demonstrate that phenol markedly influences granule stability and microbial composition, but the AGS system could adapt to moderate phenol levels while sustaining effective organic matter removal, highlighting the importance of these adaptive mechanisms for the treatment of phenolic wastewaters.