Hydrodynamic regulation drives azo dye transformation and hydrolysis acidification by reshaping microbial metabolism in the micro-aerobic hydrolysis acidification reactor.

Acid Red GR is a representative sulfonated azo dye frequently detected in dyeing wastewater, yet the role of hydrodynamic conditions in regulating its biodegradation in micro-aerobic hydrolysis acidification (MAHA) systems remains unclear. This study investigated how hydrodynamic conditions drive Acid Red GR transformation, hydrolysis acidification, and microbial metabolic responses in the MAHA reactor. The results showed that a moderate stirring speed of 120 rad/min (R2) balanced turbulence intensity and flow uniformity, with relatively high oxygen transfer, a favorable micro-aerobic niche, and the highest biodegradability (0.50) and volatile fatty acid (VFA) production (140.38 mg/L). This hydrodynamic condition enriched hydrolytic, facultative, and fermentative genera, including Klebsiella, Enterobacter, Bacteroides, Propionivibrio, and Bacillus, and was associated with higher abundances of genes encoding enzymes involved in azo reduction, aromatic intermediate transformation, pyruvate generation, and acidogenic metabolism. Integrated metagenomic, metabolomic, and functional group evidence further indicated that Acid Red GR degradation proceeded through azo reduction, deamination, desulfonation, aromatic ring cleavage, and subsequent acidogenesis. These findings demonstrate that hydrodynamic optimization regulates MAHA performance by coordinating oxygen transfer, microbial niche formation, and metabolic functional potential, providing a mechanistic basis for enhancing the biodegradability and treatment efficiency of dyeing wastewater.