Enhancing continuous-flow aerobic sludge granulation through Fe3+ external conditioning of waste activated sludge

This study presents a novel method to improve aerobic granular sludge (AGS) formation in continuous-flow reactors(CFRs), which often face slow granulation and poor stability. By externally conditioning waste activated sludge with Fe 3+ and recycling it into a two-zone sedimentation tank reactor, the process accelerates granulation and enhances stability. Complete granulation occurred in 60 days (from 0.28 mm to 0.60 mm of the initial sludge), and the sludge volume index at 5 min (SVI 5 ) stayed below 30 mL/g. The CFR achieved average removal efficiencies of 87.17% for COD and 91.55% for NH 4 + -N. Additionally, sludge activity improved, with dehydrogenase activity rising from 6.70 to 16.90 mg/(g MLVSS) and the specific oxygen uptake rate increasing from 7.80 to 36.80 mg O 2 /(g MLVSS·h). The analysis of microbial communities indicated an enrichment of genera associated with the secretion of extracellular polymeric substances (EPS) and the stability of granular structures, such as Proteobacteria, Zoogloea and Flavobacterium . Additionally, the secondary messenger c-di-GMP was found to further accelerate granulation by modulating the secretion of EPS. Its concentration reached a peak of 380.50 μg/(g MLVSS) during the granulation phase, mirroring the trend observed for EPS. In conclusion, the proposed strategy effectively enhances the formation and stability of AGS in CFRs, providing a viable approach for upgrading existing continuous-flow wastewater treatment processes.