Operational reliability of full-scale activated sludge systems under carbon-source substitution with fermentation-derived yellow water: organic risk attenuation and microbial community stability.

Fermentation-derived waste streams have attracted increasing attention as sustainable alternative carbon sources for biological nitrogen removal in municipal wastewater treatment plants (WWTPs). However, their operational reliability remains insufficiently understood because previous evaluations have focused primarily on conventional treatment performance while overlooking the fate of complex organic constituents and microbial responses to chemically heterogeneous carbon inputs. In this study, a full-scale municipal WWTP was investigated to evaluate carbon-source substitution with fermentation-derived yellow water. Carbon-source substitution maintained stable COD, NH4+-N, TN, and TP removal throughout operation, with effluent quality consistently meeting the Chinese Class IA discharge standard. Fifteen representative organic constituents with different predicted hazard potentials were identified in yellow water, all of which exhibited substantial attenuation (approximately 50-99%) during biological treatment, including compounds with relatively higher predicted toxicity. Microbial community composition remained relatively stable during carbon-source substitution, while genes associated with nitrogen cycling, aromatic compound degradation, and oxidative stress response remained consistently represented. Genome-resolved metagenomic analysis further demonstrated that representative microbial populations encoded genes spanning these three functional categories. These findings demonstrate the feasibility of using fermentation-derived yellow water as an alternative external carbon source in full-scale WWTPs and extend current evaluation of waste-derived carbon sources beyond conventional treatment performance by incorporating organic risk attenuation and microbial functional adaptation.