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Due to the refractory of 1 H-1,2,4-triazole (TZ), conventional anaerobic biological treatment technology is usually restricted by low removal efficiency and poor system stability. In this study, TZ biodegradation and nitrate reduction was coupled to improve the removal efficiency of TZ from polluted wastewater. Batch assay was performed with pure culture strain Raoultella sp. NJUST42, which was reported to have the capability to degrade TZ in our previous study. Based on batch assay result, complete removal of TZ could be achieved in the presence of nitrate, whereas only 50% of TZ could be removed in the control system. Long-term stability experiment indicated that the relative abundance of microorganisms (Bacteroidetes_vadinHA17 , Georgenia , Anaerolinea , etc) was obviously enhanced under nitrate reduction condition. During long-term period, major intermediates for TZ biodegradation such as [1,2,4]Triazolidine-3,5-diol, hydrazine dibasic carboxylic acid and carbamic acid were detected. A novel TZ biotransformation approach via hydration, TZ-ring cleavage, deamination and oxidation was speculated. PICRUSt1 and KEGG pathway analyses indicated that hydration (dch), oxidation (adhD , oah , pucG , fdhA) of TZ and nitrate reduction (Nar , napA , nrfA , nirBK , norB , nosZ) were significantly enhanced in the presence of nitrate. Moreover, the significant enrichment of TCA cycle (gab , sdh , fum , etc.) indicated that carbon and energy metabolism were facilitated with the addition of nitrate, thus improved TZ catabolism. The proposed mechanism demonstrated that TZ biodegradation coupled with nitrate reduction would be a promising approach for efficient treatment of wastewater contaminated by TZ. TZ is widely used in pesticides, herbicides and fungicides, and also is an important environmental pollution. Microbial metabolic mechanism is considered as an important indicator for removing contaminants from contaminated environment. However, information about metabolic mechanism involved in TZ biodegradation under nitrate reduction is still unclear. In this study, TZ hydration and oxidation and nitrate reduction were significantly enhanced, carbon and energy metabolism were significantly facilitated with the addition of nitrate, and thus improved TZ catabolism. The proposed mechanism demonstrated that TZ biodegradation coupled with nitrate reduction would be a promising approach for efficient treatment of wastewater contaminated by TZ. [Display omitted] • Complete biodegradation of TZ could be achieved under nitrate-reducing condition. • Simultaneous TZ biodegradation and nitrate reduction was technically feasible. • Hydration and oxidation of TZ were facilitated under nitrate reduction condition. • Functional genes involved in TZ biodegradation and nitrate reduction were elucidated. • Microbial metabolic mechanism dominating TZ biodegradation and nitrate reduction was revealed. [ABSTRACT FROM AUTHOR]