Display omitted •nZVI addition can enhance both AD performance and TC degradation.•Anaerobic digestion has high removal rate of TC in systems with low TC concentration.•Low TC concentration addition has positive effect on anaerobic methane production.•Addition of nZVI can alter the microbial communities’ structure during AD process.•The increase of Syntrophobacterale and Methanosarcinales resulted in high CH4 production. Supplementing nano zero valent iron (nZVI) is an attractive technology for wastewater treatment due to its advantages in accelerating the hydrolysis, fermentation and anaerobic digestion (AD) process. In this present study, nZVI was added to investigate its effects on enhancing tetracycline (TC) wastewater anaerobic treatment and the changes of microbial community, especially for underestimated syntrophic-methanogenic associations. The TC concentrations were 1, 10, 30, 50, 80, 100 and 150 mg/L with 0.38 g nZVI (with iron g/g VS of 0.50) complemented into reactors. Results revealed that nZVI could enhance AD process in both control and TC dosed systems, and the promoting effect on methanogenesis was more significant in systems of high concentration TC, with 100 and 150 mg/L. In addition, cumulative CH4 production for all TC added systems without nZVI were higher than the control indicating TC had positive effect instead of expected negative effect on AD process, high TC concentration of 100 and 150 mg/L only affecting the increase factor rather than causing inhibitory effect. After digestion, TC was largely removed in with/without nZVI systems. And also, nZVI evidently altered the bacterial and methanogenic community structure, with an increase abundance of syntrophic-methanogenic associations (Syntrophobacterales and Methanosarcinales) and resulting in the enhancement of methane generation. This research provides an efficient method for TC wastewater anaerobic treatment.