Biochar enhanced the stability and microbial metabolic activity of aerobic denitrification system under long-term oxytetracycline stress

[Display omitted] •Long-term stress of OTC had obvious effects on AD reactor.•Biochar addition enhanced the metabolic activity of the AD bacteria.•Biochar addition improved the stability of sludge structure.•Biochar mitigated inhibition of OTC on AD bacteria. Reactors were established to study the feasibility of the direct addition of modified biochar to alleviate the long-term stress of oxytetracycline (OTC) on aerobic denitrification (AD) and improve the stability of the system. The results showed that OTC stimulated at μg/L, and inhibited at mg/L. The higher the concentration of OTC, the longer the system was affected. The addition of biochar, without immobilization, improved the tolerance of community, alleviated the irreversible inhibition effect of OTC, and maintained a high denitrification efficiency. Overall, the main mechanisms of AD enhancement by biochar under OTC stress were: enhancing the bacteria metabolic activity, strengthening sludge structure and substrate transport, and improving the community stability and diversity. This study confirmed that direct addition of biochar could effectively alleviate the negative effect of antibiotics on the microorganisms, strengthen the AD, which provided a new idea to broaden the application of AD technology in livestock wastewater.