Impact of ampicillin on the nitrogen removal, microbial community and enzymatic activity of activated sludge

•Ampicillin decreased the nitrifying rate and denitrifying rate of activated sludge.•Ampicillin exerted obvious inhibitory effects on the microbial enzymatic activity.•Ampicillin affected the production and composition of EPS from activated sludge.•Ampicillin decreased the PN/PS ratio of EPS and affected the sludge settleability.•Ampicillin degraded the abundance of nitrifying- and denitrifying bacteria in SBR. The performance, nitrogen removal rate, extracellular polymeric substances (EPS), microbial community and enzymatic activity of activated sludge have been assessed in a sequencing batch reactor under ampicillin stress. The chemical oxygen demand and ammonia removal kept relatively stable at 0–30 mg/L ampicillin. No obvious nitrite and nitrate accumulation was found in the effluent. However, the oxygen utilization rate, nitrification rate and denitrification rate declined with the increment of ampicillin concentration. The activities of dehydrogenase and microbial enzymes relating to nitrogen removal were inhibited under ampicillin stress. Ampicillin at 20 and 30 mg/L heightened the microbial lactate dehydrogenase release and reactive oxygen species production. Ampicillin promoted the production of EPS, loosely bound EPS and tightly bound EPS and affected their chemical composition. Additionally, the protein/polysaccharide ratios in the EPS and the sludge settleability reduced with the increment of ampicillin concentration. Ampicillin obviously affected the relative abundance of nitrifying- and denitrifying bacteria.