Inhibition of the partial nitritation by roxithromycin and Cu(II)

• The IC50 of ROX and Cu(II) on PN sludge were 346 and 74.3 mg L−1, respectively.
• The response–concentration effects were simulated by various inhibition models.
• NOB was more sensitive than AOB to ROX.
• The combined toxicity of ROX and Cu(II) was shown to be synergistic.
• FTIR was used to investigate changes in the functional groups of the PN sludge.

To facilitate the application of partial nitritation (PN) – anaerobic ammonium oxidation process in nitrogen removal from livestock wastewater, the inhibition of roxithromycin (ROX) and Cu(II) on the PN sludge was examined using a respirometric method. The results showed that the IC50 of ROX and Cu(II) on PN sludge were 346 and 74.3 mg L−1, respectively. The relative specific respiration rate (SRR) of ammonia-oxidizing bacteria (AOB) decreased from 87.4% to 17.7% with the ROX concentration increased from 0 to 500 mg L−1. When the concentration of Cu(II) increased from 0 to 160 mg L−1, the SRRs of AOB and nitrite-oxidizing bacteria decreased by 85.5% and 11.2%, respectively. According to the isobole plots analysis, combined suppression by ROX and Cu(II) was synergistic. Fourier transform infrared spectroscopy analyses showed that ROX exposure altered the positions of CO bonds, and the intensity of the absorption peak at 2100 cm−1 changed under Cu(II) exposure.