Long-Term Inhibition of the Antibiotic Oxytetracycline on an Upflow Low-Matrix Anammox Biofilter

The long-term effects of the antibiotic oxytetracycline (OTC) on the process of anaerobic ammonium oxidation (anammox) were studied in an upflow low-matrix anammox biofilter. When OTC was increased to 40 mg/L, the nitrogen removal rate (NRR) reduced from 0.536 to 0.395 kg/(m3·day), and then decreased further with OTC concentration. Although extracellular polymeric substances (EPS) increased from 36.3 to 177.6 mg/g of suspended solids, this was insufficient to resist the toxicity of OTC to anaerobic ammonia-oxidizing bacteria (AAOB). The total nitrogen removal efficiency decreased from the initial 85.2% to 17.3%, and the relative abundance of AAOB decreased from 16.40% to 9.35%. When the OTC concentration exceeded 40 mg/L, some microorganisms died, which led to a decrease in EPS, but an increase in soluble microbial products. The relative abundance of AAOB decreased from 16.40% to 4.6% when the OTC concentration reached 90 mg/L, and remained at 4.8% when OTC was no longer added. The anammox process could not be recovered during the four generation cycles after OTC addition ceased. The IC50 of OTC toward the NRR of the anammox process was calculated to be approximately 10.47 mg/L.