Kinetic responses of enriched nitrifying systems to acute erythromycin and tetracycline exposure

This study investigated the acute inhibitory impacts of two widely used antibiotics, erythromycin (ERY) and tetracycline (TET), on ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) within an enriched nitrifying culture. For this purpose, a lab-scale wastewater treatment system sustaining enriched nitrifiers was exposed to antibiotic concentrations ranging from 25 to 100 mg L^-1. Model evaluations of oxygen uptake rate (OUR) evolution, together with ammonia and nitrite oxidation profiles, revealed distinct metabolic disruptions. ERY severely impaired the system, dropping overall nitrification efficiency to as low as 25% and halving the growth rates for both AOB and NOB. In contrast, TET exposure largely maintained overall nitrification performance with slightly reduced NOB activity. Under TET stress, the half-saturation constants for ammonia (K_NH) and nitrite (K_NO) increased to 32 and 2 mg N L^-1, respectively, indicating competitive inhibition and decreased substrate affinity. Both antibiotics reduced heterotrophic activity - more severely with ERY-triggering the release and utilisation of hydrolyzable biomass fractions. The results demonstrate that acute antibiotic toxicity and inhibitory impacts are markedly less pronounced in enriched nitrifying systems compared to conventional mixed activated sludge systems.