Unveiling the threshold values of organic and oxytetracycline loadings for nitrification recovery of a full-scale pharmaceutical wastewater treatment system

Nitrification is a crucial step in removing nitrogen from wastewater, but it often fails in industrial wastewater treatment due to the lack of knowledge regarding the impacts of environmental factors on nitrifying bacteria. This study aimed to recover the nitrification performance of a full-scale pharmaceutical wastewater treatment system inhibited by high organic and oxytetracycline (OTC) loadings. A bench-scale nitrifying reactor was used to treat OTC production wastewater under various organic loading rates (OLRs) and influent OTC concentrations, and the derived threshold values were used to guide the optimization of the full-scale system. Changes in physicochemical and biological parameters including antibiotic resistome were monitored to track the recovery process. After simultaneously reducing the OLR and influent OTC concentration to 0.111 +/- 0.025 g center dot COD/g center dot MLSS/d and 9.69 +/- 1.54 mg/L, respectively, the abundances of amoA gene and Nitrobacter quickly increased to (4.87 +/- 1.88) x 10(7) and (1.07 +/- 0.41) x 10(7) copies/L, and did not change much thereafter. Partial nitrification (about 50%) occurred 18 days later, while it took another 18 days to achieve complete nitrification. Batch nitrification test revealed that the adsorbed OTC at a concentration above 1,630 mg center dot OTC/kg center dot MLSS could inhibit the nitrification performance, which should be the main reason responsible for the slow recovery of complete nitrification considering the relatively high adsorbed OTC in activated sludge (from 23,280 +/- 2204 to 2936 +/- 221 mg center dot OTC/g center dot MLSS). The relative abundances of resistome and high-risk resistant determinants decreased by 63.2% and 67.8% along with the recovery process. This study deepened our understanding of the impacts of environmental factors on nitrification, and could guide the optimization of the pharmaceutical wastewater treatment processes.