Recovery of biological wastewater treatment system inhibited by oxytetracycline: rebound of functional bacterial population and the impact of adsorbed oxytetracycline on antibiotic resistance

Abstract This study revealed the recovery process of an anaerobic–aerobic wastewater treatment system, which was seriously inhibited by oxytetracycline at 200 mg/L, over a period of 91 days after the antibiotic potency in the wastewater was removed through enhanced hydrolysis pretreatment (85 ℃ for 6 h). The organic loading rate (OLR) of the upflow anaerobic sludge bed (UASB) reactor was maintained at 2 g COD/L/d for 36 days in the first recovery stage, and gradually increased to 10 g COD/L/d in 4 steps. The UASB reactor fully restored its soluble chemical oxygen demand (SCOD) removal performance in the first stage, which was accompanied by the rebound of the hydrolysis/fermentation/acetogenesis bacteria, including vadinHA17_norank, Mesotoga, Longilinea, Anaerolineaceae_norank, Lentimicrobiu, Blvii28_wastewater-sludge_group, and Synergistaceae_norank. In the aerobic reactor receiving the effluent from the UASB reactor, stable SCOD removal occurred on day 54 and complete nitrification occurred on day 64 at an OLR and ammonia load of 0.147 g COD/L/d and 0.015 g N/L/d, respectively. The resistome abundances in the UASB and aerobic reactors decreased from 1.01 (copies/16S rRNA gene) and 1.11 to 2.7 × 10−1 and 3.0 × 10−1, respectively, over the whole experimental period. The relatively high resistome levels in the final stage might be associated with the adsorbed oxytetracycline in both reactors (7.4 ± 0.3 g/Kg and 0.7 ± 0.2 g/Kg, respectively). The result demonstrated that, for the biological systems damaged by high concentrations of antibiotics, their wastewater treatment functions could be recovered easily by removing influent antibiotics, but the further reduction of resistant determinants depended on the removal of adsorbed antibiotics.