Removal and degradation characteristics of quinolone antibiotics in laboratory-scale activated sludge reactors under aerobic, nitrifying and anoxic conditions

This work describes the removal of 6 quinolone antibiotics from wastewaters under different redox conditions (aerobic, nitrifying and anoxic) through batch experiments in laboratory scale activated sludge reactors using mixed liquor from a membrane bioreactor pilot plant (MBR). The main removal pathways for antibiotics from wastewaters involved in each treatment are described. Mass balances indicated that sorption on sludge played a dominating role in the elimination of antibiotics. Sorption potential depended on the redox conditions, being lower in nitrifying (Kd, 414–876 L kg−1) and anoxic (Kd, 471–930 L kg−1) sludge in comparison with aerobic sludge (Kd, 534–1137 L kg−1). Kd was higher for piperazinylic quinolones. Redox conditions also influenced biodegradation, a secondary pathway, which followed first-order kinetics with degradation rates constants ranging from 1.8·10−3 to 8.2·10−3 h−1. Biodegradation rates under anoxic conditions were negligible. The experimental results have also demonstrated much higher removal efficiency by biodegradation (36.2–60.0%) under nitrifying conditions in comparison with aerobic conditions (14.9–43.8%). The addition of allylthiourea, an ammonia monooxygenase inhibitor, inhibited nitrification completely and reduced significantly the biodegradation of target antibiotics (16.5–29.3%). The residual biodegradation in the presence of allylthiourea may be due to the activity of heterotrophs in the enriched nitrifier culture. The removal of the selected antibiotics under the studied redox conditions depended significantly on the bacteria composition of the sludge. These results suggest that despite the known persistence of this group of antibiotics it is possible to enhance their degradation using nitrifying conditions, which at adequate working conditions as high SRT, typical in MBR, become a promising alternative for improving quinolones removal from environment.

► Sorption and biodegradation are the main pathways for quinolone removal.
► The nitrifying conditions improve biodegradation and reduce sorption potential.
► Biodegradation does not significantly occur under anoxic conditions.
► The inhibition of nitrification reduces the removal of antibiotics by biodegradation.
► Removal of target antibiotics depends on the composition of bacterial populations.