A membrane bioreactor (MBR) was used to treat ciprofloxacin (CIP)-contaminated artificial wastewater. The pollutant removal performance and the microbial community structure of the MBR were studied at three different CIP dosages (0 mg·L-1, 5 mg·L-1, and 10 mg·L-1). The results showed that the sludge concentration in the reactor decreased and then levelled off as the dosage of CIP was increased from 0 mg·L-1 to 5 mg·L-1 and further to 10 mg·L-1. The mean removal of TOC and COD decreased from 98.40% and 97.80% to 84.20% and 94.10%, respectively, indicating that the CIP negatively influenced the organic removal but the effect was minor. In contrast, the ammonium removal was greatly influenced by the dosage of CIP. When the CIP dosage increased from 0 mg·L-1 to 5 mg·L-1 and further to 10 mg·L-1, the ammonium removal efficiency decreased from 96.91% to 84.14% and then to 77.80%, and the activity of Nitrosomonas, Alcaligenes, Nitrospira, and Nitrobacter were greatly inhibited. The CIP removal initially increased and then decreased. The mass balance revealed that the removal of CIP in the MBR was principally attributed to biodegradation and sludge adsorption, which accounted for 30.13% and 0.25%, respectively, at a CIP dosage of 5 mg·L-1 and 7.55% and 1.81% at a CIP dosage of 10 mg·L-1.
Keywords: COD; ammonium nitrogen; ciprofloxacin; mass balance; membrane bioreactor (MBR); nitrifying microorganism community.