Batch tests for measurements of FQs biodegradation and bacterial activities
An anoxic–aerobic MBR with an effective volume of 10 L (5 L for anoxic tank and 5 L for aerobic tank, see Fig. S1 In Supplementary materials) was operated at Adriamycin solids retention time (SRT) of 20 d and a hydraulic retention time (HRT) of 10 h, respectively. A hollow fiber membrane module (0.1 m2, 0.01 μm, PVDF, Litree Co., Suzhou, China) was submerged in the aerobic tank where aeration rate was set at ca. 100 L/h to maintain a dissolved oxygen (DO) concentration of 1.5–2.0 mg/L. Seeding sludge was collected from a local wastewater treatment plant and incubated for 2 weeks using the synthetic wastewater mentioned below before startup of the MBRs. Sludge recirculation from the aerobic tank to the anoxic tank was set at 200% of the influent flow. Synthetic wastewater with average concentrations of chemical oxygen demand (COD), total nitrogen (TN) and phosphorous (P) of about 350 mg/L, 60 mg/L and 9 mg/L was prepared ( Table 1) and used as nutrients for the MBR biomass. The organic carbon of synthetic wastewater was composed of Na-acetate (35 mg/L), starch (162 mg/L), milk powder (200 mg/L), sucrose (141 mg/L), peptone (33 mg/L), yeast extract (77 mg/L) and beef extract (80 mg/L). When the MBR achieved steady and satisfactory performance on nutrient removal (day 0–129), mixtures of FQs (1:1:1 of enrofloxacin, norfloxacin and ciprofloxacin obtained from Zhejiang Guobang Pharmaceutical Co., Ltd. with >98% purity) were added to the synthetic wastewater at a secondary (lateral) meristems final concentration of 0.9 mg/L (day 130–249) and 9 mg/L (day 250–360). Thus, the entire MBR operation can be divided into three runs: Phase I (without FQs addition, day 0 – day 129), Phase II (with low dosage of FQs, day 130 – day 249) and Phase III (with high dosage of FQs, day 250 – day 360). The operating days of each phase were at least 5 times of the SRT, which can warrant the reactor reach steady performance. The permeate was sucked continuously from the membrane by a peristaltic pump without any physical backwashing or relaxation over the entire operation. When the transmembrane pressure (TMP) increased to ca. 0.025 MPa, the membrane module was flushed with high pressure water and then soaked in 0.1 M NaOH solution for 12 h to recover the permeability.