*3.3. Repeated Addition of Antibiotics and Their Degradation in Sludge*

To test the long-term degradation ability of aerobic strains SF1 and A12 and anaerobic strains B and SANA in sludge, experiments involving the repeated addition of antibiotics were performed (Table 2).


**Table 2.** The removal rates (%) after the first (1st), second (2nd), and third (3rd) additions of antibiotics.

Note(s): <sup>a</sup> aerobic conditions for 8 days. <sup>b</sup> anaerobic conditions for 15 days. Data from three independent experiments are presented as the means ± SE.

In the first addition experiments, in the sludge under aerobic conditions, 46.3, 41.6 and 35.6% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. In the sludge under anaerobic conditions, 42.2, 37.4 and 33.5% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. In the sludge under aerobic conditions containing aerobic strains SF1 and A12, 90.6, 88.2 and 82.6% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. In the sludge under anaerobic conditions containing anaerobic strains B and SANA, 92.3, 88.1 and 85.2% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. In the third addition experiments, in the sludge under aerobic conditions, 52.5, 50.4 and 46.1% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. In the sludge under anaerobic conditions, 50.4, 46.4 and 42.7% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. In the sludge under aerobic conditions containing aerobic strains SF1 and A12, 97.5, 95.3 and 90.3% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. In the sludge under anaerobic conditions containing anaerobic strains B and SANA, 98.1, 96.4 and 92.4% of SMX, AMO and CTC initially present at 2 mg/L was degraded, respectively. These results suggest that the degradation rate increased with the repeated addition of SMX, AMO and CTC. The repeated addition of antibiotics might increase the abundance of antibiotic-degrading microbes. These findings are similar to the results of our previous study of SA degradation in sludge [38].
