## 3 Results and Discussion  
### Effect of Antibiotics on the Decontamination Performance of BRC-MFC  
#### 3.1.1 Nutrient Removal Performance  
In stage I, the COD removal effect of the five groups of reactors (R0-R4) was good, and the COD removal rate of BRC-MFC under different concentrations of antibiotic stress remained above 90%. Compared with R0, R1, and R2 showed a slight increase in COD removal rate, while R3 and R4 showed no significant changes. This showed that OFLX stress did not inhibit the COD removal ability of BRC-MFC. There was also a positive effect on COD removal at 0.2 \begin{table}
\begin{tabular}{c c c} \hline \hline
**Target Gene** & **Primer Name** & **Primer Sequence** \\ \hline
16S rRNA & 341F & CCTACCGGAGCCAG \\  & 518R & ATTACCCGCGCTGCCTG \\  & narG 1960F & TAYTGSCGSCARGARAA \\  & narG 2650R & TTYCRTACGBRGTBC \\  & cd3 aF & GTACACGTGAAAGAACCGG \\  & R3 cd & GAGTCCGGTCTTTGA \\  & Ns-norB-454F & TACTAYGARCCCTGACTTACRA \\  & Ns-norB-710R & ATGCCYGGSAWRTAGAAGWAMAMSA \\  & nosZ-F & CGYTGTTCMTCGACAGCCAG \\  & nosZ1622R & CGSACCTSTITGCCTSYGCG \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The sequences of specific primers in real-time qPCR analysis. and 1.2 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) OFLX stress. It has been found that antibiotics also have the effect of promoting microbial metabolic activity and stimulating microbial induction of respiration in the low concentration range very commonly [38]. Therefore, microbial activity in the reactor under OFLX loading conditions also increased accordingly, increasing its efficiency in utilizing COD. As shown in Figure 2a, the COD concentration of effluent was less than 30 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\), meeting the surface water class IV standard. There was no significant difference (\(p>0.05\)) between the TN removal rate of the test group R1-R4 and the control group R0, and only a slight inhibition effect was observed. From Figure 2b, it can be seen that the microorganisms were in the growth and reproduction period in the early operation period (0-10 days), so their life activities consumed plentiful nutrients. The microorganisms used the nitrogen source to assimilate nitrate-nitrogen reduction, so the TN concentration in the effluent was low. Some scholars point out that the nitrogen source assimilated by the microorganisms themselves can account for up to 80% of the nitrogen removal rate [39], which was the result of the joint action of most microorganisms in the system. During operation (10-45 days), the effluent TN concentrations from R0 to R2 without antibiotics or a low concentration of antibiotic stress were generally lower than those from R3 to R4 under a high concentration of antibiotic stress, which may be due to the biomass advantage of a low antibiotic concentration, which enhances the nitrogen removal effect. In the OFLX stress group, the removal of NH\({}_{4}\)\({}^{+}\)-N in R1-R4 was increased by 26.57%, 18.35%, 8.63% and 7.44% compared to the control group R0, respectively. In stage I, the removal of NH\({}_{4}\)\({}^{+}\)-N by R1 and R2 was significantly enhanced (\(p<0.01\)), which may be due to the prevalence of resistance genes of fluoroquinoline antibiotics in municipal wastewater [40]. Nitriifying bacteria carrying resistance genes were already present in the reactor during the biofilm formation stage, so the removal rate of NH\({}_{4}\)\({}^{+}\)-N was not inhibited by OFLX. Meanwhile, the presence of antibiotics may promote the activity of nitrifying microorganisms [41]. While the nitrification and ammonia oxidation reactions mainly occurred in the surface layer of the BRC-MFC, the denitrification reactions mainly occurred in the submerged layer. Although it was found that the removal of NH\({}_{4}\)\({}^{+}\)-N was enhanced by iron filings in the submerged layer under antibiotic stress, the accumulation of more nitrate-nitrogen in the BRC-MFC system was detrimental to the removal of TN.