## 3 Results and Discussion
### Effect of Antibiotics on the Decontamination Performance of BRC-MFC
#### 3.1.1 Nutrient Removal Performance
This was consistent with the conclusion of this experiment that TN removal was slightly inhibited by exogenous antibiotics. The approaches consistently taken to enhance TN removal by BRC-MFC in the academia are to increase the submerged layer's depth and design the submerged layer with an anaerobic environment to enhance the denitrification reaction. When the submerged layer's depth is increased from 0 cm to 60 cm, the TN removal rate can be increased from \(-\)23% to 62% [42], which has a better enhancement of TN removal. In stage II, the COD removal effects on R0-R4 were good, and the COD removal rates were all maintained above 90%. Compared with the control group R0, there was no significant change in the COD removal rates of R1 and R2, while there was a slight decrease in the COD removal rates of R3 and R4. It can be seen that the COD removal ability of BRC-MFC was not inhibited by the low concentration of OFLX + TC stress. Only 2.4 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) OFLX + 1.2 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) TC and 3.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) OFLX + 1.8 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) TC created a slight inhibition of COD removal. This may be due to the prevalence of resistance genes for fluoroquinoline/tetracycline antibiotics in municipal wastewater [40]. The reactor accumulated a certain amount of resistance-gene-carrying bacteria during the start-up period of the hanging film; therefore, there is some resistance to shock under antibiotic stress. Under combined OFLX + TC stress, there was no significant difference (\(p>0.05\)) in the TN removal rate for each reactor from R1 to R4, but there was a slight decrease relative to the control group R0, which reached a high level compared to other domestic and international studies (TN removal rates from \(-\)133% to 99%) [43]. In the downflow influent BRC-MFC, pollutants pass through the planting layer, filtration layer, electrode layer cathode, transition layer, submerged layer, electrode layer, and drainage layer from top to bottom. A large amount of root secretions and microorganisms exist in the inter-root microenvironment of the planting layer, and some studies have shown that the organic acid component of root secretions can increase the total number of bacteria in the soil, as well as increase the percentage of effective state nitrogen in the soil [44,45] and promote plant uptake of nitrogen nutrients, which also has a positive effect on TN removal and explains the reason why planting plants in BRC-MFC contributes to TN removal from the perspective of the inter-root microenvironment. The submerged layer was supplemented with an additional carbon source by adding 5% wood chips, and the device has a high outlet to keep the submerged zone anaerobic at all times, which facilitates the denitrification reaction. The external wires between the two electrode layers constitute a complete electron transfer system, and the anode in the submerged layer receives electrons conducted from the cathode, which are used to replenish the denitrifying microorganisms in the submerged layer to ensure the balance of nitrogen conversion [46]. Meanwhile, the activated carbon felt at the anode of the electrode layer and the iron filings in the submerged layer act as an electronics shuttle-like body within the anaerobic environment [47], providing better conditions for denitrification reactions. The drainage layer can also control the extension of hydraulic retention time to increase the microbial concentration in the system, which was an important reason for the high level of TN removal in the effluent. Under the combined stress of OFLX and TC, the removal rates of NH4\({}^{*}\)-N from RI to R4 increased by 20.99%, 15.40%, 9.41% and 5.08% compared with the control group R0, especially the removal of NH4\({}^{*}\)-N by RI and R2, which increased significantly (_p_ < 0.01). As shown in Figure 2c, the NH4\({}^{*}\)-N concentrations in the effluent from R1 to R4 were lower than the control group R0.