## 3 Results and Analysis
### Study on the Nitrogen Removal Performance of a Biological Sponge Iron Reactor  
The trends of various nitrogen species during the operation of R1 and R2 are shown in Figure 3. The average influent NH\({}_{4}\)+-N concentration for R1 and R2 was 49.44 mg/L, with average effluent NH\({}_{4}\)+-N concentrations of 12.39 \(\pm\) 1.20 mg/L and 9.45 \(\pm\) 1.60 mg/L, respectively, corresponding to average NH\({}_{4}\)+-N removal rates of 74.97 \(\pm\)0.02% and 80.93 \(\pm\) 0.03%. This indicates that under long-term aerobic conditions, SFe addition enhanced autotrophic nitrifier activity, facilitating the conversion of NH\({}_{4}\)+-N to NO\({}_{3}\)\({}^{-}\)-N or other nitrogen oxides. Initially, R2 showed an accumulation of NO\({}_{2}\)\({}^{-}\)-N, which can be attributed to NO\({}_{2}\)\({}^{-}\)-N being an intermediate product of nitrification that is easily degraded but requires a longer reaction time. In the early stages of R2 operation, the microorganisms were still adapting to the environment, and nitrification was not yet fully activated. As the microorganisms gradually adapted to the environment and enzyme activities increased, NO\({}_{2}\)\({}^{-}\)-N was rapidly converted to other forms of nitrogen, and NO\({}_{2}\)\({}^{-}\)-N no longer accumulated [28]. R1, having already adapted to the system environment during previous long-term cultivation, exhibited less fluctuation in effluent NO\({}_{2}\)\({}^{-}\)-N concentration. Ultimately, the effluent NO\({}_{2}\)\({}^{-}\)-N concentrations in both R1 and R2 stabilized, with effluent NO\({}_{2}\)\({}^{-}\)-N concentrations basically at 0.01 mg/L, indicating that nitrification was thorough and NO\({}_{2}\)\({}^{-}\)-N did not accumulate substantially. The effluent NO\({}_{3}\)\({}^{-}\)-N concentration in R2 was substantially lower than that in R1. The reason for this could be that iron, as an essential trace element for microbial growth and metabolism, substantially promotes the physiological activities of nitrifying and denitifying bacteria, thereby effectively mitigating the accumulation of NO\({}_{3}\)\({}^{-}\)-N during simultaneous nitrification and denitrification. The average effluent TN concentrations for R1 and R2 were 34.39 \(\pm\) 1.76 mg/L and 29.16 \(\pm\) 3.03 mg/L, respectively, with average TN removal rates of 30.45 \(\pm\) 0.03% and 41.05 \(\pm\) 0.06%. The TN removal efficiency of R2 was substantially better than that of R1. This superior denitrification performance can be attributed to the important role of iron in microbial metabolism. The incorporation of SFe provided the system with abundant zero-valent iron (Fe\({}^{0}\)), Fe\({}^{2+}\), and Fe\({}^{3+}\), which greatly promoted the development and proliferation of the microbial community, enhancing the biodiversity and metabolic functions of the system. The oxygen-consuming corrosion of  
Figure 2: The concentration and removal rate of COD\({}_{\sigma}\) in influent and effluent during the operation of each reactor.  
SFe created a gradient environment ranging from microoxic to anaerobic on its surface and in adjacent areas [29], providing favorable ecological niches for the coexistence of aerobic and anaerobic microorganisms. Additionally, Fe\({}^{0}\) can undergo electrochemical, hydrolysis, redox, and Fenton reactions, further enhancing the denitrification performance of R2.  
Overall, R2 exhibited significantly better nitrogen removal capacity than R1, indicating that the addition of SFe enhanced the denitrification efficiency of activated sludge. The reasons for this improvement are likely due to SFe being a porous material that provides microenvironments for aerobic, facultative, and anaerobic microorganisms, which are conducive to microbial growth and metabolism. Additionally, iron, as an essential element for cell growth, not only promotes the acceleration of electron transfer rates but also enhances the activity of dehydrogenase. The combined effects of these factors substantially improve the denitrification efficiency.