Insight into enhanced adaptability of iron-carbon biofilter in treating low-carbon nitrogen mariculture wastewater for nitrogen removal and carbon reduction.

Iron-carbon (Fe-C) based biofilters have shown significant advantages in treating mariculture wastewater by facilitating the mixotrophic heterotrophic nitrification-aerobic denitrification (HNAD) process. However, the effects of Fe-C materials and varying carbon-to-nitrogen (C/N) ratios on N removal and C reduction performance remain insufficiently explored. This study demonstrated that the Fe-C biofilter (R-Fe) achieved significantly higher NO-N removal efficiency (65.1-96.0 %) compared to the control filter (-12.1-76.9 %) across all tested C/N ratios. Furthermore, the NO emission proportion in R-Fe was reduced by 37.4-42.4 % compared to the control. Increasing the influent C/N ratio enhanced N removal efficiency while reducing the proportion of NO emissions. This improvement correlated with enhanced electron transfer activity and an increased abundance of heterotrophic nitrifying-aerobic denitrifying bacteria (HNADB) and heterotrophic denitrifying bacteria (DNB), while the abundance of autotrophic denitrifying bacteria declined. Strong correlations were observed among microbial electron transfer activity, denitrifying microbial communities, Fe transport genes, denitrification-related functional genes, N removal efficiency, and NO emission proportion, highlighting the critical role of electron transfer activity in microbial N removal processes.