Simultaneous low C/N ratio wastewater treatment and solid waste utilization: Enhanced nitrogen removal in red mud-derived iron-carbon substrate floating bed
Iron-carbon micro-electrolysis (ICME) materials have been demonstrated considerable efficiency in deep nitrogen removal for low-polluted wastewater. However, their widespread application was constrained by economic cost and unsatisfied activity. Herein, a novel ICME material (FeC-RM) was prepared by solid waste (red mud and corn stalks) to achieve low-cost and high-efficiency nitrogen removal, as well as solid waste utilization. When employed in near-natural ecological bed, FeC-RM achieved 57.5% higher total nitrogen (TN) removal efficiencies compared to conventional gravel-based systems for treating low-polluted wastewater at C/N ratio of 2. The rough structure and high hydrophilicity of FeC-RM enhanced microbial adhesion. The excellent iron-carbon micro-electrolysis capacity of FeC-RM enhanced the relative abundance of nitrate reduction and Fe(II) oxidation (NAFO) bacteria (Ferritrophicum, Silanimonas, and Rubrivivax) for autotrophic denitrification. With superior electrochemical performance, FeC-RM enhanced electroactive bacteria enrichment and promoted c-type cytochrome and type IV pilus assembly, which established an electron transfer network for efficient electron utilization between electroactive bacteria (Rhodobacter and Thauera) and denitrifying bacteria (Azospira and Dechloromonas). Besides, FeC-RM facilitated microbial iron uptake and enhanced Fe-S clusters synthesis in respiration chain for remarkable microbial metabolic activity and denitrification process. Compared to commercial ICME materials, FeC-RM reduced the synthesis cost by 90%, and realized the idea of "treating waste with waste". This research presented a promising strategy for resource utilization of red mud and advanced the development of ICME materials for treating low C/N ratio wastewater.