Overcoming carbon scarcity: S⁰-Fe⁰ metabolic synergy enhances nitrogen and phosphorus removal in A/O/A-SBR systems...

Aiming at the technical bottlenecks of insufficient carbon source and poor synergistic efficiency of nitrogen and phosphorus removal in the conventional biological treatment of wastewater with a low carbon-to-nitrogen (C/N) ratio, this study developed an enhanced sequencing batch reactor‑anaerobic/oxic/anoxic (SBR-A/O/A) process synergistically strengthened by elemental sulfur (S⁰) and zero-valent iron (Fe⁰). A static magnetic field (MF) (10 mT) was introduced to mitigate Fe⁰ passivation. The performance and mechanisms of the system for nitrogen and phosphorus removal from low C/N (C/N = 4) wastewater were investigated. The results demonstrated that after adding S⁰ and Fe⁰ carriers, the total nitrogen (TN) removal efficiency increased from 69.63% to 78.03%. With MF application, sustained iron ion release further improved average TN and total phosphorus (TP) removals to 82.26% and 81.56%, respectively, while effluent sulfate remained below 250 mg/L. Microbial community analysis revealed that autotrophic functional bacteria, such as sulfur oxidizing bacteria (SOB) (Thiobacillus) and nitrate-dependent ferrous oxidizing (NDFO) bacteria (Gallionellaceae and Thermomonas), were enriched inside the carriers, forming a synergistic metabolic network with glycogen‑accumulating organisms (Candidatus_Competibacter and Defluviicoccus) and denitrifying phosphorus‑accumulating organisms (Paracoccus, Dechloromonas, and Acinetobacter) in the suspended sludge. Additionally, Fe⁰ achieved chemical phosphorus removal by releasing iron ions to form Fe-P precipitates. The phosphorus content in the system sludge increased significantly (19.5 wt%), indicating considerable potential for phosphorus resource recovery. The proposed S⁰-Fe⁰ synergistically enhanced A/O/A process offers the dual benefits of sludge reduction and phosphorus resource recovery, providing a promising solution for efficient nitrogen and phosphorus removal from low C/N wastewater.