Magnetite-Enhanced Activated Sludge Process for Improving Denitrification Under Low C/N Conditions: A Review

Declining influent carbon-to-nitrogen (C/N) ratios in modern wastewater treatment plants (WWTPs) increasingly limit conventional heterotrophic denitrification, resulting in incomplete nitrate removal, accumulation of nitrite and nitrous oxide (N₂O), and challenges in meeting strict total nitrogen (TN) limits. To overcome these issues, process intensification strategies that decouple nitrogen removal from organic carbon availability are gaining attention. Among these, the addition of conductive materials particularly magnetite (Fe₃O₄) to activated sludge and similar biological systems shows strong potential due to magnetite’s mixed-valence (Fe²⁺/Fe³⁺) properties, semiconductivity, biocompatibility, and ease of magnetic recovery. This review critically evaluates magnetite-enhanced denitrification under low C/N conditions, focusing on activated sludge, biofilm, and hybrid reactor configurations. After detailing heterotrophic and autotrophic denitrification and their electron transfer processes, four primary mechanisms of magnetite-enhanced denitrification are identified: (i) limited abiotic nitrate/nitrite reduction by surface-bound Fe(II); (ii) Fe(II) oxidation-driven autotrophic denitrification; (iii) magnetite-facilitated direct interspecies electron transfer (DIET) between fermenters and denitrifiers; and (iv) stimulation of denitrifying enzymes and enrichment of functional microbial groups such as iron-cycling bacteria and key denitrifiers. Experimental results demonstrate that magnetite supplementation can increase nitrate removal rates by 20–200%, enhance TN removal under low C/N conditions, suppress N₂O formation, and improve sludge granulation and settling. Denitrification gene enrichment (narG, napA, nirS, nirK, nosZ, and Fe-cycling genes) supports these mechanisms. Optimal magnetite dosages range from 0.1–2 g·L⁻¹, with particle size, surface properties, and reactor design influencing outcomes. The review addresses operational and economic aspects, including C/N ratio, dissolved oxygen, redox potential, pH, temperature, and magnetite recovery. It evaluates challenges such as nanoparticle toxicity, iron leaching, phosphorus removal interference, and scale-up feasibility. Future research should focus on distinguishing DIET from Fe-driven pathways, conducting long-term pilot studies, integrating with anammox or membrane-aerated systems, and performing life cycle and techno-economic analyses.