Effect of Magnet Powder (Fe₃O₄) on Aerobic Granular Sludge (AGS) Formation and Microbial Community Structure Characteristics

Magnet powder (Fe₃O₄) could affect the growth and biodegradation ability of microbes by producing a magnetic field and iron ion. In this study, the enhancement of aerobic granulation by adding Fe₃O₄ was performed to evaluate the effects of different Fe₃O₄ concentrations (0, 0.4, 0.8, 1.2, and 1.6 g/L) on sludge granulation. Fe₃O₄ had a positive effect on the formation and growth of aerobic granular sludge (AGS) during the start-up period. In addition, the Fe₃O₄ concentration at 0.4–1.2 g/L promoted COD removal compared to the sequencing batch reactor (SBR) without Fe₃O₄. The three-dimensional–excitation emission matrix (3D–EEM) indicated that 0.8 g/L Fe₃O₄ addition could accelerate the granulation by stimulating extracellular polymeric substance (EPS) secretion which was an advantage for enhancing granule size. Meanwhile, microbial richness and diversity of AGS was significantly affected with Fe₃O₄ addition by high-throughput sequencing. Furthermore, dominant groups contributing to granule formation, COD removal, and nitrifying–denitrifying were identified under different Fe₃O₄ concentrations.