The influence of Fe2+, Fe3+ and magnet powder (Fe3O4) on aerobic granulation and their mechanisms

This study aimed to develop an aerobic granular sludge and understand the granulation process of the multi-iron ions. Four sequencing batch reactors (SBRs) were applied to elucidate the effect of Fe2+, Fe3+ and Fe3O4 addition on aerobic granulation. The results confirmed that the start-up time of aerobic granulation with Fe304 addition (11 days) was notably less than that with Fe2+ (16 days) and Fe2+ (27 days) addition. Larger granules achieved with Fe3O4 addition with a sludge volume index (SVI30) of 28.50 mL/g and settling velocity of 49.68 m/h. Scanning electron microscope (SEM) analysis further revealed that the presence of mineral crystal in the granule core with Fe2+ and Fe3O4 addition accelerated the granule formation and maintained the stability of the structure. Extracellular polymeric substances (EPS) were studied using three-dimensional-excitation emission matrix (3D-EEM) fluorescence spectra technology to gain a comprehensive view of the interactions between EPS and Fe2+, Fe3+ and Fe3O4. Around 94.76% and 97.68% removal rate was noted for COD and ammonia in the granulation process. Finally, the dominant functional species involved in biological nutrients removal and granule formation were identified by high throughput sequencing technology to assess the effects of Fe2+, Fe3+ and Fe3O4 to granule at the molecular level.