The effect of nano-Fe3O4 addition on the nitrogen transfer pathways and biomass morphology in floc-granule coexistence system

Various materials such as activated carbon, zeolite, mature granular sludge, and nano-composites are used as inert nucleus to cultivate aerobic granular sludge (AGS), this study operated parallel reactors to cultivate AGS with and without nano-Fe3O4 addition inoculated by activated sludge. This study systematically compared the granulation process, nitrogen removal efficiency, transfer pathways, and microbial community structure under long-term operation. Results demonstrated that nano-Fe3O4 addition significantly promotes AGS formation with over 74.04 % of biomass exceeding 200 μm, and the average SVI and granule size were 59.08 mL/g and 361.50 μm after 60 days. The nano-Fe3O4 induced the sludge to secret more humic acid to accelerate electron transfer, and enhanced the nitrification process, particularly the transfer pathway from NO2--N to NO3--N. The specific NO2--N oxidation rate increased from 5.26 mg/(g·h) to 9.98 mg/(g·h). Furthermore, nano-Fe3O4 significantly altered the microbial community structure, the relative abundance of Nitrosomonas and Nitrospira with nano-Fe3O4 were 0.54 % and 1.54 %, compared to 0.30 % and 0.15 % without nano-Fe3O4. Furthermore, Thauera increased from 3.64 % to 14.58 %, which is crucial for AGS formation. Overall, the innovative application of nano-Fe3O4 not only enhances sludge granulation but changing nitrogen transfer pathways, underscoring its potential as a groundbreaking approach in wastewater treatment. •The granulation process was accelerated by the collision and magnetic force of nano-Fe3O4.•Nano-Fe3O4 enhanced the nitrification process, particularly the conversion from NO2--N to NO3--N.•Nano-Fe3O4 stimulated the secretion of humic acid in the EPS and strengthened the electron transfer.•Nitrosomonas and Nitrospira were enriched by nano-Fe3O4.