Insight into the impact of air flow rate on algal-bacterial granules: Reactor performance, hydrodynamics by Computational Fluid Dynamics (CFD) and microbial community analysis

This study evaluated the impact of air flow rates on nitrogen removal and the formation of algal-bacterial granules in domestic wastewater treatment, revealing that sodium acetate addition and aeration shifted the microbial community composition. Two separate feedings and the addition of external organic carbon significantly enhanced inorganic nitrogen removal efficiency (over 90 %), which might have contributed to the increased microbial diversity and the relative abundance of denitrifiers such as Acinetobacter. This study also demonstrates that the higher air flow rate (0.5 LPM) resulted in higher nitrification rates (up to 6.4 mg N/L/h) and produced smaller and denser granules. The higher air flow rate also led to the increasing relative abundances of nitrogen-related genera (such as Nitrospira) and the decreasing relative abundances of cyanobacteria and Chlorella. Computational Fluid Dynamics (CFD) simulations revealed that mechanical mixing predominantly generated shear force. Increasing the air flow rate from 0.2 LPM to 0.5 LPM only yielded a 12 % increment in the volume-averaged strain rate, suggesting diminishing returns from higher air flow rates in terms of shear force enhancement. Moreover, the decrease in total abundance of grazers and pathogens along with the operation, including Chytridiomycetes, Sessilida, and Operculariidae, might result from the shear force and the decrease of Chlorella spp. This study underscores the critical roles of aeration and carbon source management in optimizing the performance and microbial ecology of algal-bacterial systems in wastewater treatment while minimizing energy inputs.