Outdoor tubular photobioreactor microalgae-microorganisms biofilm treatment of municipal wastewater: Enhanced heterotrophic assimilation and synergistic aerobic denitrogenation

[Display omitted] •MMBF enhances heterotrophic assimilation and synergistic aerobic denitrogenation.•Increasing pollutant loads induce an imbalance of NCMs and destabilization of MMBF.•Variation of NCMs determines whether the MMBF can successfully regrow.•Multi-functional HN-AD bacteria are crucial for maintaining the equilibrium of MMBF. This research evaluated a microalgae consortium (MC) in a pilot-scale tubular photobioreactor for municipal wastewater (MWW) treatment, compared with an aeration column photobioreactor. Transitioning from suspended MC to a microalgae-microbial biofilm (MMBF) maintained treatment performance despite increasing influent from 50 L to 150 L in a 260 L system. Carbon and nitrogen removal were effective, but phosphorus removal varied due to biofilm shading and the absence of phosphorus-accumulating organisms. High influent flow caused MMBF detachment due to shear stress. Stabilizing and re-establishing the MMBF showed that a stable phycosphere influenced microbial diversity and interactions, potentially destabilizing the MMBF. Heterotrophic nitrification-aerobic denitrification bacteria were crucial for MC equilibrium. Elevated gene expression related to nitrogen fixation, organic nitrogen metabolism, and nitrate reduction confirmed strong microalgal symbiosis, highlighting MMBF’s treatment potential. This study supports the practical application of microalgae in wastewater treatment.