Rapid formation of partial denitrification biofilm using gas–liquid separation membrane as carrier: Performance and mechanism

Partial denitrification (PD) can ensure stable supply of electron acceptors for anaerobic ammonia oxidation, and biofilm is an effective method to prevent biomass loss, which are crucial for stable operation of PD. In this study, hydrophobic hollow-fiber gas–liquid separation membranes were placed in a denitrification sequencing batch reactor, and dense biofilms were formed within just 3 days. Confocal laser microscopy showed the preferential attachment of the protein (PN) content in extracellular polymeric substances (EPS) to the membrane surface, followed by exopolysaccharides. Further analyses showed the decrease in the types of signal molecules from six to two (i.e., C4-HSL, C6-HSL) due to negative pressure operation. Importantly, the concentration of C4-HSL increased dramatically with the increase in PN concentration, suggesting that negative pressure promoted the synthesis of C4-HSL signal molecules, which further mediated the secretion of PN for biofilm formation. In addition, biofilm formation was accompanied by nitrite accumulation, leading to successful achievement of PD. Furthermore, 60 % of nitrate-to-nitrite transformation ratio was obtained even when COD/N was increased from 4.5 to 5.0 and influent nitrate concentration was reduced to 25 mg/L. This confirmed the stability of PD, which was mainly attributed to a change in the microbial community and a decrease in nitrite reductase (Nir) activity, with microorganisms enriched through the gas–liquid separation operation exhibiting low Nir activity. This study provides a new method for rapid formation of biofilm for wastewater treatment and stable operation of PD.