Effects of Light Energy with Different Densities on Denitrification: Metabolic Intermediates, Electron Behavior, Enzyme Activities, and Sludge Settleability

Light energy, as one of the most influencing parameters in microalgal–bacterial consortium (MBC) systems, can regulate activities and metabolisms of microalgae and bacteria consortia. However, its effects on denitrifying bacterial remain unclear. Herein, variations of denitrification, microbial (denitrifiers') metabolism, sludge surface characteristics, and settleability were systematically investigated with five light energy density conditions. Results showed that light energy decreased the denitrification rate with obvious accumulation of denitrifying intermediates (NO2– and N2O). This could be associated with the inhibition of light energy on electron behavior, evidenced from reduced nicotinamide adenine dinucleotide (by 67.0–71.3%), lowered electron transport system activity (by 66.7%), decreased denitrifying enzyme activities (e.g., NAR, NIR, and NOS), and downregulated gene expressions. Moreover, light energy worsened sludge settleability by regulating sludge extracellular polymeric substance (EPS) types, compositions, and surface characteristics. Specifically, light energy stimulated the soluble and lightly bounded EPS, as well as the hydrophilic and electronegative functional groups in EPS. These findings advance our understandings on denitrifiers activities and their electronic behavior responses to short-term light energy. Ultimately, this may promote the development of MBC as a sustainable approach for wastewater treatment.