Increasing the light-dark ratio enhances nitrogen removal performance by altering the mechanism in photogranules

Photogranules provide a cost-effective solution for treating mariculture wastewater. The impact of light: dark ratios on nitrogen removal needs further study. We tested four photogranular reactors with different light: dark ratios and found that higher ratios increased total inorganic nitrogen (TIN) removal rate, achieved 99 % every 48 h at a 5.5 h:0.5 h ratio. Kinetic and metagenomic analyses showed that increased TIN removal was mainly due to a significant transformation in the nitrogen removal mechanism of photogranules. At 5.5 h:0.5 h, diatoms replaced the outer cyanobacteria, causing nitrifying bacteria to disappear via direct and indirect inhibition. In addition, the mechanisms by which photogranules remove nitrate nitrogen are diverse. Adjusting the light: dark ratio could change the nitrogen removal mechanism of photogranules in mariculture wastewater treatment and enhance their nitrogen removal performance. This offered insights into controlling light - related parameters of photogranules for practical engineering applications.