Influence of carbon to phosphorus ratio on the performance of single-stage aerobic simultaneous nitrogen and phosphorus removal by bioflocs

Effluents from intensive aquaculture typically contain high nitrate and phosphate concentrations. Biofloc technology has demonstrated the potential for simultaneous removal of nitrate and phosphate without ammonium nitrogen, and further optimization is needed to enhance the nitrogen and phosphorus removal efficiency. In this study, we investigated the efficiency of bioflocs in treating highly concentrated aquacultural wastewater at different carbon to phosphorus (C/P) ratios of 20 (G20), 30 (G30), and 40 (G40). The results showed that the nitrate removal rate in group G40 (1.25±0.07 mgN/gTSS/h) was significantly higher than in groups G20 and G30 (p < 0.05). However, there was no significant difference between the phosphate removal rates of groups G40 and G30, while both exhibited superior G20. The relative abundance of Thauera in G40 was significantly higher (p < 0.05), accounting for 8.74 % of the microbial community. Additionally, the copy counts of denitrification-related genes (napA, nirS, nirK, nosZ) and inorganic phosphate transport genes (pqqC) were significantly higher in G40, correlating positively with an increased C/P ratio. These results suggest that the excessive carbon source in G40 enhanced denitrification and reduced biofloc assimilation, thus failing to significantly enhance the phosphate removal rate. This study demonstrates that adjusting the C/P ratio alone can improve the efficiency of nitrogen and phosphorus removal by bioflocs, and that a C/P ratio of 30 may be the most appropriate for enhancing the rate of nutrient removal while minimizing the use of carbon source.