[Effect of the Influent C/P Ratio on the Nutrient Removal Characteristics of the SNEDPR System]

This study focuses on the nitrogen (N) and phosphorus (P) removal characteristics in a simultaneous nitrification-endogenous denitrification and phosphorus removal (SNEDPR) system at different influent C/P ratios. An extended anaerobic/low aerobic (dissolved oxygen:0.5-1.0 mg·L^-1) sequencing batch reactor (SBR) fed with municipal sewage was studied by adjusting different C/P ratios (10, 15, 20, 30, and 60). The experimental results show that the proper reduction of the influent C/P ratio (C/P ratio reduced from 60 to 30) enhances the competitive advantages of phosphorus-accumulating organisms (PAOs) in the SNEDPR system. The highest phosphorus removal efficiency was achieved at a C/P ratio of 30, with the anaerobic phosphorus release rate (PRR) and aerobic phosphorus uptake rate (PUR, used as P/MLSS) reaching 3.5 mg·(g·h)^-1 and 4.2 mg·(g·h)^-1 respectively, and an average effluent PO₄^3--P concentration below 0.3 mg·L^-1. The percentage of PAOs contributing to the storage of endogenesis carbon (P_{PAO, An}) reached 88.1%. However, a poor phosphorus removal performance was observed with further reduction of the influent C/P ratios to 10; both the PO₄^3--P removal efficiency and P_{PAO, An} decreased from 38.1% and 82.4% to 3.1% and 5.3%, respectively. The PRR and PUR were 0.2 mg·(g·h)^-1 and 0.24 mg·(g·h)^-1, respectively. The COD removal performance was not affected by the decreasing influent C/P ratios; the average COD removal efficiency stabilized at 85%. In addition, the nitrification performance became worse with decreasing C/P ratios (from 60 to 20) because the effluent NH₄⁺-N and NO₂^--N concentrations increased from 0 and 6.9 mg·L^-1 to 5.1 mg·L^-1 and 16.2 mg·L^-1, respectively. The nitrificaton performance recovered when the C/P ratios further decreased to 10, but the nitrite accumulation was disturbed as both the effluent NH₄⁺-N and NO₂^--N concentrations reduced to 0. The effluent NO₃^--N concentration increased from 0.08 mg·L^-1 to 14.1 mg·L^-1. The SNED efficiency first decreased from 62.1% to 36.4% and then increased to 56.4%. The advantageous competition of glycogen accumulating organisms (GAOs) improved when the influent C/P ratio was lower than 15. The enhancement of the endogenous denitrification ability of GAOs might explain the recovery denitrification performance of the system when the influent C/P ratios decreased from 20 to 10.