[Effect of HRT on Denitrifying Phosphorus and Nitrogen Removal in Modified A²/O-BAF]

The effect of hydraulic retention time (HRT) on denitrifying phosphorus and nitrogen removal in a modified two sludge A²/O-BAF system was studied. The influent COD, NH₄⁺-N, and TP were 189.6, 60.4, and 5.1 mg·L^-1, respectively. When HRT was 9, 8, 7, and 6 h, the average effluent COD was less than 42 mg·L^-1. The average effluent NH₄⁺-N levels were 2.4, 2.8, 3.3, and 6.5 mg·L^-1, respectively. The average effluent TP values were 0.3, 0.4, 0.7, and 0.8 mg·L^-1, respectively. The ratio of the denitrifying phosphorus accumulation organisms (DPAOs/PAOs) in the system of anoxic zone was reduced from 76.8% to 48.8%. When HRT was 8 h, the ratio of denitrifying phosphorus to nitrogen (ΔPO₄^3-/ΔNO₃^--N) was increased by 37.5% by a mathematical statistics method. The ΔPO₄^3-/ΔNO₃^--N in the anoxic zone was 1.24 (the theoretical value is 1.41). At this time, the effect of denitrifying phosphorus to nitrogen was the best. The SVI value was lower than 100 mL·g^-1 throughout the experiment, and the MLVSS/MLSS gradually decreased from 0.74 to 0.63, indicating that the sludge activity was reduced.