The combined effects of COD/N ratio and nitrate recycling ratio on nitrogen and phosphorus removal in anaerobic/anoxic/aerobic (A2/O)-biological aerated filter (BAF) systems

• Control strategies have a potential for flexible and efficient N and P removal.
• Different Rs should be employed based on the influent qualities and quantities.
• N and P removal requires the balance of C/N ratio, Rs, microbial populations and DO.
• P removal efficiency was more sensitive to R than C/N ratio.
• R = 400% could be a good option of balancing nutrient removal and economic potential.

The combined effects of chemical oxygen demand (COD) to total nitrogen ratio (COD/N) and nitrate recycling ratio (R) on simultaneous nitrogen, phosphorus, and COD removal were studied in a laboratory-scaled anaerobic/anoxic/aerobic (A2/O)-biological aerated filter (BAF) system. Variation in the COD/N ratio alone had no noticeable effect on COD removal and nitrification, with the effluent COD concentration of 37 mg/L and ammonium concentration below the detection level. However, low COD/N ratio (3.0) with high R (200–600%) deteriorated the total nitrogen (TN) removal due to insufficient carbon source and oxygen presence (dissolved oxygen concentration: 0.31–0.72 mg/L) in the anoxic zones of A2/O. At the COD/N ratio of 4.0, the TN removal efficiency steadily increased from 66% to 81% when R was increased from 100% to 400%, but decreased to 70% at the highest R (600%). At the high COD/N ratio of 5.5, the TN removal efficiency increased from 68% to 90% with R increasing from 100 to 600%. The phosphorus release amounts in the anaerobic zone of the A2/O increased from 16.2 mg/L to 55 mg/L with the COD/N ratio increasing from 3.0 to 5.5. Low COD/N ratios (3.0 and 4.0) had little effect on phosphorus removal, while high COD/N ratio (5.5) caused the phosphorus release in sedimentation tanks due to the residual carbon in effluent.

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