Full scale optimization of an anaerobic/aerobic/anoxic process for nitrogen removal from low-strength municipal...

Achieving advanced nitrogen removal in municipal wastewater treatment plants remains challenging under low-strength influent and seasonal temperature fluctuations. Here, we optimized a full-scale anaerobic/aerobic/anoxic process (40,000 m³/d) and evaluated its year-round performance for influent with biochemical oxygen demand of 49.1 ± 9.9 mg/L and total nitrogen of 18.2 ± 1.4 mg/L. By reducing the air-water ratio to 0.39 and shortening the anaerobic hydraulic retention time to 1.8 h, the system achieves effluent total nitrogen as low as 3.3 mg/L and maintains 4.1 mg/L even as the temperature declines from 24.6 °C to 11.3 °C. The combined measures preserve intracellular carbon in the anaerobic zone and limit aerobic over-oxidation, enabling sustained endogenous denitrification in the downstream anoxic zone and clarifier. Endogenous denitrifiers and hydrolytic bacteria are selectively enriched, and functional genes associated with intracellular carbon metabolism and denitrification show higher abundance. Compared to the parallel anaerobic/anoxic/oxic process treating the same influent, chemical use decreases by 40%, sludge production by 8%, and aeration energy by 73%. This work demonstrates that full-scale implementation of this process provides an energy-efficient solution for low-strength municipal wastewater treatment.