Comparison and strategy of nitrogen removal at different low temperatures in a pilot-scale A²/O system

To ensure low effluent total nitrogen (TN ≤ 15 mg L^-1) and NH₃-N( ≤ 5 mg L^-1) at low temperatures (8.8-14.7°C), a pilot-scale A²/O system with a low return activated sludge (200%) was employed to seek the optimal operation strategy. In the 90 days' experiment, the tests were separated into three stages (14.2°C, 11.0°C, and 9.1°C), and the 49 samples were analysed. Our results showed that the nitrification was remarkably inhibited when the temperature was below 10°C, while the denitrification was remarkably inhibited at the temperatures of 10-15°C. Once the effluent NH₃-N concentration was over 2.5 mg L^-1, or $$ concentration was over 13 mg L^-1, the effluent TN would be in excess of 15 mg L^-1. When the temperature was over 10°C, increasing dissolved oxygen (DO) was an optimal strategy to reduce the effluent NH₃-N; nevertheless, when the temperature was below 10°C, simultaneously increasing DO and MLVSS would be carried out due to the low max. nitrification rates (36.0 mg (g^-1 d^-1)). If the carbon source (CODcr/ $$ ratio ≥ 7.3) was enough, increasing MLVSS was an effective method to relieve the negative influence of low temperature on denitrification.