Abstract
The effects of BOD5/TKN ratio in raw night soil and dissolved oxygen concentration in a U-tube bioreactor on nitrogen removal and N2O formation were studied using a single-sludge acivated sludge system including a U-tube bioreactor with a membrane filter for the separation of activated sludge from purified night soil. The advantage of this system is that high nitrogen removal efficiency is achieved even at a low BOD5/TKN ratio in night soil without additional carbon source by the high MLSS (average MLSS of approximately 20 kg·m-3) operation. Night soil treated by the U-tube bioreactor in a pilot plant had an average BOD5/TKN ratio of 1.75. It was found out that accumulation of NO2- in the U-tube bioreactor, high N2O formation and inhibition of nitrifying organisms took place at BOD5/TKN ratio of 2 in night soil by the conventional intermittent aeration mode in which the dissolved oxygen concentration was kept over 2 mg·l-1 during the aerobic period. However, the reduction of NOx-N and control of N2O formation were achieved completely by the low dissolved oxygen concentration (between 0 and 1 mg·l-1) mode in which the average BOD5/TKN ratio and TKN load were 1.75 and 0.05 kg·kg-1·d-1, respectively.
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