Practical Performance of Nitrogen Removal in Small-Scale Sewage Treatment Plants Operated in Intermittent Aeration Mode

Research was carried out to clarify the number and the scale of small-scale sewage treatment plants operated in intermittent aeration mode by asking questions to maintenance workers in Chiba Prefecture, Japan. The percentage of the number of plants operated in intermittent aeration mode was 26.5%. This percentage was high in the plants larger than 200 population equivalent (p.e.). The practical performance of nitrogen removal was evaluated in nine existing plants operated in intermittent aeration mode. A high performance of nitrogen removal was noticed in the plants us ing extended aeration process. The performances of nitrogen removal in two plants operated in intermittent aeration mode were compared with those operated in continuous aeration mode. The characteristics of denitrification taking place in the aeration tank in a plant operated in intermittent mode were investigated by measuring the change in nitrogen concentration in the tank during the intermittent period. The seasonal performanee of denitrification was studied by a long term practical experiment for 16 months. By operating the length of intermittent time according to the change of the water temperature in the aeration tank, a high performance of nitrogen removal was obtained throughout the year.