This study was performed using the Johkasou system installed at the Gunma National College of Technology, which uses a contact aeration process to effect water treatment. This system does not have denitrification capabilities. Several intermittent aeration modes were surveyed to investigate their effects on the total nitrogen (T–N) removal performance and energy efficiency. The water temperature was significantly correlated with the decreased velocity of dissolved oxygen (DO); the
decreases at 25 and 23°C were 1.4 and 1.2 mg・(L∙hr)−1, respectively. The aeration time required to achieve DO 0 mg・L−1 was
more than 6 h. The time required upon restarting to reacquire the DO value observed during aeration was 30 min. It was thought that an aeration period exceeding 3 h was necessary for denitrification. Intermittent aeration times of 3 and 6 h exhibited good biochemical oxygen demand (BOD) values at more than 25°C. However, at water temperatures lower than 20°C, the effluent BOD was approximately 3 times higher, denitrification did not proceed, and the NH4-N concentration became high. The denitrification effect was not able to expect the big effect in comparison with consecutive aeration mode.
As a result of this study, we concluded that the most suitable operating conditions for intermittent aeration would employ a 3 h application from December to April, and a 6 h application from May to November. The energy costs were calculated to be 1,120,000 yen∙year−1 based on the most suitable operating conditions. In comparison to the consecutive aeration mode, the energy, environmental, and cost savings with intermittent aeration were calculated to be 38,500 kWh, 15.6t-CO2, and 730,000 yen per year
