Abstract
Effects of high circulation ratio on nitrification and denitrification processes were investigated. In bench-scale Johkasou model system, increase in hydraulic retention time was effective for BOD removal. However, T－N removal efficiency was not increased because of NO2＋3－N remaining in effluent. Because both nitrification and denitrification rates were high, effluent concentration of T－N should depend on the operational parameter, circulation ratio. Therefore, circulation ratio was set from 0 to 7 in bench-scale systems although circulation ratio is normally set from 3 to 4 empirically. In the case of higher circulation ratio such as 6, denitrification was affected by the large volume of circulation water causing increase in dissolved oxygen in anaerobic tank. Meanwhile, lower circulation ratio decreased dilution rate of influent domestic wastewater and increased NH4－N concentration in the inlet water of aerobic tank. The effect of NH4－N concentration on ammonia oxidation rate was investigated by batch test, and it was found that ammonia oxidation rate was not affected by NH4－N concentration ranging from 5.6 to 11.25 mg l－1. According to these results, optimum circulation ratio should be 5. The effect of high circulation ratio was also demonstrated in the full scale Johkasou systems.
