Abstract
In this study, the performance of on-site treatment system for domestic wastewater (Johkasou) was evaluated. The qualities of effluent from actual nitrogen removal type Johkasou which has begun to become popular recently and removal efficiencies were examined. The nitrogen removal type Johkasou showed better performance for BOD and nitrogen removal than the conventional type Johkasou. Both BOD and total nitrogen (T－N) concentrations were 15 mg/L or less in 75－80% of effluent water samples investigated. The high performance of the treatments system was attributed to a mixed liquor recycle system for denitrification. The reduction of BOD concentration in the effluent water was also observed accompanied by the high efficiency of nitrogen removal. Recycling of treated water would result in improving reduction of not only NO3－N but also dissolved Kjeldahl nitrogen (DKN) reduction, and the decrease in DKN led to a decrease in both N－BOD and BOD as well. The denitrification activity was declined when circulation ratio drop to 1 Q or less where Q is the influent flow quantity. The effluent T－N seemed to be controlled below 20 mg/L with the recycle ratio of 2 Q－15 Q. The average removal rates of T－N and total phosphorus (T－P) of the surveyed Johkasous were 67% and 58%, respectively.
