The circulated aeration type biological denitrification system, named the Kubota Nitro-cycle System, has been developed for removing total nitrogen in night soil. It has the following features:
In the circulated aeration tank, BOD removal, nitrification and denitrification can be simultaneously conducted by activated sludge treatment without dividing the tank into an aerobic zone and an anaerobic zone, and it is possible to remove 80–90% of the total nitrogen and BOD in night soil influent.
Denitrification and re-aeration tanks are provided after the circulated aeration tank and over 99% removal of total nitrogen and BOD can be attained with the whole system.
In this system, not only nitrogen, but also phosphorus can be removed to the extent that there remains only a trace and this is effective to prevent eutrophication.
This system was approved by the Ministry of Health and Welfare of the Japanese Government in 1978 as a new technology for night soil treatment.