PURPOSE: To perform stable biotreatment, while enhancing the efficiency of purification per weight of sludge, by subjecting a part of an outflow liquid from the second denitrification tank to a centrifugal thickener, and handling a part of the conc. sludge as returned sludge and the remainder as excess sludge.
CONSTITUTION: Organic filthy water 1 of high concentration, e.g. night soil one, is let flow into the first denitrification tank 2 and treated therein by mixing it with a nitrified liquid 9 circulated from a nitrification tank 3 and conc. returned sludge 10 from a centrifugal thickener 5, to reduce nitrate and nitrite nitrogens in the nitrified liquid 9 into nitrogen gas. A part of the nitrified liquid 9 is let flow into the second denitrification tank 4 and reduced therein into nitrate and nitrite and nitrogen gases. A part 4a of an outflow liquid from the second denitrification tank 4 is treated by the centrifugal thickener 5. A part of the resulting conc. sludge 5' is returned as conc. returned sludge 10 to the first denitrification tank 2, while a separated liquid 5" is transferred to the next reaeration tank 6.
KOIKE YOUICHIROU
KOJIMA RIYOUZOU
NARAHARA HIDEO
