PURPOSE: To suppress formation of nitrate nitrogen which is a by-product having a problem in the case of removal of ammoniac nitrogen in waste water containing the ammoniac nitrogen by incorporating ozone in the presence of bromine ions into this waste water.
CONSTITUTION: The treated water from which the ammoniac nitrogen after incorporation of the ozone is removed is partly subjected to a reduction treatment and thereafter, the ozone is added to the ammoniac nitrogen-contg. waste water while the treated water is circulated and mixed to and with the ammoniac nitrogen-contg. waste water prior to the incorporation of the ozone. For example, a discharge pipe 10 in which the treated water flows is provided with a reduction mechanism 12 and a return pipe 16 is installed between the downstream side of the reduction mechanism 12 of the discharge pipe 10 and an introducing pipe 4 into which the raw water flows to circulate a part of the treated water subjected to the reduction treatment by the reduction mechanism 12 through the return pipe 16 to this introducing pipe 4.
AKEGA HARUKI
TAKADA KATSUO
IWAI TOMIO
