Abstract: To provide an apparatus for treating a nitrate waste liquid that includes a denitrification tank (12A) which accommodates active sludge that adsorbs or takes in the radioactive substance in a nitrate waste liquid (11) and in which an anaerobic microorganism that reduces the nitric acid to nitrogen gas grows, and a reaeration tank (14) in which a denitrification-treated liquid (24) treated in the denitrification tank (12A) is aerated and mixed with active sludge.

Abstract: To provide an apparatus for treating a nitrate waste liquid that includes a denitrification tank (12A) which accommodates active sludge that adsorbs or takes in the radioactive substance in a nitrate waste liquid (11) and in which an anaerobic microorganism that reduces the nitric acid to nitrogen gas grows, and a reaeration tank (14) in which a denitrification-treated liquid (24) treated in the denitrification tank (12A) is aerated and mixed with active sludge.

Abstract: A nitrate solution which is a substance to be treated is heated and denitrated using microwaves, by introducing the substance directly into a cylindrical oven to which microwaves are applied, horizontally rotating the oven about a vertical axis, detecting the amount of reflected waves of the applied microwaves with time to observe the heating condition of the substance, and controlling microwave power to be applied according to the detected amount of the reflected waves and the change in the amount of reflected waves with time. The cylindrical oven preferably has a construction capable of being separated into a stationary upper part and a horizontally rotatable lower part constituting a closed-bottom vessel. These two parts are combined through a choke coupling mechanism to form the oven. According to the detected values of the reflected waves, the precise control of the microwave power applied to the substance to be treated can be conducted at a suitable time.