A jet aerator header assembly is provided for the deep oxygen contact duct of a partial, total, or non-barriered oxidation ditch having an endless channel. The header assembly may be vertically installed in the bottom of the contact duct or at an angle in the intake portion thereof as a mid-duct header assembly; it can additionally be installed in a horizontal position at the inlet of the contact duct as an inlet header assembly. Each header assembly comprises a liquid header, an air header, a plurality of jet aerators which are flow connected to both headers, stiffener plates which rigidly attach the headers to each other and to the jet aerators, and pipes connecting the headers to respective liquid and air supply lines. The mid-duct header assembly is slideably mounted within an access duct which is accessible from the surface. The mid-duct header assembly can be operated to provide: (1) a base level of dissolved oxygen in the mixed liquor being discharged from the contact duct, and (2) a minimum circulation velocity within the endless channel of the oxidation ditch. The inlet header assembly can then be selectively operated to provide the additional amount of dissolved oxygen in the discharge mixed liquor that is required for aerobic treatment of the wastewater entering the endless channel at that time and season of the year. Air and liquid can be separately and selectively controlled for both mid-duct and inlet headers. Such operation provides separate control of mixed-liquor circulation velocity and of mixed-liquor aeration.