An apparatus and method for denitrification of treated wastewater from an aerobic wastewater treatment system in which oxygen levels in the aeration chamber of an aerobic digestion vessel are cycled between an anoxic environment, i.e., when the treated wastewater in the aeration vessel contains from just above 0 to just less than 1 ppm dissolved oxygen to an aerobic environment wherein the treated wastewater in the aeration vessel has a dissolved oxygen content of about 1 ppm or higher.