An evaluation of the pure oxygen activated sludge system (UNOX) has been underway since May, 1970. During the first 16 months of test operation on the 100,000 gpd facility at the Blue Plains Wastewater Treatment Plant, over five different phases of operation were tested to demonstrate the performance of the system under varying conditions. The report describes the pilot plant facilities, the character of the wastewater, and the mode of operation and process information. The oxygen activated sludge system (UNOX) consisted of a unique, four stage, gas tight biological reactor that employed cocurrent gas-liquid contacting. In less than 1.85 hours of oxygenation, the system removed 90 percent of the influent BOD5 and utilized over 95 percent of the supplied oxygen. The effluent quality was as good or better than that obtained from a 3.6 hour step aeration system operating in parallel with the oxygenation system. The microbial organisms visually were essentially the same as those found in a typical conventional system. Their rate of activity, however, was greater than those of the air system. Satisfactory solid-liquid separation was achieved.