The pilot plant for this study consisted of one oxygenation basin and two clarifiers. The system treated primary clarifier effluent from the Englewood, Colorado, treatment facility. The influent flow rate was adjusted to attain average aeration reactor detention times ranging from 0.94 to 3.3 hr. The pilot plant operation was conducted in two phases. Treatment performance during both phases was excellent. Final effluent BOD5 concentration and secondary BOD5 removals averaged less than 20 mg/l and greater than 90 percent, respectively. No degradation in process removal efficiency occurred even at organic loadings as high as 1.23 kg BOD5 applied/day/kg MLVSS (1.23 lb BOD5 applied/day/lb MLVSS) and volumetric loadings as high as 4.07 kg BOD5 applied/day/cu m (254 lb BOD5 applied/day/1000 cu ft) of reactor capacity. Analysis of pilot plant operations indicated that somewhat less sludge was produced with the oxygen system when compared to literature-cited, typical air sludge production. This decreased sludge production occurred and became more pronounced at higher organic loadings. Comparisons were made between literature-cited, typical air-sludge and oxygen-sludge settling characteristics. In all cases, the oxygen sludge initial settling velocity was greater than typical air sludge at given TSS concentrations. Average oxygen utilization efficiency was 92 percent.