A plug flow, O2-activated sludge process was operated with alum addition to remove phosphorus and with lime addition to prevent the process pH from decreasing below 6.4. The O2 reactor was operated at F/M ratios between 0.18 to 0.24 gm of BOD5/gm of MLVSS/day in a typical co-current O2-liquid contacting system. The alum dosages for the five steady-state operating periods increased from 84 mg/l to 184 mg/l was used to maintain the process pH at 6.4. The pollutant removals from the primary effluent ranged from 82 to 92% for BOD5 (7.4 to 19 mg/l of residual BOD5) and from 54 to 86% for phosphorus (1.05 to 3.26 mg/l of residual P). The optimum P removal occurred at a 1.8 to 1 average Al/P mole ratio with an average total residual P of 1.05 mg/l. In a second study, the O2 process was operated in a step-feed configuration consisting of a sludge oxygenation stage followed by three stages of oxygen aeration with equal portions of the primary effluent fed to each stage. In typical operation, the process with a F/M ratio of 0.23 exhibited a MLSS concentration profile in the four stages of 10,700 mg/l; 7,060 mg/l; 5,020 mg/l; and 4,150 mg/l. The step configuration clearly reduced the solids loading to the clarifier and provided an average MLSS of more than 6,500 mg/l. The step operation produced excellent BOD5 removals (89% from primary). The O2 usage in the step configuration was significantly less than the 90% typically achieved in co-current contacting.