The results of operating a 10,000 to 20,000 gal per day staged rotating disk pilot plant have been presented for three modes of operation, namely, (1) the slimes in all stages exposed to ambient atmospheres, (2) the slimes in stage 1 exposed to an atmosphere enriched with oxygen, and (3) the slimes in stages 1 and 3 of a 3-stage operation exposed to atmospheres enriched with oxygen. In the first mode of operation about 30 minutes of contact with the slimes was sufficient to remove 90% of the B.O.D. while during the second mode of opertaion, 18 minutes accomplished the same rate of removal of B.O.D. In the third mode of operation, not only was 90% of the B.O.D. removed during 18 minutes of contact with the slimes but a substantial portion of the nitrogenous oxygen demand was satisfied. (Modified author abstract)