Abstract: Improvements to an upflow biological reactor (UBR) waste water treatment system include preoxygenating the waste water in a separate vessel before submitting the preoxygenated waste to the biological reaction tower. A unique distribution means is described which maximizes the distribution of the preoxygenated waste across the cross-sectional area of the UBR tower. This results in an optimization of the reaction process between the bacteria culture in the tower and the contaminants contained in the waste water. Anaerobic conditions are thus inhibited and to a large extent, eliminated. Further, the invention provides for a "polishing" system which permits for maximum elimination of the contaminant by bubbling through any remaining oxygen containing air after the process in the UBR tower is complete.