A design approach for plastic media bio-oxidation towers is presented. This paper considers that the rate of mass transfer of oxygen is the rate controlling factor in the overall rate of reduction of BOD. A simplified mathematical model is presented which permits the prediction of the BOD removal which can be obtained in plastic media towers of a given size and loading. This model permits a design engineer to evaluate the capabilities of a proposed design, and it gives plant operators a guide for trouble-shooting plants with poor removals.