Computer programs based on mechanistic mass transfer models have been developed by the author to predict sBOD removal, ammonia removal, and oxygen transfer in plastic media trickling filters. Although the use of these models requires the numerical solution of a set of differential equations, the availability and speed of modern personal computers makes these programs accessible to design engineers. Application of the program that predicts sBOD removal (TRIFIL2), using a default molecular weight distribution, predicted lower removal (68%) than observed (95%) for a trickling filter in Tucson AZ. Size distributions of sBOD were therefore measured using two ultrafiltration membranes with molecular weight cutoffs of 1,000 and 10,000 daltons. Using the observed sBOD size distributions as input into the computer model we calculated 93% removal which compared very well to the observed removals. We conclude that high sBOD removals at the Tucson plant are due to the high concentration of low molecular weight organic compounds in the wastewater.