For more than 20 years, engineers have used fairly simple design equations, such as the modified Velz equation, to design trickling filters.1,2 In developing such equations, the assumption was that soluble BOD (sBOD) removal followed a first order kinetic expression. However, different plastic media with the same specific surface can have different first order rate coefficients.2,3 Faced with a range in model coefficients for the same media type, engineers have usually conducted expensive and site-specific pilot studies or have resorted to excessive conservatism in final design.