A simplified design method is developed for submerged filter and expanded bed biofilm reactors. The method incorporates limitations of both the electron donor and electron acceptor based upon Monod kinetics, and the stagnant liquid layer at the biofilm surface based upon the packing media and the waste flow rate. The model has been programmed so that calculations can be accomplished on a hand-held calculator, offering wide-scale availability to designers. The model consists of two basic processes: diffusion of substrate from the bulk liquid into the biofilm, and transformation of the substrate by bacteria within the biofilm. Conceptually, substrate must diffuse first from the bulk liquid through a stagnant liquid layer at the biofilm surface, and then into the biofilm until reaching a constant concentration, close to zero, at the effective depth.