The performance characteristics of the unsaturated granular media were described using Monod type biological kinetics and mass transfer concepts within microbial films. For this purpose computer techniques were first developed for the numerical evaluation of the normalized biofilms mathematical model. The effects of wetted surface area and the other parameters on substrate removal were then numerically evaluated. The theory enables one to transfer the experimental findings obtained from a pilot plant to another filter for design purposes. Practical application of the theory to experimental results was also demonstrated.