In this paper an attempt has been made to set up a relationship between the removal efficiency and the hydraulic regimes of biological filters. For granular-media trickling filters effluent concentrations have been calculated with the equation of complete mixing at low flow rates and by means of dispersed plug-flow model for large hydraulic loadings. Between the mixed flow and the dispersed flow regimes there is a transition zone. For coarse media with a specific surface area of 40 to 70 m2m-3 the mixed flow zone disappears and the substrate removal is obtained using the equation of dispersed plug flow. The above mentioned flow limits have been given as a function of the specific surface area of the media and the application of this theory to the practical problems has also been demonstrated.