Relationships between the removal efficiency and the hydraulic regimes of trickling filters were investigated. At low flow rates, where break-up of liquid jets occurs and drops form, completely mixed conditions prevail. For large hydraulic loadings effluent concentrations were calculated by means of a dispersed plug flow model. The ranges of validity of these models were studied theoretically. The effects of drop formation and breakage of liquid jets on the substrate utilization are expressed in terms of hydraulic and physical properties of the media and liquid.