In this study, biofilm annular reactors (BARs) were used to evaluate the effectiveness of various treatment processes, such as coagulation, direct filtration, biofiltration (with granular activated carbon or anthracite filter media), primary disinfection, and dissolved air flotation on biostability. The BARs are continuous-flow, dynamic systems that were used to simulate two hours of residence time in a 12-inch diameter pipe. The biofilm reactors were placed at the effluent of a given treatment process to determine the biological stability and regrowth potential of the treated water. The surface of the biofilm reactors become inoculated and colonized by the indigenous bacterial populations that escape treatment. An important advantage of the BAR method is that it provides for the direct sampling of the biofilm and the continuous evaluation of the biostability of a specific water over time. Includes 17 references, tables, figures.