The concept of residence time distribution (RTD) can also be applied to incompressible fluids in closed-closed systems under nonsteady conditions, when the residence time distribution expressed as a function of a residence time, defined in this paper, is independent of the volume and/or the flow rate. In this paper, an analysis of the hydrodynamics of the plug flow reactor (PFR), the continuous stirred tank reactor (CSTR), the plug flow reactor with axial dispersion (DFR) and a series of n-CSTR under unsteady-state conditions is made. A generalized residence time distribution is proposed for analyzing the behavior of the system. The proposed RTD is applied to the experimental data obtained when a tracer is introduced as a pulse in a sewage system. Three runs, with different outlet flow ranges, were carried out. By means of the generalized RTD, a disperse flow reactor model for correlating the experimental data was proposed. In this way, the variation deduced in the tracer outlet concentration can be explained, despite the fact that the outlet flow range is different from one run to another.