Residence time distribution (RTD) studies were executed on a water – softening reactor consisting of three physically separated units in order to characterize its hydrodynamical behaviour. The measured RTD curves and the actual mean residence times were compared with the theoretical values set up by modelling the reactor as a hydrodynamical system. The experiments were carried out by means of radiotracer technique. Neither the mixing, nor the sedimentation reaches ideal flow. Flow type in the settling cylinder is far of that is expected.