The article deals with the processes of complex waste water treatment, which are carried out with the sets of hair brushes and aeration, taking into account the hydrodynamic resistance of brushes to liquid stream with the definition of brushes sticking time. The simplified physical and mathematical model which considers liquid as elementary liquid cubes was developed for the calculations. These cubes interact with hair brushes and air bubbles resulting in the removal of impurities. During the study it was found that the number of hairs per unit volume and density of air bubbles greatly affects the stepping rate of impurities. It was established that it is necessary to consider the hydrodynamic resistance of brushes for a flow velocity more than 0.2 m/s. All the proposed calculations make it possible to design new devices and to upgrade the existing ones to provide a high degree of purification of liquid medium with the help of straining and aeration and to facilitate their maintenance.