Water flow approaches the step screen along with solid screenings with it. The solid screenings are retained on the screen which form a carpet by time. The carpet creates an additional filtering effect and helps to filter increasingly finer particles. As soon as the level difference between upstream and downstream reaches to the predetermined point, the cycle automatically starts to clean the screen. During the cycle, steps move upwards and mobile lamella lifts the screenings onto the next step until the screenings reach the discharge point.