The classical screen in waste water purification is the bar screen. A grid of flat bars (bar spacing 4-30 mm), which is angled at approx. 60°, retains the screened material. The latter is lifted off with a mechanically operated rake and thrown onto a conveying device (screw conveyor, conveyor belt) by a scraper. The screen is controlled by means of the difference in water level upstream and downstream of the screen array, and a minimum clearing frequency (night operation) is controlled by means of a time program.
The multi-raked screen is a continuously cleaning bar screen. To this end there are many rakes mounted on a chain (as far as the channel bed).