Bar Screen
 

The screen must be cleaned periodically so it can operate smoothly and allow water to pass though without impediments.  In a water treatment plant, the bar screen is designed to catch large objects before the influent enters the intake pipe.  In a sewage treatment plant, the bar screen catches large objects in the influent before the flow reaches the grit chamber.  In nature, running water washes the soil away from the roots of large trees and rocks allowing them to fall in the stream.  This creates a natural filter for larger objects and slows the water flow.