First stage filtration of water is done by Bar Screens (also known as Trash Racks). These remove bulky and coarse material. The spacing of bars on such bar screens depends upon the quality of filtered water required. To prevent material too coarse from reaching the travelling band screens, bar screens may be used in two stages where the first screen with wider bar spacing remove bulky solids; a second bar screen with smaller bar spacing then remove debris still coarse enough to clog travelling band screens installed downstream for finer screening.