Granular media filters are commonly used to remove particulate solids from water in water and wastewater treatment plants. These filters typically employ sand, crushed coal, granular activated carbon (GAC), crushed garnet or a combination of these or similar granular materials as filter media. The water to be filtered is passed through the filter media and the particulate solids are trapped at the surface of the filter media and in the interstitial spaces within the filter media. The entrapped particulate solids are removed from the filter media by a periodic washing operation called backwashing in which water or a water/air combination is passed upwardly through the filter media at sufficient velocities to carry the particulate solids upwardly. The backwash water and entrained particulate solids are intercepted by a backwash water collector typically consisting of a pipe or trough. Because of turbulence caused by the backwashing process, grains of the filter media can become entrained in the backwash water stream and are carried over the overflow weir edge of the backwash trough and are removed or lost from the filter. The media loss is particularly severe when using low density media such as crushed coal or when using air to assist in scouring the entrapped particulate solids from the filter.
One prior art granular media filter is shown in U.S. Pat. No. 4,076,625 and includes means intended to reduce media loss and consisting of baffles placed adjacent to or surrounding the backwash trough to intercept and deflect the air used in the backwash process. The baffles reduce the velocity and turbulence of water currents near the overflow weir edge of the backwash trough to thereby reduce media loss.