Following the clarification phase of water treatment, pre-filtered water passes from an inlet channel and onto a filtration bed. As it passes through the bed’s media—sand or anthracite in depth-filtration types; cloth or synthetic membranes in surface-filtration types—the water is cleansed of its fine-grained suspended solids.
Filter tank level triggers the backwash cycle. As solids accumulate within the filter, headloss begins to build up and water level increases in the filter tank. A pre-determined tank level indicates that terminal headloss value has been reached, and that the filter must now be backwashed to remove the suspended solids.