Rapid Gravity Filters
Rapid sand filters can be constructed as gravity filters. They are used for water purification purposes and essentially consist of an open-topped box (usually made of concrete), drained at the bottom, and partly filled with a filtering medium. Fresh water is admitted to the space above the medium and flows downward under the action of gravity. Purification takes place during this downward passage and the treated water is discharged through the under-drains.